Ask the Experts: All Questions

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Results (1355)

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: March 24, 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: August 31, 2024

Acceptable presumptive evidence of immunity against measles includes at least one of the following:

  • written documentation of adequate vaccination:
    • one or more doses of a measles-containing vaccine administered on or after the first birthday for preschool-age children and adults not at high risk
    • two doses of measles-containing vaccine for school-age children, adolescents, and adults at high risk, including college students, healthcare personnel, and international travelers
  • laboratory evidence of immunity
  • laboratory confirmation of measles (verbal history of measles does not count)
  • birth before 1957

Although birth before 1957 is considered acceptable evidence of measles immunity, healthcare facilities should consider vaccinating unvaccinated personnel born before 1957 who do not have other evidence of immunity with 2 doses of MMR vaccine (minimum interval 28 days).

During an outbreak of measles, healthcare facilities should recommend 2 doses of MMR vaccine at the appropriate interval for unvaccinated healthcare personnel regardless of birth year if they lack laboratory evidence of measles immunity.

Last reviewed: June 19, 2023

Yes. Several studies have measured the effect of vaccination on preventing RSV-associated hospitalizations among people vaccinated in the first RSV season after the recommendation for use of the vaccine. Estimates were similar for both the GSK and Pfizer products and showed that they reduced the risk of hospitalization among recipients overall by between 75% and 82%. For additional details, see the CDC MMWR published in August 2024: www.cdc.gov/mmwr/volumes/73/wr/pdfs/mm7332e1-H.pdf .

Last reviewed: August 25, 2024


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Last reviewed: May 23, 2023

Yes. Several studies have measured the effect of vaccination on preventing RSV-associated hospitalizations among people vaccinated in the first RSV season after the recommendation for use of the vaccine. Estimates were similar for both the GSK and Pfizer products and showed that they reduced the risk of hospitalization among recipients overall by between 75% and 82%. For additional details, see the CDC MMWR published in August 2024: www.cdc.gov/mmwr/volumes/73/wr/pdfs/mm7332e1-H.pdf.

Last reviewed: August 25, 2024

People who are at increased risk for acquiring HAV infection include the following:

  • Travelers to countries that have high or intermediate endemicity of HAV infection
  • Men who have sex with men (MSM)
  • Users of injection and non-injection drugs (in other words, all who use illegal drugs)
  • People with occupational risk of exposure (those who work with HAV-infected non-human primates or researchers handling hepatitis A virus)
  • People who anticipate close contact with an international adoptee coming from a country with high or intermediate endemicity of HAV infection
  • People living with HIV infection
  • People experiencing homelessness, including temporary shelters and other unstable living arrangements
  • People living in group settings for those with developmental disabilities and other settings where hygiene is difficult to maintain
  • People who are incarcerated
Last reviewed: June 25, 2023

No vaccine component, including the diluent used to reconstitute a vaccine, should be stored in vegetable bins, nor in the space occupied by vegetable bins of a household-style refrigerator. CDC recommends that you position vaccines and diluents 2 to 3 inches from the unit walls, ceiling, floor, and door. If using a household-style unit, avoid storing vaccines and diluents in any part of the unit that may not provide stable temperatures or sufficient air flow, such as directly under cooling vents; in deli, fruit, or vegetable drawers; or on refrigerator door shelves. The instability of temperatures and air flow in these areas may expose vaccines to inappropriate storage temperatures. For more information, refer to page 14 of the CDC Vaccine Storage and Handling Toolkit available at www.cdc.gov/vaccines/hcp/admin/storage/toolkit/storage-handling-toolkit.pdf.

Last reviewed: July 26, 2023

No. ACIP recommends giving a dose of MMR to infants age 6 through 11 months before international travel, but not varicella vaccine. Varicella vaccine is neither approved nor recommended for children younger than age 12 months in any situation.

Last reviewed: May 16, 2023

Zero, one, or two doses of MMR vaccine are needed for the adults described below.

