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 (1379)

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

Last reviewed: February 27, 2025

Medicare Part B (which primarily covers medical outpatient care costs) covers four recommended vaccines for all Medicare beneficiaries: COVID-19, hepatitis B (whether or not a risk factor is reported), influenza, and recommended pneumococcal conjugate and pneumococcal polysaccharide vaccines. Medicare Part B does not cover other adult vaccinations (e.g., Tdap) 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, all other vaccines, including zoster, are covered by Medicare Part D.

Medicare Part D plans (outpatient prescription drug insurance) cover the Advisory Committee on Immunization Practices (ACIP) recommended vaccines that Part B does not cover (for example, RSV and zoster). Since early 2023, the Inflation Reduction Act of 2022 has required Medicare Part D plans to cover ACIP-recommended adult vaccinations without a deductible or copay when given by an in-network provider. Medicare Part D coverage also includes ACIP-recommended vaccines recommended for an individual as a result of occupational or travel-related risks. A summary of adult vaccine insurance coverage can be found at www.izsummitpartners.org/content/uploads/NAIIS_Vaccine-insurance-coverage-2024-2025.pdf.

Last reviewed: February 14, 2025

If a provider suspects an invalid vaccination, including those from persons vaccinated outside the U.S., one of two approaches can be taken. Repeating the vaccinations is an acceptable option. Doing so is generally safe and avoids the need to obtain and interpret serologic tests. If avoiding unnecessary injections is desired, judicious use of serologic testing might be helpful in determining which immunizations are needed. This may be particularly helpful in determining tetanus and diphtheria antitoxin levels for children whose records indicate 3 or more doses of DTP or DTaP. This issue is discussed in detail in the CDC’s “General Best Practices for Immunization” chapter titled Special Situations, and summarized in Table 9-1, available at www.cdc.gov/vaccines/hcp/imz-best-practices/special-situations.html.

Last reviewed: February 16, 2025

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

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

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

There are no specific data on transmission of bloodborne viruses through oral-genital sex. Saliva has not been associated with HBV transmission unless biting has taken place. HBV is not spread by kissing, hugging, sneezing, coughing, food or water, sharing eating utensils or drinking glasses, or casual contact.

Last reviewed: January 17, 2025

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

No. ACIP does not recommend routinely checking a patient’s temperature or other vital signs before vaccination. Requiring these extra steps can be a barrier to immunization.

Last reviewed: August 29, 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, up to half of rubella virus infections cause no symptoms.

Last reviewed: March 16, 2025

Immunize.org has developed several resources that can help patients identify what they may need. These include:

Translations of these handouts are also available in several languages. To access all of Immunize.org’s clinical resources available in languages other than English, visit www.immunize.org/translations/.  All of these clinical resources can be found in the subsection of clinical resources that address adult immunization:  www.immunize.org/clinical/topic/adult-vaccination/.

Last reviewed: March 26, 2025

CDC has published the following summary of 2024–25 season recommendations for people age 6 months and older who are moderately or severely immunocompromised:

  • Unvaccinated: Should receive a multidose initial series with an age-appropriate 2024–2025 COVID-19 vaccine and 1 dose of a 2024–2025 COVID-19 vaccine 6 months (minimum interval 2 months) after completing the initial series
  • Previously completed an initial series: Should receive 2 doses of an age-appropriate 2024–2025 COVID-19 vaccine, spaced 6 months (minimum interval 2 months) apart*
  • May receive additional age-appropriate 2024–2025 COVID-19 vaccine doses based upon clinical judgment, patient preference, and circumstances

*For people ages 6 months and older who are moderately or severely immunocompromised who initiated but did not complete an initial series, consult CDC Table 2, linked below.  

See CDC Table 2 for complete details of the schedule, brands, dose, and dosing intervals for people age 6 months or older who are moderately or severely immunocompromised: www.cdc.gov/vaccines/covid-19/clinical-considerations/interim-considerations-us.html#table-02.  

Last reviewed: November 16, 2024

In June 2023, ACIP recommended that all adults (18 years and older) in the United States 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 should be administered 6–12 months after the second. Previously, ACIP did not recommend IPV vaccination of unvaccinated or incompletely vaccinated adults who lacked a specific increased risk for exposure to polio (e.g., due to travel). Most U.S. adults may be presumed to be vaccinated against polio unless there is a specific reason to believe otherwise (e.g., an adult whose parents were known to have refused vaccinations). Rates of polio vaccination among children in the United States have been extremely high for decades.

Last reviewed: July 23, 2023


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Last reviewed: December 6, 2023

CDC recommends that all doses of COVID-19 vaccine given to children age 6 months through 4 years be from the same manufacturer. In this case, and in other situations where the same brand is recommended for multiple doses, CDC sets out conditions when it is acceptable to use an age-appropriate COVID-19 vaccine from a different manufacturer:

  • Same vaccine not available at the time of the clinic visit
  • Previous dose unknown
  • Person would otherwise not receive a recommended vaccine dose
  • Person starts but unable to complete a vaccination series with the same COVID-19 vaccine due to a contraindication

Therefore, it is acceptable to vaccinate these children with the available product. Note that children age 6 months through 4 years who receive a mixed brand primary series will need a total of 3 doses to complete the primary series.  

