Ask the Experts: All Questions

Ask the Experts is one of our most popular destinations for healthcare professionals. Our experts provide clear, easy-to-understand answers to commonly asked questions about vaccines and their use.

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Results (1372)

No. Hospitalization should be used as an opportunity to provide recommended vaccinations. Hospitalized patients who are not moderately or severely acutely ill may be vaccinated during hospitalization or at discharge. Current, recent or upcoming anesthesia, surgery or hospitalization are not contraindications to vaccination, although certain factors may lead a healthcare provider to consider these situations a precaution to vaccination for a specific patient.

Last reviewed: August 29, 2022

Yes. Rotavirus infection of adults is usually asymptomatic but may cause diarrheal illness. Outbreaks of diarrheal illness caused by rotavirus have been reported, especially among elderly persons living in retirement communities. For more information on this issue see www.cdc.gov/mmwr/pdf/wk/mm6042.pdf, page 1456.

Last reviewed: June 7, 2023

Zoster is caused by reactivation of a latent varicella virus infection (from having chickenpox in the past). Zoster is not passed from one person to another through exposure to another person with zoster. However, if a person who is susceptible to chickenpox (i.e., they had never had chickenpox and were not vaccinated against chickenpox) comes in direct contact with a person with a zoster rash, the virus could be transmitted to the susceptible person. The exposed person would develop chickenpox, not zoster. Covering the zoster rash reduces the chances of transmitting varicella zoster virus.

Last reviewed: March 9, 2022

Most commercial insurance plans provide coverage of routine vaccinations with no out-of-pocket cost to the family. However, a child whose health insurance plan covers the cost of vaccinations as a benefit is not eligible for VFC vaccines, even if the plan requires a copay or requires that they meet a deductible for the year.

Last reviewed: August 26, 2022

You should record the generic abbreviation for the type of vaccine given (e.g., DTaP-IPV-HepB) in each of the sections that correspond to the separate antigens listed on the record (e.g., DTaP section, polio section, hepatitis B section). If possible, avoid using trade names, since trade names could be misinterpreted or discontinued.

Last reviewed: July 15, 2023

Immunity to pertussis following infection is not life-long. Persons with a history of pertussis should continue to receive pertussis-containing vaccines according to the recommended schedule. (Note: This answer is based upon recommendations of the AAP’s Committee on Infectious Diseases.)

Last reviewed: March 31, 2022

ACIP recommends administration of varicella zoster immune globulin (VariZIG, Saol Therapeutics) to certain people up to 10 days following exposure to varicella or herpes zoster. People for whom VariZIG is recommended are those without evidence of immunity to varicella who are at high risk of severe disease and complications of varicella illness and are ineligible for varicella vaccination. VariZIG given up to 10 days after an exposure can modify or prevent clinical varicella disease. See the varicella zoster immune globulin section below, and www.cdc.gov/mmwr/pdf/wk/mm6228.pdf, pages 574–6, for more information on this topic.

 

Patients recommended by ACIP to receive VariZIG include:

  • Immunocompromised patients without evidence of immunity
  • Newborn infants whose mothers have signs and symptoms of varicella around the time of delivery (i.e., 5 days before to 2 days after)
  • Hospitalized premature infants born at 28 weeks (or more) of gestation whose mothers do not have evidence of immunity to varicella
  • Hospitalized premature infants born at less than 28 weeks of gestation or who weigh 1,000 grams or less at birth, regardless of their mothers’ evidence of immunity to varicella
  • Pregnant people without evidence of immunity

If a susceptible person exposed to varicella or zoster is age 12 months or older, and has no contraindications to varicella vaccination, varicella vaccine can prevent or reduce the severity of infection when administered as post-exposure prophylaxis (PEP) as soon as possible, within 5 days after exposure. There is no evidence that vaccination after infection increases the risk of vaccine-associated adverse reactions. If the patient’s exposure does not result in infection, vaccination can protect against future exposures. See the MMWR for details: www.cdc.gov/mmwr/preview/mmwrhtml/rr5604a1.htm.

Last reviewed: May 16, 2023

With one exception, you do not need to consider the timing of this vaccine relative to other vaccines. According to CDC’s “General Best Practice Guidelines for Immunization”, concerns about spacing between doses of live vaccines not given at the same visit applies only to live injectable or intranasal vaccines. The one exception is administration of Ty21a oral typhoid vaccine and oral cholera vaccine. The CVD 103-HgR buffer might interfere with the enteric-coated Ty21a formulation. For this reason, ACIP recommends that the cholera vaccine should be administered at least 8 hours before ingesting the first dose of Ty21a vaccine.

Last reviewed: August 21, 2023

Stand-alone units that only refrigerate or only freeze are recommended by CDC. Household-style combination refrigerator/freezer units are less capable of simultaneously maintaining proper storage temperatures in both compartments. In addition, some areas of the refrigerator space may also be unusable due to uneven temperatures in the refrigerator section interior. If a household-style combination refrigerator/freezer must be used, only refrigerated vaccines should be stored in the unit: a separate stand-alone freezer should be used if the clinic also provides frozen vaccines. Pharmaceutical grade combination units designed for vaccine storage may be acceptable for use because they are engineered not to circulate air from the freezer directly into the refrigerator compartment in the way that a household-style unit does. Stand-alone units can vary in size from compact, under-the-counter (not dormitory) style to large, stand-alone, pharmaceutical grade units (which may be labeled as purpose-built for vaccine storage). For additional information see the CDC Storage and Handling Toolkit, page 9, at www.cdc.gov/vaccines/hcp/admin/storage/toolkit/storage-handling-toolkit.pdf.

One way to have confidence that the refrigerator or freezer unit you purchase will reliably maintain proper vaccine storage temperatures is to look for a unit labeled as meeting the NSF/ANSI 456 standard for vaccine storage. This voluntary certification indicates that the model has been tested and certified to maintain proper vaccine storage conditions under a range of normal clinic conditions.

Last reviewed: July 26, 2023

Appropriate site and needle length depends on age, route of injection, and body mass. Most injected vaccines are administered by the intramuscular route.