Zero doses:

  • adults born before 1957 except healthcare personnel*
  • adults born 1957 or later who are at low risk (i.e., not an international traveler or healthcare worker, or person attending college or other post-high school educational institution) and who have already received one or more documented doses of live measles vaccine
  • adults with laboratory evidence of immunity or laboratory confirmation of measles

One dose of MMR vaccine:

  • adults born in 1957 or later who are at low risk (i.e., not an international traveler, healthcare worker, or person attending college or other post-high school educational institution) and have no documented vaccination with live measles vaccine and no laboratory evidence of immunity or prior measles infection

Two doses of MMR vaccine:

  • high-risk adults without any prior documented live measles vaccination and no laboratory evidence of immunity or prior measles infection, including:
    • healthcare personnel*
    • international travelers born in 1957 or later
    • people attending colleges and other post-high school educational institutions

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, should be revaccinated with either one (if low-risk) or two (if high-risk) doses of MMR vaccine.

* Healthcare personnel born before 1957 should be considered for MMR vaccination in the absence of an outbreak, but are recommended for MMR vaccination during outbreaks.

Last reviewed: June 19, 2023

All children should receive a series of DTaP at ages 2, 4, and 6 months, with boosters at ages 15–18 months and at 4–6 years. The fourth dose may be given as early as age 12 months if at least 6 months have elapsed since the third dose.

Last reviewed: March 31, 2022

Two or more injectable or nasally administered live vaccines not administered on the same day should be separated by at least 4 weeks to minimize the potential risk for interference. If two such vaccines are separated by less than 4 weeks, the second vaccine administered should not be counted and the dose should be repeated at least 4 weeks later. Alternatively, one can perform serologic testing to check for immunity, but this option may be more costly, may not be practical if multiple antigens are involved (such as measles, mumps and rubella), and may provide results that are difficult to interpret.

In cases where the vaccine doses given less than 28 days apart are two doses of the same live vaccine in a series (e.g., 2 doses of MMR vaccine), not different vaccines, you do not need to repeat the second dose if it was inadvertently administered within the 4-day “grace period” before day 28. If given more than 4 days earlier than day 28, the second dose should be repeated after the recommended minimum interval from the invalid dose.

The oral vaccines Ty21a typhoid, cholera and rotavirus vaccines can be administered on the same day with or at any interval before or after other live vaccines (injectable or intranasal). However, ACIP recommends that oral cholera vaccine should be administered before Ty21a vaccine, and at least 8 hours should separate the oral cholera vaccine and the first dose of Ty21a in order to minimize the risk that the oral cholera vaccine buffer might interfere with the enteric coating of the oral Ty21a vaccine.

Last reviewed: June 6, 2023

No. According to ACIP, vaccines administered outside the U.S. generally can be accepted as valid if the schedule (i.e., minimum ages and intervals) is similar to that recommended in the U.S. However, with the exception of the influenza and pneumococcal polysaccharide vaccines, only written documentation should be accepted as evidence of previous vaccination. In general, if records cannot be located or will definitely not be available anywhere because of the patient’s circumstances, children 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. More information is available in the ACIP “General Best Practice Guidelines for on Immunization”, available at www.cdc.gov/vaccines/hcp/acip-recs/general-recs/special-situations.html.

Last reviewed: August 22, 2020

Not that we know. Nearly all vaccines have been reported to be associated with the fainting. Post-vaccination fainting has been most frequently reported after three vaccines commonly given to adolescents (HPV, MenACWY, and Tdap). However, the association with these vaccines may simply reflect the fact that adolescents are generally more likely to experience fainting in response to pain or anxiety.

Last reviewed: August 31, 2022

Treatment after an exposure (PEP) in a previously unvaccinated person requires receiving a dose of human rabies immune globulin (HRIG) and four (or five if the person’s immune system is suppressed) doses of vaccine. Pre-exposure prophylaxis (PrEP) requires only two doses of vaccine and no immune globulin. If a person who is up to date with the recommended PrEP schedule is exposed to rabies, the person’s PEP treatment is completed with two doses of vaccine (on day 0 and day 3).

Last reviewed: May 14, 2023

Use the date of administration to make a presumptive determination of what type of OPV was received. Only trivalent doses count as valid for the U.S. polio vaccination schedule.