Last reviewed: November 16, 2024

Individuals with moderate to severe immunocompromise who have completed a primary series should receive one dose of 2024–2025 Formula COVID-19 vaccine this season followed by a second dose 6 months (minimum 2 months) later. Additional doses of 2024–2025 Formula COVID-19 vaccine may be administered (with a minimum two-month interval) based on the clinical judgment of the individual’s healthcare provider and the recipient’s personal preference and circumstances. 

The option to receive additional doses is offered because COVID-19 vaccine effectiveness declines most rapidly in people with moderate to severe immunocompromise. Such people also have the highest risk of hospitalization with COVID-19 if infected. Although protection against severe disease is more durable than protection against milder illness, individuals in these groups may benefit from shorter intervals between doses.

Last reviewed: November 16, 2024

In June 2023, ACIP affirmed its longstanding recommendation that adults who received a primary series of trivalent OPV (tOPV) or IPV in any combination and who are at increased risk of poliovirus exposure may receive another dose of IPV. Available data do not indicate the need for more than a single lifetime booster dose with IPV for adults.

The following are examples of vaccinated adults at increased risk of exposure who may receive a single lifetime booster dose of IPV:

  • Travelers who are going to countries where polio is epidemic or endemic (see polio information for travelers at wwwnc.cdc.gov/travel/)
  • Laboratory and healthcare workers who handle specimens that might contain polioviruses
  • Healthcare workers or other caregivers who have close contact with a person who could be infected with poliovirus
Last reviewed: July 23, 2023

Generally speaking, no. A person with laboratory evidence of resolved hepatitis B infection is considered immune. Vaccination of such individuals is not harmful but is not necessary.

Last reviewed: January 17, 2025

CDC recommends that all travelers to countries affected by wild (WPV) or circulating vaccine-derived poliovirus (cVDPV) be vaccinated fully against polio. Adults who were fully vaccinated during childhood may receive an additional (single) lifetime booster dose of polio vaccine.

WHO recommends that countries affected by wild poliovirus or cVDPV outbreaks require residents and long-term (4 weeks or more) visitors show proof of polio vaccination before leaving the country. These recommendations are regularly reviewed and updated. Visit CDC’s Travelers’ Health site for current details about country-specific requirements (wwwnc.cdc.gov/travel/).

Last reviewed: July 23, 2023

Yes. TBE is caused by a flavivirus transmitted by ticks in certain regions of Asia and Europe. Most infections are asymptomatic, but it can cause meningitis and encephalitis. In August 2021, the FDA approved a TBE vaccine, Ticovac (by Pfizer), for people age 1 year or older. The dose for people age 16 years and older is 0.5mL, and for children and adolescents up to age 15 years is 0.25mL. The primary vaccination schedule includes 3 doses, and a booster dose can be given if ongoing exposure or re-exposure to TBE virus is expected. TBE vaccines are also available in many countries overseas where TBE virus is present. For more information from CDC about the TBE vaccine, visit www.cdc.gov/tick-borne-encephalitis/hcp/vaccine/index.html.

Last reviewed: August 21, 2023

Nonsexual HPV transmission is theoretically possible but has not been definitively demonstrated. This is mainly because HPV can’t be cultured, and DNA detection from the environment is difficult and likely prone to false negative results.

Last reviewed: March 2, 2024

MenACWY is recommended for these groups:

Routine vaccination of all children and teens, age 11 through 18 years: a single dose at age 11 or 12 years with a booster dose at age 16 years

Routine vaccination of people age 2 months or older at increased risk for meningococcal disease (the primary dosing schedule and booster dose interval varies by age and indication):

  • People with functional or anatomic asplenia
  • People who have persistent complement component deficiency (an immune system disorder) or who take a complement inhibitor (examples include eculizumab [Soliris] and ravulizumab [Ultomiris])
  • People who have HIV infection
  • People who are at risk during an outbreak caused by a vaccine serogroup
  • People age 2 months and older who reside in or travel to certain countries in sub-Saharan Africa as well as to other countries for which meningococcal vaccine is recommended (e.g., travel to Mecca, Saudi Arabia, for the annual Hajj)
  • Microbiologists who work with meningococcus bacterial isolates in a laboratory
  • First-year college students living in residence halls who are unvaccinated or undervaccinated; these students should receive a dose if they have not had a dose since turning 16 or if it has been more than 5 years since their previous dose
Last reviewed: November 15, 2024

Recent Tdap vaccination does not affect PCR testing. PCR tests are used to detect DNA sequences of the Bordetella pertussis bacterium. PCR tests are very sensitive and could give a false positive result for other reasons. For more information on best practices when performing PCR testing for pertussis see www.cdc.gov/pertussis/php/pcr-bestpractices.