Please refer for details to the Immunize.org handouts on administering intramuscular and subcutaneous vaccines to children and adults at www.immunize.org/catg.d/p2020.pdf and to adults only at www.immunize.org/catg.d/p2020a.pdf.

A summary of needle length and site selection by age is below.

For intramuscular injections (use a 22- to 25-gauge needle for all ages):

  • For neonates (first 28 days of life) and preterm infants the anterolateral thigh should be used. A ⅝-inch needle usually is adequate to penetrate the thigh muscle if the skin is stretched flat between the thumb and forefinger and the needle is inserted at a 90-degree angle to the skin.
  • The anterolateral thigh is preferred for infants younger than age 12 months. For the majority of infants a 1-inch needle is sufficient.
  • For toddlers age 12 months through 2 years the anterolateral thigh muscle is preferred. The needle should be at least 1 inch long. The deltoid muscle can be used if the muscle mass is adequate.
  • For children age 3 through 10 years, the deltoid muscle is preferred; the needle length for deltoid site injections can range from ⅝ to 1 inch on the basis of technique. The anterolateral thigh can also be used. In this case the needle length should be 1 inch to 1.25 inches.
  • For adolescents 11 through 18 years, the deltoid muscle is preferred. The anterolateral thigh can also be used. For injection into the anterolateral thigh, most adolescents will require a 1-1.5-inch needle.
  • For adults age 19 years and older, the deltoid muscle is preferred. The anterolateral thigh also can be used.
    • For men and women who weigh less than 130 pounds (less than 60 kg), a ⅝-inch needle is sufficient to ensure intramuscular injection in the deltoid muscle if the injection is made at a 90-degree angle and the tissue is not bunched.
    • For men and women who weigh 130–152 pounds (60–70 kg), a 1-inch needle is sufficient.
    • For women who weigh 152–200 pounds (70–90 kg) and men who weigh 152–260 pounds (70–118 kg), a 1- to 1½-inch needle is recommended.
    • For women who weigh more than 200 pounds (more than 90 kg) or men who weigh more than 260 pounds (more than 118 kg), a 1½-inch needle is recommended.

For subcutaneous injections (use a 23- to 25-gauge needle for all ages):

Subcutaneous injections are administered at a 45-degree angle, usually into the thigh for infants younger than age 12 months and in the upper-outer triceps area of people age 12 months and older. Subcutaneous injections may be administered into the upper-outer triceps area of an infant if necessary. A ⅝-inch needle length should be used for all ages.

More information on injection technique is in the ACIP “General Best Practices Guidelines for Immunization”, available at www.cdc.gov/vaccines/hcp/imz-best-practices/vaccine-administration.html.

Last reviewed: December 28, 2022

One way to handle this is to indicate if the vaccination was given either in the “upper” or “lower” portion of the injection area selected (e.g., DTaP: right thigh, upper; Hib: right thigh, lower; or PCV13: left thigh, upper; HepB: left thigh, lower). It is helpful if everyone in your office or clinic uses the same sites for each vaccine. Use of a standardized site map can facilitate this. Here are some helpful site maps for different ages so you can record where shots were given:

For infants and toddlers: www.eziz.org/PDF/IMM-718.pdf

For adults: eziz.org/assets/docs/ADA/IMM-718A.pdf

Last reviewed: June 6, 2023

CDC estimated that 36,500 people developed cancers attributable to HPV infections each year between 2015 to 2019. Of these annual cases, about 94% could have been prevented by the 9-valent HPV vaccine, including about 30,100 cases caused by HPV types 16 and 18 and 4,300 cases caused by HPV types 31, 33, 45, 52, and 58.

HPV types 6 or 11 cause 90% of anogenital warts (condylomata) and most cases of recurrent respiratory papillomatosis.

For additional details, see www.cdc.gov/cancer/uscs/about/data-briefs/no31-hpv-assoc-cancers-UnitedStates-2015-2019.htm.

Last reviewed: March 2, 2024

You can download electronic versions of the schedules from CDC’s website at www.cdc.gov/vaccines/hcp/imz-schedules. Immunize.org has also created laminated versions of the child and adolescent schedule, as well as the adult schedule, which make an excellent resource for placement in each exam room. Each is based on the immunization schedules recommended by ACIP and approved by CDC, AAP, AAFP, ACP, ACOG, and the American College of Nurse-Midwives. You can find them by going to shop.immunize.org.

Last reviewed: August 22, 2020

Multiple complex mechanisms likely contribute to increased disease severity during a second DENV infection. The published ACIP recommendation provides a detailed description of these mechanisms at www.cdc.gov/mmwr/volumes/70/rr/pdfs/rr7006a1-H.pdf. In addition, CDC has developed simple illustrations and descriptions to explain this phenomenon at https://www.cdc.gov/dengue/hcp/vaccine/eligibility.html.

Last reviewed: January 17, 2025

Healthcare providers should suspect measles in patients with a febrile rash illness and the clinically compatible symptoms of cough, coryza (runny nose), and/or conjunctivitis (red, watery eyes). The illness begins with a prodrome of fever and malaise before rash onset. A clinical case of measles is defined as an illness characterized by

  • a generalized rash lasting 3 or more days, and
  • a temperature of 101°F or higher (38.3°C or higher), and
  • cough, coryza, and/or conjunctivitis.

Koplik spots, a rash present on mucous membranes, are considered pathognomonic for measles. Koplik spots occur from 1 to 2 days before the measles rash appears to 1 to 2 days afterward. They appear as punctate blue-white spots on the bright red background of the buccal mucosa. Pictures of measles rash and Koplik spots can be found at www.immunize.org/clinical/image-library/measles/.

Providers should be especially aware of the possibility of measles in people with fever and rash who have recently traveled abroad or who have had contact with international travelers.  Providers should immediately isolate and report suspected measles cases to their local health department and obtain specimens for measles testing, including viral specimens for confirmation and genotyping. Providers should also collect blood for serologic testing during the first clinical encounter with a person who has suspected or probable measles.

Last reviewed: June 19, 2023

The timing and severity of influenza seasons are always unpredictable. Although influenza viruses circulated at very low levels while measures to prevent the spread of COVID-19 were widely adopted during the pandemic, the circulation of influenza viruses has returned to pre-pandemic patterns. Current information on influenza virus circulation can be found at www.cdc.gov/fluview/.