Trivalent OPV was used throughout the world before April 2016. In April 2016, all countries using tOPV switched to bivalent OPV (bOPV). In addition, some countries also use monovalent OPV (mOPV) during special vaccination campaigns. Doses recorded as bOPV or mOPV, and unspecified OPV doses noted on an immunization record as given during a vaccination campaign, do not count as valid doses for the U.S. polio vaccination schedule.

You may count a record of an “OPV” dose as valid if the dose was administered before April 1, 2016, and was not noted as being administered as part of a mass vaccination campaign. OPV doses administered on or after April 1, 2016, should not be counted as a valid dose for the U.S. polio vaccination schedule.

Last reviewed: July 23, 2023

No, do not use the same unit. Frequent opening of the refrigerator door to retrieve food items can adversely affect the internal temperature of the unit and potentially damage the vaccines.

Last reviewed: July 26, 2023

ACIP recommends that the rotavirus vaccine series be completed with the same product whenever possible. However, vaccination should not be deferred because the product used for a previous dose is not available or is unknown. In these situations, the provider should continue or complete the series with the product available. If any dose in the series was RotaTeq, or the vaccine product is unknown for any dose in the series, a total of 3 doses of rotavirus vaccine should be administered. The minimum interval between doses of rotavirus vaccine is 4 weeks. All doses should be administered by age 8 months and 0 days.

Last reviewed: June 7, 2023

Yes. If either ActHIB, Vaxelis, Pentacel, or Hiberix is used for a routine primary series dose, a complete routine primary series consists of three doses. Vaxelis is not recommended for use as a Hib booster (4th) dose. A different Hib-containing vaccine licensed for the booster dose should be used. If Vaxelis (DTaP-IPV-Hib-HepB, MSP Company) is inadvertently given as the booster dose, the dose does not need to be repeated with another Hib-containing vaccine, if the proper spacing of prior doses is maintained.

Last reviewed: July 31, 2022

Since DTaP and pneumococcal conjugate (PCV) are the vaccines most likely to cause a local reaction, it is prudent to give DTaP and PCV in separate limbs (if possible), so there is no confusion about which vaccine caused the reaction.

Last reviewed: December 28, 2022

Yes. In clinical trials among immunocompetent adults age 50 years or older, Shingrix reduced the risk of PHN by 91%. One study among hematopoietic cell transplant recipients reported that vaccination reduced the risk of PHN by 89%.

Last reviewed: March 9, 2022
Table 1: Hepatitis B laboratory nomenclature
HBsAg: Hepatitis B surface antigen is a marker of infectivity. Its presence indicates either acute or chronic HBV infection.
anti-HBs: Antibody to hepatitis B surface antigen is a marker of immunity. Its presence indicates an immune response to HBV infection, an immune response to vaccination, or the presence of passively acquired antibody. (It is also known as HBsAb, but this abbreviation is best avoided since it is often confused with abbreviations such as HBsAg.)
anti-HBc (total): Antibody to hepatitis B core antigen is a nonspecific marker of acute, chronic, or resolved HBV infection. It is not a marker of vaccine-induced immunity. It may be used in prevaccination testing to determine previous exposure to HBV infection. (It is also known as HBcAb, but this abbreviation is best avoided since it is often confused with other abbreviations.)
IgM anti-HBc: IgM antibody subclass of anti-HBc. Positivity indicates recent infection with HBV (<6 mos). Its presence indicates acute infection.
HBeAg: Hepatitis B “e” antigen is a marker of a high degree of HBV infectivity, and it correlates with a high level of HBV replication. It is primarily used to help determine the clinical management of patients with chronic HBV infection.
Anti-HBe: Antibody to hepatitis B “e” antigen may be present in an infected or immune person. In persons with chronic HBV infection, its presence suggests a low viral titer and a low degree of infectivity.
HBV-DNA: HBV Deoxyribonucleic acid is a measure of viral load and reflects viral replication. It correlates well with infectivity. It is used to assess and monitor the treatment of patients with chronic HBV infection.
Last reviewed: July 21, 2023

Yes. As with any combination vaccine, it may be used when any of the components are indicated and none are contraindicated. Providers must observe spacing intervals such that the minimum interval between doses is equal to the greatest interval of any of the individual antigens. Pediarix may only be used in children younger than age 7 years.