Last reviewed: October 31, 2023

The incidence of hepatitis A in the US increased more than 10-fold from 2015 to 2019, with over 18,800 cases reported to CDC in 2019. The number of reported cases declined by 47% to 9,952 in 2020. This number is an underestimate of the actual number of infections: CDC estimates that about 19,900 cases actually occurred in 2020.

Between 2012 and 2015 the number of reported hepatitis A infections ranged from approximately 1200 to 1800 cases every year. Beginning in 2016, large foodborne outbreaks led to an increase in the number of cases and sustained, large person-to-person outbreaks began, primarily driven by infections among unvaccinated people who use drugs and people experiencing homelessness and their contacts. Since then, persistent person-to-person outbreaks have led to substantial increases in hepatitis A infection, with 37 states reporting almost 45,000 cases over the 7 years between the beginning of the outbreaks in 2016 and June 2023. The rate of new cases has declined substantially since the peak in 2019. More information regarding ongoing multistate outbreaks can be found here: www.cdc.gov/hepatitis/outbreaks/ongoing-hepatitis-a.

Last reviewed: June 25, 2023

In May 2023, FDA licensed two RSV vaccines: RSVPreF3 (Arexvy, GSK) and RSVpreF (Abrysvo, Pfizer). A third RSV vaccine, mRNA RSV (mResvia, Moderna), was licensed by FDA in May 2024. All three vaccines are licensed for the prevention of RSV-associated lower respiratory tract disease (LRTD) in adults 60 age years and older in the United States. Arexvy is also licensed for use in adults age 50 through 59 years who are increased risk of RSV LRTD; however, ACIP has not yet made recommendations for its use in this age group. Only Abrysvo is licensed for use during pregnancy (during 32 through 36 weeks and 6 days’ gestation) for the prevention of RSV disease in infants.

Arexvy and Abrysvo are recombinant protein vaccines that contain the prefusion form of the spike protein found on the surface of the RSV virus. The mResvia mRNA vaccine contains mRNA that encodes the prefusion form of the RSV F glycoprotein. The mRNA vaccine temporarily enables some of the vaccine recipient’s own cells to produce the prefusion form of the spike protein, causing the immune system to respond by generating antibodies to it. Because none of these vaccines contain live virus, they cannot cause RSV illness.

The GSK vaccine, Arexvy, includes an AS01adjuvant, a chemical designed to enhance the immune response to vaccination. AS01 is the same adjuvant used in GSK’s recombinant zoster vaccine (Shingrix), but Arexvy contains half the amount of adjuvant as a dose of Shingrix. Abrysvo and mResvia vaccines do not contain an adjuvant.

For further information about the vaccines, see the FDA package inserts: Arexvy (www.fda.gov/media/167805/download); Abrysvo (www.fda.gov/media/168889/download); and mResvia (www.fda.gov/media/179005/download).

Last reviewed: August 25, 2024

Janssen (Johnson & Johnson) COVID-19 vaccine is no longer available for use in the United States. The last remaining doses expired May 7, 2023. People who received 1 or 2 Janssen COVID-19 Vaccine doses are recommended to receive one 2024–2025 Formula dose (Moderna, Pfizer-BioNTech, or Novavax).

Last reviewed: November 16, 2024

Yearly influenza vaccination continues to be recommended for everyone age 6 months and older. Changes relevant to clinical practice in the CDC’s published ACIP recommendations for influenza vaccination in the 2024-25 season are summarized below:

  • ACIP now recommends high-dose inactivated influenza vaccine (HD-IIV) or adjuvanted inactivated (aIIV) influenza vaccine (each licensed by FDA for people age 65 years or older) as acceptable options for influenza vaccination of solid organ transplant recipients age 18 through 64 years who are on immunosuppressive medication regimens. There is no preference for aIIV or HD-IIV over any other age-appropriate inactivated or recombinant influenza vaccine in this group.
  • The HD-IIV vaccine, which is now trivalent, is given at a dose of 0.5 mL (the previous dose for quadrivalent HD-IIV was 0.7 mL).
  • ACIP affirmed that everyone age 6 months and older who has an egg allergy should receive influenza vaccine. Any influenza vaccine (egg based or non-egg based) that is otherwise appropriate for the recipient’s age and health status may be used. ACIP updated its recommendation to state that egg allergy alone necessitates no additional safety measures for influenza vaccination beyond those recommended for any recipient of any vaccine, regardless of severity of previous reaction to egg. All vaccines should be administered in settings in which personnel and equipment needed for rapid recognition and treatment of acute hypersensitivity reactions are available.

The current ACIP recommendations for influenza vaccination are available here:  www.cdc.gov/acip-recs/hcp/vaccine-specific/flu.html.