Last reviewed: August 11, 2024

Almost all U.S. infants and toddlers contract RSV illness within the first two years of life. RSV causes a mild respiratory illness in most, with symptoms including cough, runny nose, fever, and fatigue. Illness is more likely to be mild if the child is older at the time of first infection. Infants with RSV infection frequently develop bronchiolitis, a lower respiratory tract disease (LRTD) that can be severe.

RSV LRTD is the leading cause of hospitalization among U.S. infants, who may require supplemental oxygen, treatment for dehydration, or mechanical ventilation. Approximately 50,000–80,000 RSV-associated hospitalizations and 100–300 RSV-associated deaths occur each year among U.S. infants and children younger than age 5 years. The risk of severe RSV disease is increased by prematurity and lung disease, among other health conditions. However, RSV is also the leading cause of hospitalization among healthy, full-term infants. The large majority (almost 80%) of infants and children younger than age 2 who are hospitalized with RSV are otherwise healthy and have no underlying medical conditions.

Some otherwise healthy American Indian or Alaska Native (AI/AN) children experience higher rates of severe RSV disease than the general population. One study found that the incidence of RSV-associated hospitalization among children in their second RSV season in some AI/AN communities was 4 to 10 times higher than that of similar-aged children elsewhere in the United States. AI/AN children living in remote areas also may have difficulty accessing adequate medical care when they develop LRTD.

Last reviewed: August 25, 2024

CDC maintains a webpage with critical interim clinical considerations for vaccination of eligible recipients: www.cdc.gov/vaccines/covid-19/clinical-considerations/covid-19-vaccines-us.html. This covers important clinical details about COVID-19 vaccination. It is the first content to be updated soon after any announced change to CDC recommendations.

All of the CDC’s Advisory Committee on Immunization Practices (ACIP) vaccine recommendations published in MMWR can be accessed here: www.cdc.gov/acip-recs/hcp/vaccine-specific/covid-19.html.

CDC posts product-specific resources at this site: www.cdc.gov/vaccines/covid-19/info-by-product/index.html. It generally takes a few days or weeks following FDA approval of new products for CDC to release updated resources.

Last reviewed: November 16, 2024

Pneumococcal disease can be invasive, meaning a normally sterile part of the body is infected, or non-invasive. There are two major clinical syndromes of invasive pneumococcal disease (IPD): bacteremia (blood stream infection), and meningitis (infection of the meninges that surround the brain). They are both caused by infection with the same bacteria but produce different signs and symptoms.

Pneumococcal pneumonia is a common disease caused by pneumococcal infection. Symptoms include abrupt onset of fever, shaking chills or rigors, chest pain, cough, shortness of breath, rapid breathing and heart rate, and weakness. The case-fatality rate is 5%–7% and is higher in adults 65 years and older and people with certain underlying medical conditions.

Pneumococcal pneumonia can occur in combination with bacteremia and/or meningitis (invasive pneumococcal pneumonia), or it can occur alone (non-invasive pneumococcal pneumonia). Before the COVID-19 pandemic, at least 100,000 people were hospitalized each year for pneumococcal pneumonia. At least 30,000 people were hospitalized each year for IPD, and about 3,000 people died. Rates of pneumococcal disease declined during the COVID-19 pandemic, but rates have returned to pre-pandemic levels.

About 4,000 cases of pneumococcal bacteremia without pneumonia occur each year in the United States. Bacteremia is the most common clinical presentation among children less than two years, accounting for up to 70% of IPD in this age group. The overall case-fatality rate of pneumococcal bacteremia is about 20% and may be as high as 60% among older adults.

Pneumococci cause 50% of all cases of bacterial meningitis in the United States. There are an estimated 2,000 cases of pneumococcal meningitis each year. Symptoms and signs may include headache, tiredness, vomiting, irritability, fever, seizures, and coma. The case-fatality rate of pneumococcal meningitis is about 8% among children and 22% among adults. Permanent neurological damage is common among survivors.

Last reviewed: November 13, 2024

In addition to risk based on age, non-specific risk factors for serogroups A, C, W and Y include having a previous viral infection, living in a crowded household, having an underlying chronic illness, and being exposed to cigarette smoke (either directly or second-hand).

The following groups are at increased risk for all meningococcal serogroups:

  • People with persistent (genetic) complement component deficiencies (a type of immune system disorder)
  • People who use complement inhibitors such as eculizumab (Soliris, Alexion Pharmaceuticals), ravulizumab (Ultomiris, Alexion Pharmaceuticals), or sutimlimab (Enjaymo, Sanofi) for treatment of atypical hemolytic uremic syndrome or paroxysmal nocturnal hemoglobinuria
  • People with anatomic or functional asplenia
  • Microbiologists routinely exposed to meningococcal isolates in a laboratory
  • People at increased risk during an outbreak of meningococcal disease
  • Military recruits
  • College students

Certain groups are at increased risk of serogroups A, C, W and Y, but not serogroup B:

  • People living with HIV
  • Men who have sex with men (MSM)
  • Travelers to countries where meningococcal disease is endemic or hyperendemic, such as the meningitis belt of sub-Saharan Africa
Last reviewed: November 15, 2024

Persons with chronic HBV infection (those with persistent hepatitis B surface antigen [HBsAg] in the serum for at least 6 months) serve as the main reservoir for HBV transmission.

HBV is transmitted through percutaneous (through the skin), mucosal, or non-intact skin exposure to infectious blood or body fluids. HBV is concentrated most highly in blood, and percutaneous exposure is an efficient mode of transmission. Semen and vaginal secretions are infectious, and HBV also can be detected in saliva, tears, and bile. Cerebrospinal fluid, synovial fluid, pleural fluid, peritoneal fluid, pericardial fluid, and amniotic fluid are also considered potentially infectious. Urine, feces, vomitus, nasopharyngeal washings, sputum, and sweat are not efficient vehicles of transmission unless they contain blood because they contain low quantities of infectious HBV. Hepatitis B surface antigen (HBsAg) found in breast milk is also unlikely to lead to transmission, so HBV infection is not a contraindication to breastfeeding.