Last reviewed: July 15, 2023

In general, neither exposure to or recovery from an infectious disease is a contraindication or precaution to vaccination. In particular, recovery from varicella (chickenpox) is not a reason to withhold a live vaccine, such as MMR.

COVID-19 is the exception to this general rule. CDC recommends that routine vaccination should be deferred for persons with suspected or confirmed COVID-19, regardless of symptoms, until criteria have been met for them to discontinue isolation. The reason for this exception is that vaccination visits for these individuals should be postponed to avoid exposing healthcare personnel and other patients to the virus that causes COVID-19.

Last reviewed: August 29, 2022

There is no federal law requiring such documentation. Several major medical organizations, including the American Academy of Pediatrics, have stated that healthcare providers may decide it is in their best interest to formally document a parent’s refusal to accept vaccination for their (minor) child. To read a discussion on this topic and to access a prototype refusal form, see “Record of Vaccine Declination” that can be accessed at www.immunize.org/catg.d/p4059.pdf.

Last reviewed: June 6, 2023

In February 2022, ACIP voted on the following recommendation or TBE vaccination: TBE vaccine is recommended for people who are moving or traveling to a TBE-endemic area and will have extensive exposure to ticks based on their planned outdoor activities and itinerary.

In addition, TBE vaccine may be considered for persons traveling or moving to a TBE-endemic area who might engage in outdoor activities in areas ticks are likely to be found. The decision to vaccinate should be based on an assessment of their planned activities and itinerary, risk factors for a poorer medical outcome, and personal perception and tolerance of risk. For more information about TBE vaccine from CDC, visit www.cdc.gov/tick-borne-encephalitis/hcp/vaccine/index.html.

Last reviewed: August 21, 2023

Gardasil 9 (9vHPV, Merck) is the only HPV vaccine being distributed in the United States. Bivalent Cervarix (2vHPV, GSK) and quadrivalent Gardasil (4vHPV, Merck) are no longer available in the United States.

9vHPV is an inactivated 9-valent vaccine licensed by the FDA in 2014. It contains 7 oncogenic (cancer-causing) HPV types (16, 18, 31, 33, 45, 52, and 58) and two HPV types that cause most genital warts (6 and 11). The 9vHPV vaccine is licensed for people age 9 through 45 years.

Last reviewed: March 2, 2024

CDC defines an “additional primary dose” as a subsequent dose of vaccine administered to people who are less likely to develop a protective immune response after initial vaccination because of moderate or severe immunocompromise. All previously unvaccinated people with moderate or severe immunocompromise are recommended to receive an initial 3-dose primary COVID-19 series.

CDC defines a “booster dose” as a subsequent dose of vaccine administered to enhance or restore protection which might have declined over time after primary series vaccination.

Last reviewed: August 31, 2024

To assist with the shared clinical decision-making around the option to vaccinate against meningococcal serogroup B disease and the timing of vaccination, CDC has provided some specific considerations about the disease and the vaccine that the patient and provider may weigh:

  • Serious nature of invasive meningococcal serogroup B infection, with a high risk of death and permanent complications
  • Low level of serogroup B disease in the United States, with an average of 34 cases each year among people age 16 through 23 years between 2015 and 2018, declining to 4 cases in 2021.
  • Increased risk among college students, especially those who are freshmen, attending a 4-year university, living in on-campus housing, or participating in sorority and fraternity life
  • Protection of MenB vaccine against most strains of meningococcal serogroup B bacteria
  • Estimated relatively short duration of MenB vaccine protection, with antibody levels waning within 1–2 years of completing the primary series; however, if a booster is indicated (e.g., during an outbreak) antibody titers rise in one to two weeks after booster dose administration
  • Evidence to date suggests no impact of MenB vaccine on meningococcal B carriage (may protect an individual from invasive disease but is unlikely to impact transmission of the bacteria to others)
Last reviewed: March 24, 2024

Multiple manufacturers are producing inactivated, recombinant, and live attenuated influenza vaccines for the U.S. market for the 2024-25 season. All vaccines are trivalent (containing two influenza A and one influenza B virus vaccine antigen).