Last reviewed: August 11, 2024

Three monovalent Hib vaccines are available in the United States: PedvaxHIB (PRP-OMP, Merck), ActHIB (PRP-T, Sanofi) and Hiberix (PRP-T, GSK). These vaccines are composed of Hib purified polyribosylribitol phosphate (PRP) capsular polysaccharide chemically bound (conjugated) to a protein to enhance the quality of the immune response to PRP. All three vaccines are approved for infants in a 3- or 4-dose series (depending on brand).

Two combination vaccines containing Hib are currently available in the United States: Pentacel (DTaP-IPV/Hib, Sanofi) and Vaxelis (DTaP-IPV-Hib-HepB, MSP Company). Pentacel is licensed for use in children younger than age 5 years and contains Hib conjugate, DTaP, and inactivated polio vaccines; it is approved as a 4-dose series for infants at age 2, 4, 6, and 15 through 18 months, but it is not approved for use as the DTaP/IPV booster dose recommended at age 4 to 6 years. Vaxelis (DTaP-IPV-Hib-HepB) is licensed for use in children younger than age 5 years and is FDA-approved and recommended by CDC as a 3-dose primary series of Hib for infants at age 2, 4, and 6 months. Vaxelis is not approved for use as a Hib booster (4th) dose. Vaxelis contains the same PRP-OMP Hib antigen as PedvaxHIB, but in a reduced amount; like PedvaxHIB, it is an ACIP-preferred option for administration to American Indian and Alaska Native infants (who are at increased risk of early-onset invasive Hib disease) because it induces protective antibody levels after the first dose.

Last reviewed: January 21, 2025

There is usually very close agreement between vaccine package inserts and ACIP statements. The Food and Drug Administration (FDA) must approve the package insert and requires documentation for all data and recommendations made in the insert. Occasionally, ACIP may use different data to formulate its recommendations, or try to add flexibility to its recommendations, which results in wording different than in the package insert. ACIP sometimes makes recommendations based on expert opinion and public health considerations. Published recommendations of ACIP should be considered equally as authoritative as those on the package insert.

Last reviewed: January 20, 2025

MenB is routinely recommended for these groups:

  • People age 10 years and older who have functional or anatomic asplenia (including sickle cell disease)
  • People age 10 years and older who have persistent complement component deficiency (an immune system disorder) or who take a complement inhibitor (examples include eculizumab [Soliris], ravulizumab [Ultomiris], and sutimlimab [Sanofi])
  • People age 10 years and older who are exposed during an outbreak caused by serogroup B
  • Microbiologists who work with meningococcal isolates in a laboratory

For adolescents and young adults not otherwise at increased risk for meningococcal B disease, ACIP recommends that a MenB series may be administered to people 16 through 23 years of age (preferred age 16 through 18 years) on the basis of shared clinical decision-making. The shared clinical decision-making recommendation allows the clinician and patient to decide together based upon the risks and benefits of vaccination for the individual patient.

Last reviewed: November 15, 2024

Ideally, you can arrange to have the cast cut to administer vaccines in the anterolateral thighs. If that option is not available, the gluteal region can be used if not covered by the cast. There are no other sites recommended for vaccination; however, the inactivated polio vaccine could be given subcutaneously in either arm, if the child is large enough. Rotavirus vaccine is given orally and should be administered. If vaccines cannot be given for the 10 weeks, please advise the family to keep people with any illness away from the child until she has been vaccinated. For more information see the vaccine administration section of CDC’s “General Best Practices for Immunization”, available at www.cdc.gov/vaccines/hcp/imz-best-practices/vaccine-administration.html.

Last reviewed: February 27, 2025

All commercial insurance plans compliant with the Affordable Care Act (ACA) should cover vaccines listed on the CDC’s recommended schedules for vaccines for children and adults without any cost-sharing (e.g., no deductible or copay) when given by in-network providers.

The Inflation Reduction Act (IRA) of 2022, which was fully enacted during 2023, substantially reduced cost-related barriers to vaccination for adults. The law eliminated cost-sharing for ACIP-recommended vaccination of all adults covered by traditional Medicaid and Medicaid expansion plans, as well as adults with Medicare Part D.  More information about the ACA and IRA legislation impact on vaccine payment is available in this pdf report published by Avalere on October 2, 2023:  https://avalere.com/wp-content/uploads/2023/10/Guide-to-Vaccine-Coverage-Policies.pdf.

The National Adult and Influenza Immunization Summit (NAIIS) has created a web section with resources on billing and coding for adult vaccinations at www.izsummitpartners.org/naiis-workgroups/access-provider-workgroup/coding-and-billing.

Last reviewed: February 14, 2025

In May 2022, CDC published updated ACIP recommendations for rabies PrEP based on risk categories. All people in categories 1-4 should receive a 2-dose primary PrEP rabies vaccine series. People in categories 1-3 require additional long-term follow-up with periodic antibody titer checks and/or booster doses, depending on the category. See the table on pages 622-623 of the MMWR publication for details: www.cdc.gov/mmwr/volumes/71/wr/pdfs/mm7118a2-H.pdf.