Among adults in the U.S., HBV is transmitted primarily by percutaneous exposure to blood (for example, injection drug use) and sexual contact. HBV is transmitted efficiently by sexual contact both among heterosexuals and among men who have sex with men (MSM). Transmission can occur from interpersonal contact (e.g., sharing a toothbrush or razor, contact with exudates from dermatologic lesions, or contact with HBsAg-contaminated surfaces) and in settings such as schools, child care centers, and facilities for developmentally disabled persons. Transmission of HBV from transfusion of blood or blood products is rare because of donor screening and viral inactivation procedures. Other possible sources of infection include contaminated medical or dental instruments, unsafe injections, needle-stick injuries, organ transplantation, and dialysis.

In 2022, a total of 2,126 cases of acute hepatitis B were reported to CDC, corresponding to 13,800 estimated acute infections (based on the estimated ratio of acute cases reported to actual acute cases). In 2022, 52% of all acute hepatitis B cases were people age 40 through 59 years.
In 2022, only 26% of reported acute cases documented the presence of one or more risk factors for hepatitis B infection; 28% reported no risk factor and 46% of reports were missing risk factor data.

Last reviewed: January 17, 2025

S. pneumoniae bacteria are serotyped based on the polysaccharides in the outer capsule of the bacteria. The more than 100 known serotypes vary in how common they are and in what percentage of pneumococcal disease they cause. Pneumococcal vaccines are designed to target specific serotypes.

Our understanding of which serotypes are currently causing invasive pneumococcal disease (IPD) in the United States comes primarily from CDC’s Active Bacterial Core surveillance (ABCs) program, which routinely collects serotype data on IPDs leading to hospitalization in 10 states. From ABCs and similar systems in Alaska and the Navajo Nation, CDC has determined that certain adult populations in the western United States have a high percentage (30% or higher) of IPD caused by serotype 4. These areas include Alaska, Colorado, the Navajo Nation, New Mexico, and Oregon. Serotype 4 is not commonly detected in other regions of the United States.

Typically, people in these geographic areas who develop serotype 4 IPD are adults younger than age 65 years who have specific underlying conditions or risk factors, such as alcoholism, chronic lung disease, cigarette smoking, homelessness, and injection drug use. Affected adults typically have not received a pneumococcal vaccination targeting serotype 4.

Serotype 4 is included in all but one of the current pneumococcal vaccines offered to children and adults (PCV13, PCV15, PCV20, and PPSV23). Serotype 4 is not included in the adult PCV21 (Capvaxive, Merck). While ACIP has not expressed a preference for a specific pneumococcal vaccine schedule for adults, ACIP notes that vaccine schedules that include serotype 4 (PCV20 alone or PCV15 followed by PPSV23) are expected to provide broader serotype coverage for adults in these western states with underlying conditions or risk factors for serotype 4 IPD. ACIP and CDC have indicated they will continue to monitor the prevalence of serotype 4 and provide additional guidance if necessary.

For more details of clinical considerations for selection of pneumococcal vaccines in communities with high proportions of serotype 4, please see the box on page 797 of the September 12, 2024, MMWR article entitled, Use of 21-Valent Pneumococcal Conjugate Vaccine Among U.S. Adults: Recommendations of the Advisory Committee on Immunization Practices — United States, 2024: www.cdc.gov/mmwr/volumes/73/wr/pdfs/mm7336a3-H.pdf.

Last reviewed: November 13, 2024

Information about pneumococcal serotype prevalence is not available for most communities. The best available data is from communities in the 10 states that have active pneumococcal surveillance programs through the CDC’s Active Bacterial Core surveillance (ABCs), available at www.cdc.gov/abcs/index.html, as well as similar surveillance systems in Alaska and the Navajo Nation. Your local or state health department is your best source for guidance or additional information.

Last reviewed: November 13, 2024

Antibiotics administered before or after receiving the CVD 103-HgR vaccine might diminish the effectiveness of the vaccine because the immune response to the vaccine relies on the live attenuated vaccine organisms replicating within the small intestine. We do not know what the optimal interval is between receipt of antibiotics and the Vaxchora vaccine. CDC refers clinicians to the package insert which specifies that CVD 103-HgR should not be given to patients who have received oral or parenteral antibiotics during the preceding 14 days. A duration of fewer than 14 days between stopping antibiotics and giving CVD 103-HgR might be acceptable under certain circumstances, such as if travel cannot be avoided during that 14-day interval.

The package insert, however, does not specify an optimal minimum duration between the completion of CVD 103-HgR vaccination and starting antibiotics. In certain circumstances, antibiotics might be clinically necessary after the vaccine (to treat an unrelated infection), thus clinical discretion is recommended.

Last reviewed: August 21, 2023


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Last reviewed: May 23, 2023

Chloroquine might diminish the immune response to CVD 103-HgR. The vaccine manufacturer, Emergent BioSolutions, recommends that CVD 103-HgR be administered 10 days or more before starting chloroquine. Doxycycline, a tetracycline antibiotic, is often used for malaria prophylaxis. The manufacturer does not recommend administration of the vaccine with oral antibiotics and does not recommend administering CVD 103-HgR to a person within 14 days prior to vaccination. The optimal duration between completion of CVD 103-HgR and then starting doxycycline is unknown.

Last reviewed: August 21, 2023

The minimum age for varicella vaccine is 12 months. Vaccination is not recommended for infants younger than 12 months of age even as post-exposure prophylaxis. CDC recommends that a healthy infant should receive no specific treatment or vaccination after exposure to VZV. The child can be treated with an appropriate antiviral medication if chickenpox occurs.

See the Varicella Zoster Immune Globulin section for details on the recommended use of VariZIG in immunocompromised children, infants exposed to varicella around the time of birth and some hospitalized preterm infants.

Last reviewed: May 16, 2023

HAV can produce either asymptomatic or symptomatic infection in humans after an average incubation period of 28 days (range: 15–50 days).