Immunize.org offers a 1-page printable document that summarizes each of the products available for the current influenza vaccination season: www.immunize.org/catg.d/p4072.pdf.

Last reviewed: August 11, 2024

Currently, protection from serious lower respiratory tract disease (LRTD) after RSV vaccination is expected to last for about two RSV seasons, but exactly how well it protects over a longer period of time is not yet known. In clinical trials, the ability of the vaccine to reduce the risk of serious lower respiratory tract disease was tested in some people over 2 RSV seasons, and all RSV vaccines provided meaningful protection in the second season.

RSV vaccination is currently recommended as a single dose: revaccination is not yet recommended. ACIP will update its recommendations concerning whether to revaccinate, and when, as more data become available over time concerning how long protection from a single dose lasts and how effective revaccination is in boosting protection. A small study by GSK did not show a meaningful benefit of revaccination with Arexvy just one year after dose 1; however, revaccinating after two or three years, or longer, might be helpful. Studies are underway to evaluate the benefit of revaccination at different intervals.

Last reviewed: August 25, 2024

FDA licensed the first pneumococcal conjugate vaccine against seven serotypes (PCV7, Prevnar7, Pfizer) in 2000. A large clinical trial showed PCV7 reduced invasive disease caused by vaccine serotypes by 97%. Compared to unvaccinated children, children who received PCV7:

  • Had 20% fewer episodes of chest X-ray confirmed pneumonia
  • Had 7% fewer episodes of acute otitis media
  • Underwent 20% fewer tympanostomy tube placements

FDA licensed PCV13 based on studies comparing the serologic response of children who received PCV13 to those who received PCV7. Substantial evidence demonstrates that routine infant PCV7 and PCV13 vaccination reduces the carriage and transmission of vaccine serotypes.

Researchers conducted a randomized placebo-controlled trial (CAPiTA trial) in the Netherlands among approximately 85,000 adults 65 years or older from 2008 through 2013. This trial evaluated the clinical benefit of PCV13 in the prevention of pneumococcal pneumonia. The results of the CAPiTA trial demonstrated:

  • 46% efficacy against vaccine-type pneumococcal pneumonia
  • 45% efficacy against vaccine-type non-bacteremic pneumococcal pneumonia
  • 75% efficacy against vaccine-type invasive pneumococcal disease (IPD, i.e., bacteremia or meningitis)

FDA licensed PCV15 and PCV20 in 2021 based on studies comparing the serologic response of adults who received either PCV15 or PCV20 to those who received PCV13. These studies showed PCV15 and PCV20 induced antibody levels comparable to those induced by PCV13 and shown to be protective against invasive disease. FDA subsequently expanded the indication for use of PCV15 and PCV20 to include children starting at age 6 weeks in 2022 and 2023, respectively, based on serologic studies. PCV21 was licensed in 2024 based on a similar evaluation of serologic response to vaccination.

Last reviewed: November 13, 2024

Yes. Dengvaxia is included in the VFC program. The most current VFC resolution is available at www.cdc.gov/vaccines-for-children/about/index.html.

The cost of the pre-vaccination screening test will be covered by Medicaid for those who are eligible.

Last reviewed: February 16, 2022

No. ACIP considers a dose of MenACWY given to a 10-year-old child to be valid for the first dose in the adolescent series. Doses given before age 10 years should not be counted. The child should receive the second (booster) dose at age 16 years as usual.

Last reviewed: November 15, 2024


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Last reviewed: May 24, 2023

ACIP has not made a routine recommendation for COVID-19 vaccination of older adults every 6 months. Although vaccine effectiveness against hospitalization in this age group wanes significantly by 6 months after vaccination, the ACIP recommendation made in February 2024 for adults in this age group to receive an additional dose of the 2023–2024 Formula COVID-19 vaccine was based upon conditions at the time and the expectation that COVID-19 would continue to circulate through the summer of 2024. ACIP will evaluate data and make a decision concerning an additional dose during the 2024-25 respiratory virus season as the season progresses.