In brief, the risk categories are as follows:

  • Risk category 1: elevated risk of unrecognized or recognized exposures, including unusual or high-risk exposures (e.g., laboratory or rabies vaccine production settings)
  • Risk category 2: elevated risk, exposures typically recognized but may be unrecognized (e.g., people who frequently interact with bats or collect suspected rabies samples)
  • Risk category 3: elevated risk, exposure nearly always recognized (e.g., veterinary workers, people who handle wildlife reservoir species, spelunkers [because of bats], and certain travelers who have increased risk and may have trouble obtaining safe post-exposure prophylaxis)
  • Risk category 4: same type of risk as category 3, but risk period is time-limited (no more than 3 years)
  • Risk category 5: low risk, typical person living in the United States (PrEP not recommended)
Last reviewed: May 14, 2023

Pediarix contains the vaccine components DTaP, IPV, and HepB. The primary series is 3 doses (0.5 mL) given intramuscularly at 2, 4, and 6 months of age. Pediarix is licensed by the Food and Drug Administration (FDA) for only the first 3 doses of the DTaP series. It should not be given to infants younger than 6 weeks of age or to children 7 years or older.

Last reviewed: January 27, 2025

No. If you turn off the freezer portion of a household-style combination refrigerator/freezer, the refrigerated compartment will not maintain the proper temperature.

Last reviewed: July 26, 2023

The two rotavirus vaccine products differ in composition and schedule of administration. RotaTeq was approved by the Food and Drug Administration (FDA) in 2006. It contains five reassortant rotaviruses developed from human and bovine parent rotavirus strains; 3 doses are given in the series. Rotarix was approved by the FDA in 2008 and contains an attenuated human rotavirus strain; 2 doses are given in the series.

Last reviewed: June 7, 2023

All children, beginning at age 12 months, as well as adults without other evidence of immunity should be vaccinated with 2 doses of varicella vaccine. Special consideration should be given to vaccinating adults who (1) have close contact with people at high risk for severe disease (e.g., healthcare workers and family contacts of immunocompromised people), or (2) are at high risk for exposure or transmission (e.g., teachers of young children; child care employees; residents and staff members of institutional settings, including correctional institutions; college students; military personnel; adolescents and adults living in households with children; non-pregnant women of childbearing age; and international travelers).

Last reviewed: May 16, 2023

The accelerated schedule should be used when the child is more than a month behind schedule, until you get them caught up. An accelerated schedule is acceptable as long as minimum ages and minimum intervals are observed for each dose. Once you have the child back on schedule, use the recommended ages and intervals on the childhood schedule. In this case you can give the child the first set of recommended vaccines at age 3 months and then bring him back at age 4 months and give the second set of vaccinations. At this point the child will be caught up and can return to the usual schedule. Be sure to educate the parents and talk to them about the importance of bringing the child back on time.

Last reviewed: June 6, 2023

Syncope, also called fainting, is a temporary loss of consciousness as a result of decreased blood flow to the brain. The most common form of fainting is called “vasovagal” fainting. This type of fainting can be triggered by an event associated with pain or anxiety. Some people may experience jerking movements after fainting which are not seizures. Published literature reveals that about 3% of men and 3.5% of women report at least one episode of fainting during their lifetime.

Last reviewed: August 31, 2022

A polysaccharide vaccine is a type of vaccine that is composed of long chains of sugar molecules, called polysaccharides, that resemble the surface of certain serotypes of pneumococcal bacteria in order to help the immune system mount a response.

A conjugate vaccine is a type of vaccine that joins a protein to an antigen (in the case of pneumococcal vaccines, the protein is connected to unique polysaccharides from the surface of each of the pneumococcal serotypes). The protein helps improve the quality of the immune system response to the vaccine compared to the response to an unconjugated polysaccharide.

Last reviewed: November 13, 2024

Recombinant zoster vaccine (RZV, Shingrix, GlaxoSmithKline) was licensed by the Food and Drug Administration (FDA) in October 2017. It is a subunit vaccine that contains recombinant varicella zoster virus (VZV) glycoprotein E in combination with a novel adjuvant (AS01B). Shingrix does not contain live VZV. It is FDA-approved and recommended by the Advisory Committee on Immunization Practices (ACIP) for all people 50 years and older and for adults age 19 years or older who are or will be immunodeficient or immunosuppressed because of disease or therapy. It has not been evaluated and is not approved for the prevention of primary varicella infection. Shingrix is administered as a 2-dose series by the intramuscular route. The second dose should be given 2 to 6 months after the first dose, with a minimum interval of 1 month (4 weeks) between doses.

Zoster vaccine live (ZVL, Zostavax, Merck) is a live attenuated vaccine that was licensed by the FDA in 2006 for adults age 50 and older and recommended by ACIP for people age 60 and older. Zostavax has been unavailable for use in the United States since November 18, 2020.