Last reviewed: June 25, 2023

The significance of non-standard intervals probably depends on the vaccine and the dose. This is a complex issue—studies have not been done to examine the effect of various intervals between doses on the immunogenicity of those doses. But ACIP has examined the available data and made recommendations about the minimum acceptable interval between doses for that dose to be considered valid (there is no maximum interval between doses). These minimum intervals are published as Table 3-2 in ACIP’s “General Best Practice Guidelines on Immunization”, available at www.cdc.gov/vaccines/hcp/acip-recs/general-recs/timing.html. Doses with a minimum interval less than the recommended minimum, as described in Table 3-2, should not be counted as valid. More details on this topic can be found in the General Best Practice Guidelines.

Last reviewed: June 6, 2023

There are several very easy to read pieces that can be downloaded from the Immunize.org website. This includes “Vaccinations for Infants and Children, Age 0–10 Years”, “When do children and teens need vaccinations?”, “Vaccinations for Preteens and Teens, Age 11–19 Years”. These handouts can be found at www.immunize.org/handouts/discussing-vaccines-parents.asp

Last reviewed: August 22, 2020

In May 2022, CDC published new ACIP recommendations reducing the primary PrEP rabies vaccination series from 3 doses to 2 doses (administered intramuscularly on days 0 and 7) for all people at elevated risk for exposure to rabies. This was done based on strong evidence that the 2-dose schedule would provide the same protection as the previously recommended 3-dose primary series for up to three years. The less costly 2-dose schedule conserves rabies vaccine supplies, which have been subject to national shortages at times, and may increase adherence with PrEP recommendations.

Five categories of risk were created based upon variables including elevated risk of unusual, unrecognized, or recognized exposures and whether elevated risk is sustained over time, or not. For individuals in risk categories 1–3 (all with sustained elevated risk expected to last more than 3 years after the primary series), ACIP recommends periodic rabies vaccine antibody titer checks at different intervals, and/or booster vaccination. See the table on pages 622–623 of the MMWR published on May 6, 2022, (www.cdc.gov/mmwr/volumes/71/wr/pdfs/mm7118a2-H.pdf) for each risk category’s antibody titer and/or booster dose recommendations for sustained pre-exposure prophylaxis over time.

Last reviewed: May 14, 2023

Haemophilus influenzae type b is a polysaccharide-encapsulated bacteria that causes a variety of invasive diseases, such as meningitis, epiglottitis, and pneumonia. Influenza is a virus that causes the disease influenza.

Historical note: Haemophilus influenzae was first isolated in 1889 from the sputum of a patient who died of (viral) influenza disease, and the isolated organism (then called the Pfeiffer bacillus) was incorrectly assumed to have caused the patient’s illness. Haemophilus influenzae received its name in 1920, to acknowledge its historical association with influenza illness. The viral cause of influenza was not discovered until 1933.

Last reviewed: July 31, 2022

Advisory Committee on Immunization Practices (ACIP) recommendations for use of rotavirus vaccines are available at www.cdc.gov/mmwr/PDF/rr/rr5802.pdf.

Last reviewed: June 7, 2023

In general, vaccines containing adjuvants (a component that enhances the antigenic response) are administered IM to avoid irritation, induration, skin discoloration, inflammation, and granuloma formation if injected into subcutaneous tissue. This includes most of the inactivated vaccines, with a few exceptions (such as IPV and pneumococcal polysaccharide vaccines, which may be given either SC or IM). Vaccine efficacy may also be reduced if not given by the recommended route.

Last reviewed: December 28, 2022

Yes. Vaccines that are stored in the refrigerator portion of a household-style combination refrigerator/freezer should be moved away from the vent located in the refrigerator compartment. The cold air from the freezer is circulated into the refrigerator compartment to cool it, which can cause your vaccines to freeze. Inactivated vaccines must be kept between 2° and 8°C (between 36° and 46°F) and not frozen.

Last reviewed: July 26, 2023

No. The CDC states that the only criteria are age (age 18 years or younger) and the four eligibility criteria listed previously. No other factors (for example, residency status) can be considered when screening for eligibility requirements for the VFC program.

Last reviewed: August 26, 2022

The hyphen (-) is intended to indicate that the antigens are mixed together by the manufacturer before the product is sold and the forward slash (/) indicates that the two products are to be reconstituted by the user.

Last reviewed: July 15, 2023

This is not true. Pregnant healthcare personnel may administer any vaccine except the live, replication-competent smallpox vaccine (ACAM2000, Emergent Biosolutions).

Last reviewed: August 29, 2022

Yes. Adolescents or adults who have a history of pertussis disease generally should receive Tdap according to the routine recommendation. This practice is recommended because the duration of protection induced by pertussis disease is unknown (waning might begin as early as 7 years after infection) and because diagnosis of pertussis can be difficult to confirm. Administering pertussis vaccine to people with a history of pertussis presents no theoretical risk. For details, visit CDC’s published recommendations on this topic at www.cdc.gov/mmwr/volumes/67/rr/pdfs/rr6702a1-H.pdf.

Last reviewed: March 31, 2022

Vaccination providers frequently encounter people who do not have adequate documentation of vaccinations. Providers should only accept written, dated records as evidence of vaccination. CDC recommends that, with the exception of influenza and pneumococcal polysaccharide vaccines, self-reported doses of vaccine without written documentation should generally not be accepted. An attempt to locate missing records should be made whenever possible by contacting previous healthcare providers, reviewing state or local immunization information systems, and searching for a personally held record. However, if records cannot be located or will definitely not be available anywhere because of the patient’s circumstances, people without adequate documentation should be considered susceptible and should be started on the age-appropriate vaccination schedule. Serologic testing for immunity is an alternative to vaccination for certain antigens (e.g., measles, rubella, or hepatitis A).

In general, although it is not ideal, receiving extra doses of vaccine poses no medical problem. Receiving excessive doses of tetanus toxoid (DTaP, DT, Tdap, or Td) can increase the risk of a local adverse reaction, however. For details, consult the ACIP’s “Best Practice Guidelines for Immunization” chapter titled Timing and Spacing of Immunobiologics, available at www.cdc.gov/vaccines/hcp/imz-best-practices/timing-spacing-immunobiologics.html.