Last reviewed: August 31, 2024

In general, CDC recommends waiting a minimum of 8 weeks (2 months) since the last 2023–2024 Formula COVID-19 vaccine to receive an age-appropriate 2024–2025 Formula COVID-19 vaccine.

Previously unvaccinated children age 6 months through 4 years are recommended to receive a 2-dose (Moderna) or 3-dose (Pfizer-BioNTech) primary series of COVID-19 vaccine. Previously unvaccinated moderately or severely immunocompromised people are recommended to receive a 3-dose primary series of mRNA COVID-19 vaccine (either product) or a 2-dose primary series of Novavax COVID-19 vaccine. Any previously unvaccinated person who receives Novavax COVID-19 vaccine should receive a 2-dose primary series, 3-8 weeks apart. If a person in one of these categories initiated their primary series with a 2023–2024 formula vaccine, they are recommended to follow the age-appropriate recommended schedule for completion of the primary series with the 2024–2025 formula of the same brand.

Last reviewed: August 31, 2024

Varicella vaccine is neither approved nor recommended for children younger than age 12 months. Assuming that the child is not immunocompromised, varicella zoster immune globulin (VariZIG) is also not recommended.

ACIP does not have a recommendation for acyclovir for varicella postexposure prophylaxis. The American Academy of Pediatrics provide some guidance on this issue in the current edition of the Red Book.

Last reviewed: May 16, 2023

The most current ACIP recommendations for Tdap can be accessed here at www.cdc.gov/mmwr/volumes/69/wr/pdfs/mm6903a5-H.pdf.

A listing of the recommendations follows:

  • Tdap can be given regardless of the interval since the last Td was given. There is NO need to wait 2–5 years to administer Tdap following a dose of Td.
  • Adolescents should receive a single dose of Tdap (instead of Td) at the 11–12-year-old visit.
  • Adolescents and adults who have not received a dose of Tdap, or for whom vaccine status is unknown, should receive a single dose of Tdap as soon as feasible. As stated above, Tdap can be administered regardless of interval since the previous Td dose.
  • Children age 7–10 years who are not fully immunized against pertussis (i.e., did not complete a series of pertussis-containing vaccine before their seventh birthday) should receive a single dose of Tdap. If needed, they should complete their series with Td or Tdap. If a Tdap dose is administered at age 10 years or older, the Tdap dose may count as the adolescent Tdap dose.
  • All healthcare personnel, regardless of age, should receive a single dose of Tdap as soon as feasible if they have not previously received Tdap and regardless of the time since the last dose of Td.
  • Pregnant teens and adults should receive Tdap during each pregnancy, preferably between 27- and 36-weeks’ gestation. Mothers who have never received Tdap and who do not receive it during pregnancy should receive it immediately postpartum.
  • Tdap may be administered in any situations where Td only was previously recommended.
  • After receiving an initial dose of Tdap, either Tdap or Td can be used to fulfill the decennial (every 10 years) Td booster dose recommendation.
Last reviewed: March 31, 2022

Historically, HAV infection was highly endemic in institutions for people with developmental disabilities as a result of poor hand hygiene, close living conditions and diaper use. As fewer children have been institutionalized and as conditions in institutions have improved, the incidence and prevalence of HAV infection have decreased, although outbreaks can occur in these settings. All children with developmental disabilities should receive HepA vaccine according to U.S. routine vaccine recommendations, including catch up vaccination of all children through age 18 years.

As a strategy to further reduce the risk of hepatitis A outbreaks and reach adults in settings with a high proportion of people with risk factors for HAV infection, the current ACIP recommendations suggest considering HepA vaccination of residents and staff in facilities where hygiene is difficult to maintain, such as group homes for people with developmental disabilities and homeless shelters.

Last reviewed: June 25, 2023

If vaccines are given too close together, it can result in a less than optimal immune response. However, in most instances, a difference of a few days is unlikely to have a negative effect on immune response. With the exception of rabies vaccine, ACIP allows a grace period of 4 days (i.e., vaccine doses administered up to 4 days before the recommended minimum interval or age can be counted as valid). However, if a dose was administered 5 or more days earlier than the recommended minimum interval between doses, it is not valid and must be repeated. The repeat dose should be spaced after the invalid dose by the recommended minimum interval. Note that for hepatitis A vaccination, if the second dose is administered too early and must be repeated, the recommended interval between the invalid dose and the repeat dose is 6 months; however, if the repeat dose is administered earlier than 6 months no further doses are recommended as long as the interval between the first and final dose is at least 6 months.