Last reviewed: March 9, 2022

A “moderate or severe acute illness” is a precaution for administering any vaccine. A mild acute illness (e.g., diarrhea or mild upper-respiratory tract infection) with or without fever is not a precaution, and vaccines may be given. The concern in vaccinating someone with moderate or severe illness is that a fever following the vaccine could complicate management of the concurrent illness – it could be difficult to determine if the fever was from the vaccine or due to the concurrent illness. In deciding whether to vaccinate a patient with moderate or severe illness, the clinician needs to determine if deferring vaccination will increase the patient’s risk of vaccine-preventable diseases, as is the case if the patient is unlikely to return for vaccination or to seek vaccination elsewhere.

Last reviewed: August 29, 2022

No, Dengvaxia does not protect the recipient against yellow fever.

Last reviewed: January 17, 2025

The most recent comprehensive ACIP recommendations for the use of MMR vaccine were published in 2013 and are available at www.cdc.gov/mmwr/pdf/rr/rr6204.pdf. CDC published the ACIP recommendations for the use of Priorix (GSK) brand of MMR vaccine on November 18, 2022, and they are available here: www.cdc.gov/mmwr/volumes/71/wr/pdfs/mm7146a1-H.pdf.

There is no difference in recommendations between Priorix and MMRII (Merck) brands of MMR vaccine. Priorix may be used in any situation where MMR vaccination is recommended. Despite minor differences in manufacturing (MMRII contains gelatin; Priorix does not) and approved route of administration (MMRII is approved for subcutaneous or intramuscular injection; Priorix is approved for subcutaneous injection only), the two vaccines may be considered functionally identical and interchangeable.

MMR vaccine is recommended routinely for all children at age 12 through 15 months, with a second dose at age 4 through 6 years. The second dose of MMR can be given as early as 4 weeks (28 days) after the first dose and be counted as a valid dose if both doses were given after the child’s first birthday. The second dose is not a booster, but rather is intended to produce immunity in the small number of people who fail to respond to the first dose.

Adults with no evidence of immunity should get 1 dose of MMR vaccine (evidence of immunity is defined as documented receipt of 1 dose of live measles virus-containing vaccine [or 2 doses, if high risk], laboratory evidence of immunity or laboratory confirmation of disease, or birth before 1957), unless the adult is in a high-risk group. Susceptible high-risk people need 2 doses of vaccine, given 4 weeks apart. High-risk people include school-age children, healthcare personnel, international travelers, and students attending post-high school educational institutions.

Live measles vaccine became available in the United States in 1963. An ineffective, inactivated measles vaccine was also available in the United States in 1963–1967. Combined MMR vaccine (MMRII, Merck) was licensed in 1971; Priorix (GSK) MMR vaccine was licensed and recommended in 2022. For people who previously received a dose of measles vaccine in 1963–1967 and are unsure which type of vaccine it was, or are sure it was inactivated measles vaccine, that dose should be considered invalid and the patient revaccinated as age- and risk-appropriate with MMR vaccine. If the person can confirm they received a live vaccine during that era, the dose is considered valid and does not need to be repeated. At the discretion of the state public health department, anyone exposed to measles in an outbreak setting can receive an additional dose of MMR vaccine even if they are considered completely vaccinated for their age or risk status.

Last reviewed: March 16, 2025


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Last reviewed: May 9, 2023


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Last reviewed: March 28, 2023

Healthcare providers (or other covered entities) may share immunization information with schools or daycare facilities, without authorization, if permitted or required by state law. These state laws would not be preempted by the HIPAA Privacy Rule [45 CFR 160.203(c)].

Last reviewed: February 16, 2025

Tick-borne encephalitis (TBE) is a disease caused by a flavivirus transmitted by tick bite in certain areas of Europe and Asia. It may also be transmitted through the consumption of unpasteurized milk or cheese from infected cows, goats, or sheep. CDC provides details of TBE-endemic areas, but notes that the risk of TBE in endemic areas is variable within risk areas and from year-to-year. Additional geographic information is available from CDC: www.cdc.gov/tick-borne-encephalitis/data-maps/.

Travelers moving or traveling to a TBE-endemic area and likely to have extensive exposure to ticks based on their planned outdoor activities and itinerary should be vaccinated for TBE. TBE vaccination may be considered for other travelers to TBE-endemic areas based on their likely exposure to ticks during their activities, their risk of a poor health outcome, and their personal perception and tolerance of risk.