Last reviewed: June 6, 2023

Yes. Even though tattooing and body piercing are not thought to be a significant mode of transmission for HBV, tattooing and body piercing have the potential to transmit bloodborne infections, including HBV, hepatitis C virus (HCV), and human immunodeficiency virus (HIV), if the person doing the tattoos or body piercing does not use good infection control practices. The Centers for Disease Control and Prevention (CDC) recommends that instruments or materials (including ink), intended to penetrate the skin be used once, then disposed of or thoroughly cleaned and sterilized between clients. Personal service workers who do tattooing or body piercing should be educated about the transmission of bloodborne pathogens and what precautions are needed to prevent transmission.

People considering getting a tattoo or having a body part pierced should ask staff at the establishment what procedures they use to prevent the spread of bloodborne infections. They also might call the local health department to find out what sterilization procedures are required by law or ordinance for tattooing and body piercing establishments.

Last reviewed: January 17, 2025

The production of a few vaccines, including those for varicella, rubella, hepatitis A, and one of the COVID-19 vaccines (Janssen, Johnson & Johnson) involves growing the viruses in human cell culture. Two human cell lines provide the cell cultures needed for producing vaccines; these lines were developed from two legally aborted fetuses in the 1960s. These cell lines are maintained to have an indefinite life span. No fetal tissue has been added since the cell lines were originally created.

Some parents are concerned about this issue because of misinformation they have encountered on the Internet. Two such untrue statements are that ongoing abortions are needed to manufacture vaccines and vaccines are contaminated with fetal tissue.

Parents can read the facts and several thought-provoking articles about this issue at www.immunize.org/talking-about-vaccines/religious-concerns.asp and then make an informed decision.

A Catholic Pontifical Academy for Life statement that Catholic parents have no general obligation to refuse permission for these vaccines is summarized at www.immunize.org/talking-about-vaccines/vaticandocument.htm.

Last reviewed: August 31, 2022

Most adults residing in the United States are presumed to be protected against polio because they received routine childhood immunization and have only a small risk of exposure to poliovirus in the United States. For decades, routine polio vaccination of U.S. residents 18 years of age and older, including those working in healthcare or in healthcare-related training, was not recommended.

In June 2022, a case of paralytic polio caused by vaccine-derived poliovirus type 2 (VDPV2) was confirmed in an unvaccinated young adult from Rockland County, New York, and wastewater surveillance in the region repeatedly detected evidence of poliovirus over the following months, suggesting the presence of an unknown number of asymptomatically infected people in the community. People who are fully vaccinated with IPV are protected from polio illness but may shed virus in their feces, if exposed. This experience underscored the ongoing risk, however small, of paralytic polio among incompletely vaccinated people in the United States.

CDC’s ACIP recommended in June 2023, that all adults (18 years and older) who are known or suspected to be unvaccinated or incompletely vaccinated against polio should complete a primary 3-dose vaccination series with IPV: 2 doses of IPV administered at an interval of 4–8 weeks; and a third dose 6–12 months after the second. If an adult previously received only one or two documented doses of polio vaccine (either trivalent OPV or IPV), the person should receive the remaining dose(s) of IPV necessary to complete the series, regardless of the interval since the last dose. It is not necessary to restart the vaccination series.

Polio vaccination with a complete primary series is recommended for all travelers to countries with wild poliovirus (WPV) or vaccine-derived poliovirus (VDPV) circulation. WHO recommends that countries affected by WPV or cVDPV require long-term visitors (4 weeks or longer) to provide proof of polio vaccination before leaving the country. For additional information on countries with WPV or VDPV circulation, as well as country-specific requirements for documentation of vaccination, consult the CDC Travelers’ Health website (wwwnc.cdc.gov/travel/).

Adults who have completed a routine series of polio vaccine (with trivalent OPV or IPV in any combination) are considered to have lifelong immunity to poliomyelitis, but data on duration of immunity are lacking. In June 2023, ACIP affirmed and clarified its longstanding recommendation that certain adults at increased risk of exposure to poliovirus may receive a single lifetime adult booster dose of IPV. Adults in situations that put them at increased risk of poliovirus exposure include: travelers going to countries where polio is epidemic or endemic; laboratory and healthcare workers who handle specimens that might contain polioviruses; and, healthcare workers or other caregivers who have close contact with a person who could be infected with poliovirus.

Last reviewed: July 23, 2023

Following a multi-year shortage, in April 2021, the FDA-licensed yellow fever vaccine, YF-Vax (Sanofi) resumed availability for purchase in the United States. Providers with a current Yellow Fever Vaccination Stamp issued by their state or territorial health department may order YF-VAX from the manufacturer.

Locations that administer yellow fever vaccine can be found on CDC’s yellow fever vaccination clinic search page at wwwnc.cdc.gov/travel/yellow-fever-vaccination-clinics/search.

Healthcare providers should refer to the section titled “Yellow Fever and Malaria Information, by Country,” in CDC Health Information for International Travel 2024 (“The Yellow Book”) for information about the countries that require yellow fever vaccination for entry and the countries where CDC recommends yellow fever vaccination. If a country does not have an entry requirement, CDC does not recommend yellow fever vaccination if the traveler’s itinerary does not include travel to a yellow fever–endemic area. This section is available at wwwnc.cdc.gov/travel/yellowbook/2024/preparing/yellow-fever-vaccine-malaria-prevention-by-country.

Last reviewed: August 21, 2023

There is no treatment for infection with the HPV virus itself. Only HPV-associated lesions including genital warts, recurrent respiratory papillomatosis, precancers, and cancers are treated. Recommended treatments vary depending on the diagnosis, size, and location of the lesion. Local treatment of lesions might not eradicate all HPV-containing cells fully. It is unclear whether treating the lesion reduces the risk that the infected person could transmit the HPV infection to others.

Last reviewed: March 2, 2024

Pneumococcal disease is a serious disease that causes much sickness and death. Before the dip in pneumococcal disease observed in the first two years of the COVID-19 pandemic (when measures to control COVID-19 reduced the incidence of several infectious diseases), an estimated 30,000 cases and 3,000 deaths from invasive pneumococcal diseases (bacteremia and meningitis) occurred in the United States. Children younger than age two years and adults age 50 years and older have the highest incidence of serious disease. Case-fatality rates are highest for pneumococcal meningitis and bacteremia, and the highest mortality occurs among older adults and patients who have underlying medical conditions.