If the first dose in a series is given 5 days or more before the recommended minimum age, the dose should be repeated on or after the date when the child reaches at least the minimum age. If the vaccine is a live vaccine, ensuring that a minimum interval of 28 days has elapsed from the invalid dose is recommended. Avoid such errors by knowing the minimum intervals and ages for routinely given vaccines. You can look up such information in the ACIP “General Best Practices Guidelines for Immunization”, available at www.cdc.gov/vaccines/hcp/acip-recs/general-recs/timing.html#, Table 3-2.

The 4-day “grace period” should not be used when scheduling future vaccination visits, and should not be applied to the 28-day interval between different live parenteral vaccines not administered at the same visit. It should be used primarily when reviewing vaccination records (for example, when evaluating a vaccination record prior to entry to daycare or school).

Last reviewed: June 6, 2023

Falls that occur due to fainting after vaccination can be prevented by assuring that the vaccinated person is sitting in a chair or lying down and is observed for 15 minutes following vaccination.

Last reviewed: August 31, 2022

Human rabies immune globulin (HRIG) is the IgG fraction of plasma from human donors who have received multiple doses of rabies vaccine and have high levels of anti-rabies antibody. HRIG is administered once to previously unvaccinated individuals exposed to a rabid animal to provide rabies virus neutralizing antibody coverage until the patient responds to vaccination by actively producing virus-neutralizing antibodies. HRIG is administered once on day 0 at the time postexposure prophylaxis (PEP) is initiated, in conjunction with human rabies vaccine. If HRIG was not administered when vaccination was begun on day 0, it can be administered up to and including day 7 of the PEP series. If anatomically feasible, the full dose of HRIG is infiltrated around and into any wounds. Any remaining volume is injected intramuscularly at a site distant from vaccine administration. HRIG should not be administered in the same syringe or at the same anatomic site as the first vaccine dose. However, subsequent doses (i.e., on days 3, 7, and 14) of vaccine in the 4-dose PEP vaccine series can be administered in the same anatomic location in which HRIG was administered.

Last reviewed: May 14, 2023

First-year college students living in residence halls should be vaccinated against meningococcal ACWY disease. Before enrollment, administer a dose of MenACWY vaccine to those previously unvaccinated, to those who have not had a dose of MenACWY since turning 16, and to those whose most recent MenACWY dose (given after turning 16) was not given within the past 5 years. Some schools, colleges, and universities have policies requiring vaccination against meningococcal disease as a condition of enrollment.

Last reviewed: November 15, 2024

ACIP recommends that infants who have had rotavirus gastroenteritis before receiving the full series of rotavirus vaccination should still start or complete the schedule according to the age and interval recommendations because the initial rotavirus infection might provide only partial protection against subsequent rotavirus disease.

Last reviewed: June 7, 2023

Children with confirmation of previous DENV infection need three doses of Dengvaxia, given 6 months apart, for complete protection (0, 6 months, 12 months).

Recipients achieve significant protection following dose 1. Ongoing evaluation will determine whether the schedule could be simplified in the future.

Last reviewed: February 16, 2022

According to CDC, more than 80% of healthy adults who receive PPSV23 develop antibodies against the serotypes contained in the vaccine that persist for at least 5 years. Older adults and people with some chronic illnesses or immunodeficiency may not respond as well and their antibody levels may decline more quickly.

Overall, the vaccine is 60% to 70% effective in preventing invasive pneumococcal disease caused by serotypes in the vaccine. PPSV23 shows less effectiveness among immunocompromised people; however, because of their increased risk of invasive pneumococcal disease, CDC recommends PPSV23 for people in these groups who receive PCV15. There has not been consensus regarding the ability of PPSV23 to prevent non-bacteremic pneumococcal pneumonia; however, recent observational studies reported 21%–46% effectiveness against PPSV23-type pneumococcal pneumonia when PPSV23 was given less than 5 years before illness onset.