Last reviewed: August 21, 2023
  • If a person is infected with an HPV strain that does not clear (that is, the person becomes persistently infected) the person cannot be reinfected because they are continuously infected.
  • If a person is infected with an HPV strain that clears, some but not all people will have a lower chance of reinfection with the same strain. Data suggest that females are more likely than males to develop immunity after clearance of natural infection.
  • Prior infection with an HPV strain does not lessen the chance of infection with a different HPV strain.
Last reviewed: March 2, 2024

The terminology change from “Category B” to “shared clinical decision-making” was done to describe more clearly the intent of the recommendation that the patient should be informed of the option to be vaccinated against meningococcal serogroup B disease and that the decision to vaccinate against MenB should be made by the provider and patient together. ACA requires coverage of vaccines as indicated on the recommended immunization schedule, including vaccines with shared clinical decision-making recommendations. The Vaccines for Children (VFC) program also covers vaccines recommended for shared clinical decision-making.

Last reviewed: November 15, 2024

The polysaccharide vaccine includes the different polysaccharides (chains of complex sugars) from different serotypes as the antigen. The conjugate vaccines have the polysaccharides for different serotypes attached (or conjugated) to a carrier protein. The immune response to the PPSV23 vaccine is a T-cell independent immune response, while the immune response to PCV vaccination is a T-cell dependent response that produces memory B-cells and reduces carriage of the bacteria in the respiratory track. The PPSV23 does not reduce bacterial carriage.

Last reviewed: November 13, 2024

All three RSV vaccines were effective in preventing RSV-associated lower respiratory tract disease (LRTD) in clinical trial participants, over the course of two RSV seasons.

The global clinical trials for RSVPreF3 (Arexvy, GSK) vaccine involved nearly 25,000 participants and some participants were followed for 2 RSV seasons. One dose reduced the risk of laboratory-confirmed RSV-associated LRTD with two or more symptoms by 82.6% during the first RSV season and 56.1% during the second season.

The RSVpreF (Abrysvo, Pfizer) vaccine global clinical trials involved nearly 37,000 participants, and some participants were followed for 2 RSV seasons. One dose reduced the risk of symptomatic, laboratory-confirmed RSV-associated LRTD with 3 or more symptoms by 88.9% during the first RSV season and 78.6% during a partial second season.

The mRNA RSV (mResvia, Moderna) vaccine global clinical trials involved nearly 37,000 participants. One dose reduced the risk of symptomatic laboratory-confirmed RSV LRTD with three or more symptoms by 80.9% in the first season. After a median follow up time of 18.8 months, its efficacy declined to 48.4%.

Few people enrolled in the clinical trials were either frail or of advanced age (80 or older), and none lived in long-term care facilities. People with immunocompromising conditions were excluded from clinical trials. For this reason, the clinical trials did not measure how well the vaccines would work in the people at highest risk of serious RSV disease.

Last reviewed: August 25, 2024

Yes. Vaccination should be offered regardless of history of prior SARS-CoV-2 infection.

Viral testing to assess for current SARS-CoV-2 infection or serologic testing to assess for prior infection for the purposes of vaccine decision-making is not recommended. However, as with all vaccines, vaccination should be deferred until after recovery from moderate to severe illness. In addition, to minimize the risk of exposing others to SARS-CoV-2 virus, vaccination of a person diagnosed with COVID-19 generally should be deferred until the person does not pose a risk of infection to others.

Although not required, people who recently had SARS-CoV-2 infection may consider delaying a 2024–2025 COVID-19 vaccine dose by up to 3 months from symptom onset or positive test (if infection was asymptomatic). Among vaccine recipients at increased risk of post-vaccination myocarditis (such as males age 12 through 39 years), an increased interval may reduce the rare risk of myocarditis after vaccination. A recipient’s individual risks for severe disease and current COVID-19 conditions in the community should be taken into account when deciding whether to delay vaccination up to 3 months after infection.

Last reviewed: November 16, 2024

ACIP recommends routine administration of a conjugate Hib vaccine series for all infants beginning at age 2 months. Infants 2 through 6 months of age should receive a 3-dose series of ActHIB, Hiberix, Pentacel, or Vaxelis, or a 2-dose series of PedvaxHIB. The first dose can be administered as early as age 6 weeks, but not earlier. Hib-containing vaccine should not be given before 6 weeks of age. Doses given before 12 months of age should be separated by at least 4 weeks. A booster dose (which will be dose 3 or 4 depending on vaccine type used in primary series) of any Hib-containing vaccine is recommended at age 12 through 15 months and at least 8 weeks after the most recent Hib dose. Vaxelis is recommended only for the primary Hib series and is not recommended for use as a booster (4th) dose. A different Hib-containing vaccine licensed for a booster dose should be used.

Medically stable preterm infants should be vaccinated beginning at age 2 months according to the schedule recommended for other infants, on the basis of chronological age. For special situations in children, refer to the current CDC recommended immunization schedule Hib notes: www.cdc.gov/vaccines/hcp/imz-schedules/child-adolescent-notes.html.