Last reviewed: November 13, 2024

Measles is highly contagious. A person with measles is infectious up to 4 days before through 4 days after the day of rash onset. Patients with suspected measles should be isolated for 4 days after they develop a rash. Airborne precautions should be followed in healthcare settings by all healthcare personnel. The preferred placement for patients who require airborne precautions is in a single-patient airborne infection isolation room. Providers should immediately isolate and report suspected measles cases to their local health department and obtain specimens for measles testing, including serum sample for measles serologic testing and a throat swab (or nasopharyngeal swab) for viral confirmation.

Measles is a nationally notifiable disease in the U.S.; healthcare providers should report all cases of suspected measles to public health authorities immediately to help reduce the number of secondary cases. Do not wait for the results of laboratory testing to report clinically-suspected measles to the local health department.

More information on measles disease, diagnostic testing, and infection control can be found at www.cdc.gov/measles/hcp/clinical-overview/index.html.

Last reviewed: June 19, 2023

In most temperate regions of the continental United States and other areas with similar climates, RSV season typically begins in the fall and peaks in the winter, generally circulating between October and the end of March. However, the timing and severity of RSV season in a given community can vary from year to year. RSV seasonality was temporarily disrupted by the COVID-19 pandemic and widespread changes in social interactions and infection control practices (e.g., mask wearing, social distancing).

U.S. states and territories with very different weather patterns from the continental United States (e.g., Alaska, and areas with tropical weather patterns, such as Hawaii, southern Florida, Puerto Rico, and other island territories) may experience very different patterns of local RSV circulation. For this reason, public health authorities in those areas may issue different recommendations for appropriate timing of RSV prevention strategies (vaccines, monoclonal antibodies).

Last reviewed: August 25, 2024

In a school setting, an immunocompetent person with zoster (staff or students) can remain at school as long as the lesions can be completely covered. People with zoster should be careful about personal hygiene, wash their hands after touching their lesions, and avoid close contact with others. If the lesions cannot be completely covered and close contact avoided, the person should be excluded from the school setting until the zoster lesions have crusted over. See archive.cdc.gov/#/details?url=https://www.cdc.gov/chickenpox/outbreaks/manual.html for more information. If your program is licensed by a state or county, you should check their regulations as well.

Last reviewed: March 9, 2022

All people age 6 months and older in the United States are recommended to receive one or more doses of an age-appropriate 2024–2025 Formula COVID-19 vaccination. Product options include two mRNA vaccines (Pfizer-BioNTech, Moderna) and one protein subunit vaccine (Novavax). Schedules vary by age and immunocompromised status. 

Last reviewed: November 16, 2024

The vaccines for meningococcal serogroups A, C, W, and Y (MenACWY-TT, MenQuadfi [Sanofi]; MenACWY-CRM, Menveo [GSK]) contain meningococcal conjugate in which the surface polysaccharide is chemically bonded (“conjugated”) to a protein to produce a robust immune response to the polysaccharide. The MenACWY vaccine products are considered interchangeable; the same vaccine product is recommended, but not required, for all doses.

Two meningococcal vaccines were used in the recent past but are no longer available. Menactra (Sanofi) is a discontinued MenACWY conjugate vaccine. The last doses of Menactra expired in 2023. Menactra was considered interchangeable with Menveo or MenQuadfi. An older meningococcal polysaccharide vaccine (MPSV4, Menomune, Sanofi) was available in the United States until the last doses expired in 2017: it was never routinely recommended for children or teens.

Since late 2014, vaccines have become available that offer protection from meningococcal serogroup B disease (MenB; Bexsero by GSK; Trumenba by Pfizer). These vaccines are composed of proteins found on the surface of the bacteria. Bexsero and Trumenba are not interchangeable; the same vaccine product is required for all doses.

A pentavalent MenACWY and MenB vaccine, abbreviated MenABCWY (Penbraya, Pfizer) contains a conjugated MenACWY vaccine mixed with the MenB vaccine contained in Trumenba (Pfizer).

MenACWY vaccines provide no protection against serogroup B disease, and MenB vaccines provide no protection against serogroup A, C, W, or Y disease. For protection against all 5 serogroups of meningococcus, it is necessary to receive both MenACWY and MenB, either as separate vaccines or as the combination MenABCWY vaccine, Penbraya.

Trade Name Type of Vaccine Serogroups Year Licensed Approved Ages
Penbraya Conjugate A, B, C, W, Y 2023 10–25 years*
Menveo (two vial)
Menveo (one vial)		 Conjugate
Conjugate		 A, C, W, Y
A, C, W, Y		 2010
2022		 2 mos.–55 years*
10–55 years*
MenQuadfi Conjugate A, C, W, Y 2020 2 years and older
Trumenba Protein B 2014 10–25 years*
Bexsero Protein B 2015 10–25 years*

*May be given to adults at increased risk older than the FDA-approved upper age limit (see ACIP recommendations, Table 11, page 41, www.cdc.gov/mmwr/volumes/69/rr/pdfs/rr6909a1-H.pdf)

Last reviewed: November 15, 2024

Dengvaxia (Sanofi) is a live-attenuated, chimeric tetravalent dengue vaccine built on a yellow fever 17D backbone. It is designed to protect against all four DENV serotypes. It was licensed by the FDA in May 2019 for children and adolescents age 9 through 16 years with a confirmed history of previous dengue infection. FDA lowered the youngest age for licensed use from 9 years to 6 years in 2023; however, CDC’s recommendations to begin routine vaccination of children with serologic evidence of prior dengue infection at age 9 years remains unchanged. The recommendation for vaccination is restricted to people age 9 through 16 years with confirmed previous DENV infection because Dengvaxia is associated with an increased risk for severe dengue in those who experience their first natural infection (i.e., primary infection) after vaccination.

In 2024, Sanofi announced plans to discontinue production of Dengvaxia in the third quarter of 2026 due to low global demand. Other dengue vaccines are in late stages of development; however, there are no other licensed dengue vaccines available in the United States.

Last reviewed: January 17, 2025


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Last reviewed: June 7, 2024

Two vaccines containing varicella virus are licensed for use in the United States. Both vaccines contain live, attenuated varicella zoster virus (VZV) derived from the Oka strain.

  • Varivax (VAR, Merck) contains only varicella vaccine virus.
  • ProQuad (MMRV, Merck) is a combination measles, mumps, rubella, and varicella vaccine

Both vaccines may be administered either by subcutaneous injection or intramuscular injection. VAR is approved by the Food and Drug Administration (FDA) for people 12 months of age and older. MMRV is approved for people 12 months through 12 years of age. MMRV should not be administered to people age 13 years or older.

Last reviewed: May 16, 2023

In infected people, HAV replicates in the liver, is excreted in bile, and is shed in stool. Peak infectivity occurs during the 2-week period before onset of jaundice or elevation of liver enzymes, when concentration of virus in stool is highest. Concentration of virus in stool declines after jaundice appears, with most people no longer infectious about a week after jaundice appears. Children can shed HAV for longer periods than adults, up to 10 weeks or longer after onset of clinical illness.

Last reviewed: June 25, 2023

There are many excellent websites that have information about vaccine safety, including the American Academy of Pediatrics, the CDC, Vaccinate Your Family, Immunize.org, the National Academies of Science, Engineering, and Medicine, and the Vaccine Education Center of the Children’s Hospital of Philadelphia.

Last reviewed: August 31, 2022

The most recent Advisory Committee on Immunization Practices (ACIP) recommendations for Hib vaccination were published in 2014 and are available on the CDC website at www.cdc.gov/acip-recs/hcp/vaccine-specific/hib.html. Guidance for Hib vaccination is also provided in the annual childhood immunization schedule, available at www.cdc.gov/vaccines/schedules/hcp/child-adolescent.html.

Last reviewed: July 31, 2022

With some exceptions, there aren’t VISs for combination vaccines. Instead, providers should provide a separate VIS for each vaccine component in the combination (e.g., DTaP-IPV-HepB or DTaP-IPV/Hib). There is a combined VIS (the multi-vaccine VIS) that can substitute for any or all of the routine vaccines given from birth–6 months (DTaP, IPV, Hib, PCV and HepB vaccines). VISs in English and many other languages are available at www.immunize.org/vis/.

Last reviewed: July 15, 2023

Immunity to one of the serotypes of polio does not produce significant immunity to the other serotypes. A history of having recovered from polio disease should not be considered evidence of immunity to polio. You should complete a primary series of IPV if he is known or suspected to have been unvaccinated or incompletely vaccinated against polio. If he received a primary series in childhood, a single adult booster dose may be given before travel to an area where polio vaccination is recommended.

Last reviewed: July 23, 2023

Generally speaking, CDC recommends avoiding the top shelf and the areas near vents due to temperature fluctuations. However, most purpose-built or pharmaceutical-grade units use a fan to circulate air within the storage area and create more uniform temperatures than household units. During a power outage, the top shelf is an area of caution for all units as the temperatures increase most quickly there. In this instance, it would be best to check with the manufacturer or owner’s manual to see if the top shelf is appropriate for storage in your unit. Units that meet the NSF/ANSI 456 voluntary certification standard are designed to deter the user from placing vaccines in areas where proper storage temperatures cannot be maintained, so any shelf available storage in a certified unit would be usable.

Last reviewed: July 26, 2023

Two rotavirus vaccines are available in the United States. RotaTeq (RV5; Merck) is recommended for routine oral administration for all infants as a 3-dose series. The usual schedule is at ages 2, 4, and 6 months. Rotarix (RV1; GlaxoSmithKline) is recommended as a 2-dose series at ages 2 and 4 months.

The minimum interval between doses of rotavirus vaccine is 4 weeks. The minimum age for the first dose is 6 weeks and the maximum age for dose #1 is 14 weeks 6 days. Vaccination should not be initiated for infants age 15 weeks 0 days or older because there are insufficient data on the safety of dose #1 in older infants. The maximum age for the last dose of rotavirus vaccine is 8 months and 0 days.

Last reviewed: June 7, 2023

Yes. There is no age limit for use of the anterolateral thigh for either subcutaneous or intramuscular vaccines.

Last reviewed: December 28, 2022

Medicare Part B (medical insurance) statutorily covers four recommended vaccines for Medicare beneficiaries: influenza, pneumococcal polysaccharide (Pneumovax 23, Merck), pneumococcal conjugate (PCV20, Prevnar 20, Pfizer, or PCV15, Vaxneuvance, Merck), and hepatitis B (for patients at high or intermediate risk). Medicare Part B does not cover other adult vaccinations (e.g., Tdap and zoster) unless they are directly related to the treatment of an injury or direct exposure to a disease, such as anti-rabies treatment or tetanus prevention due to an injury. In the absence of injury or direct exposure, preventive immunization against diseases such as tetanus, pertussis, or diphtheria is not covered by Part B.

Medicare Part D plans (outpatient prescription drug insurance) generally cover vaccines that Part B does not cover (for example Tdap and zoster), as long as the vaccine is recommended by the Advisory Committee on Immunization Practices (ACIP). Payment for Part D-covered vaccines and their administration is currently determined solely by the patient’s prescription drug plan. The Inflation Reduction Act of 2022 will require Medicare Part D plans to begin covering ACIP-recommended adult vaccinations without a deductible or copay beginning later in 2023.

Last reviewed: August 26, 2022

For measles, there is an average of 10 to 12 days from exposure to the appearance of the first symptom, which is usually fever. The measles rash doesn’t usually appear until approximately 14 days after exposure (range: 7 to 21 days), and the rash typically begins 2 to 4 days after the fever begins. The incubation period of mumps averages 16 to 18 days (range: 12 to 25 days) from exposure to onset of parotitis. The incubation period of rubella is 14 days (range: 12 to 23 days). However, as noted above, up to half of rubella virus infections cause no symptoms.

Last reviewed: June 19, 2023

All healthcare personnel should ensure they are immune to varicella regardless of the setting in which they work. For healthcare personnel, accepted evidence of varicella immunity includes any of the following: 1) documentation of age-appropriate vaccination with a varicella vaccine, 2) laboratory evidence of immunity or laboratory confirmation of disease; 3) diagnosis or verification of a history of varicella disease by a healthcare provider; or 4) diagnosis or verification of a history of herpes zoster by a healthcare provider.

Last reviewed: March 9, 2022

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