Last reviewed: January 21, 2025

Vaccines must always be dispensed with a prescription or order from a physician or other healthcare provider authorized by the state to prescribe medications. However, there are situations where vaccines can be administered using a standing order or vaccine protocol that is not patient-specific. In these situations, a physician or other healthcare provider does not need to be physically present for the vaccine to be administered. Several studies have shown that the use of standing orders can improve vaccination rates, and ACIP recommends the use of standing orders programs in both outpatient and inpatient settings. A comprehensive set of standing orders for the routine vaccines given to children and adults can be found at www.immunize.org/clinical/topic/standing-orders-templates/.

Last reviewed: January 20, 2025

ACIP does not address this issue. However, CDC experts have previously recommended that these children should each be vaccinated, notwithstanding they are conjoined. We believe even in conjoined twins who share organs and/or blood supply, vaccination of each child would also be indicated. The rationale is one cannot be sure, even in the latter case, that the common organs/blood supply would eliminate vaccine antigens less quickly, or the immune system(s) would respond adequately, to one dose of each vaccine for the two children. Therefore, administering a dose to each child, as if they were not conjoined, seems appropriate.

Last reviewed: February 27, 2025

The Inflation Reduction Act of 2022 (IRA) requires “first-dollar” coverage (no out-of-pocket cost to patient) for all recommended vaccines for adults covered by traditional Medicaid or Medicaid expansion plans. This coverage requirement applies to all states; however, enforcement policies and implementation may vary by state. Providers should check with their state Medicaid program directly about vaccine payment coverage and amounts paid by specific state plans.

 

Last reviewed: February 14, 2025

The ACIP statement (“Human Rabies Prevention-United States, 2008, Recommendations of the Advisory Committee on Immunization Practices”) was published in MMWR on May 23, 2008. This document updated the status of rabies and anti-rabies biologics in the United States. To view this document, go to www.cdc.gov/mmwr/PDF/rr/rr5703.pdf.

In March 2010, ACIP eliminated the fifth dose of vaccine given post-exposure to previously unvaccinated persons with no immunosuppression. To view these recommendations, go to www.cdc.gov/mmwr/pdf/rr/rr5902.pdf.

In May 2022, ACIP eliminated the third dose of vaccine given in the primary series for pre-exposure prophylaxis and introduced updated risk categories with customized recommendations for long-term follow up after completion of the 2-dose primary series. To view this document, visit www.cdc.gov/mmwr/volumes/71/wr/pdfs/mm7118a2-H.pdf.

Last reviewed: May 14, 2023

It is common practice in many developing countries to administer oral polio vaccine (OPV) to children during both routine visits and periodic vaccination campaigns, so a child’s record may indicate more than 4 doses. Some of these doses may not be valid according to the U.S. immunization schedule.

Doses are considered valid if written documentation indicates that doses of polio vaccine were given after 6 weeks of age and the vaccine received was listed as IPV or trivalent OPV (tOPV). A record simply indicating “OPV” also is acceptable if the OPV was administered before April 1, 2016, and it was not noted as being administered during a vaccination campaign.

There specific criteria for OPV documentation because only trivalent polio vaccine doses count as valid for the U.S. polio vaccination schedule. Trivalent OPV ceased to be used globally in April 2016. OPV administered before April 1, 2016, generally was tOPV. However, “OPV” doses recorded as given during a mass vaccination campaign before April 2016 do not count as valid because such campaigns may have used monovalent or bivalent OPV.

If the history is of a complete series of IPV or tOPV in any combination, at least one dose should be administered on or after 4 years of age and at least 6 months after the previous dose. If a complete series cannot be identified that meets these criteria, then the child should receive as many doses of IPV as needed to complete the U.S. recommended schedule.

Last reviewed: July 23, 2023

All adolescents should receive a dose of MenACWY at 11 or 12 years of age. A second (booster) dose is recommended at 16 years of age. Adolescents who receive their first dose at age 13 through 15 years should receive a booster dose at age 16 years. The minimum interval between MenACWY doses is 8 weeks. Adolescents who receive a first dose after their 16th birthday do not need a booster dose unless they become at increased risk for meningococcal disease. Colleges may not consider a second dose given even a few days before age 16 years as valid, so keep that in mind when scheduling patients. People 19 through 21 years of age are not recommended routinely to receive MenACWY. However, MenACWY may be administered to people age 19 through 21 years as catch-up vaccination for those who have not received a dose after their 16th birthday.

Last reviewed: November 15, 2024

Vaccines should not be stored in vegetable bins nor in the space occupied by vegetable bins of a household-style refrigerator. This area is commonly closer to the motor of the unit and the temperature is different from that in the body of the refrigerator. We recommend that you remove the vegetable bins and put bottles of water in that space to help maintain a constant temperature in your refrigerator. Vaccines should be placed in the center of the refrigerator, away from the walls and floor of the unit in open containers so air can circulate around the vaccines. If using the refrigerator section of a household-style combination refrigerator/freezer unit, you do not want the top storage shelf in the refrigerator to be too close to the vent that comes from the freezer because this can expose your vaccines to freezing temperatures.

Last reviewed: July 26, 2023

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