Unlike conjugate vaccines, PPSV23 vaccination has not been shown to decrease nasal carriage of pneumococcal bacteria among those vaccinated.

Last reviewed: November 13, 2024

Shingrix is recommended for the prevention of herpes zoster and related complications for immunocompetent adults 50 years of age and older, including those who previously received Zostavax. On October 20, 2021, ACIP recommended 2 doses of RZV for the prevention of herpes zoster and related complications in adults age 19 years or older who are or will be immunodeficient or immunosuppressed because of disease or therapy.

ACIP published its zoster vaccination recommendations for immunocompetent adults age 50 years and older in January 2018: www.cdc.gov/mmwr/volumes/67/wr/pdfs/mm6703a5-H.pdf.

ACIP published its recommendations for the use of recombinant zoster vaccine in adults age 19 years or older who are or will be immunocompromised in January 2022: www.cdc.gov/mmwr/volumes/71/wr/pdfs/mm7103a2-H.pdf.

Last reviewed: March 9, 2022
Table 2
Tests Results Interpretation Vaccinate?
HBsAg
anti-HBc
anti-HBs		 negative
negative
negative		 susceptible vaccinate if indicated
HBsAg
anti-HBc
anti-HBs		 negative
negative
positive with >10mIU/mL*		 immune due to vaccination (or may represent passive transfer of antibodies from receipt of HBIG) no vaccination necessary
HBsAg
anti-HBc
IgM anti-HBc
anti-HBs		 negative
positive
negative
positive		 immune due to natural infection no vaccination necessary
HBsAg
anti-HBc
IgM anti-HBc
anti-HBs		 negative
positive
positive
positive		 acute resolving infection no vaccination necessary
HBsAg
anti-HBc
IgM anti-HBc
anti-HBs		 positive
positive
positive
negative		 acutely infected no vaccination necessary
HBsAg
anti-HBc
IgM anti-HBc
anti-HBs		 positive
positive
negative
negative		 chronically infected no vaccination necessary (may need treatment)
HBsAg
anti-HBc
anti-HBs		 negative
positive
negative		 four interpretations possible† use clinical judgment

* Postvaccination testing, when it is recommended, should be performed 1-2 months after the last dose of vaccine. Infants born to HBsAg-positive mothers should be tested for HBsAg and anti-HBs after completion of at least 3 doses of a licensed hepatitis B vaccination series, at age 9-18 months (generally at the next well child visit).

†1. May be distantly immune, but the test may not be sensitive enough to detect a very low level of anti-HBs in serum

2. May be susceptible with a false positive anti-HBc

3. May be chronically infected and have an undetectable level of HBsAg present in the serum

4. Passive transfer of antibody following HBIG administration or from an HBsAg-positive mother to her newborn

Last reviewed: July 21, 2023

Although birth before 1957 is considered acceptable evidence of measles immunity for routine vaccination, healthcare facilities should consider vaccinating unvaccinated healthcare personnel (HCP) born before 1957 who do not have laboratory evidence of measles immunity, laboratory confirmation of disease, or vaccination with 2 appropriately spaced doses of MMR vaccine.

However, during a local outbreak of measles, all healthcare personnel, including those born before 1957, are recommended to have 2 doses of MMR vaccine at the appropriate interval if they lack laboratory evidence of measles.

Healthcare facilities should check with their state or local health department’s immunization program for guidance. Access contact information here: www.immunize.org/coordinators.

Last reviewed: June 19, 2023

CDC published a revised ACIP recommendation for IPV on August 6, 2009, to include a dose given at age 4 through 6 years (and at least 6 months after the preceding dose), regardless of the number of doses given before the fourth birthday. In general, you do not need to administer an additional IPV dose to older teens (or adults) who had already completed a routine 4-dose schedule of polio vaccination before their fourth birthday before the change was published in August 2009; however, if a dose given after the fourth birthday is needed for compliance with state requirements, you may want to check with your state immunization program or immunization registry manager to see what they accept/expect in this case. Contact information for state immunization managers can be found at www.immunize.org/coordinators.

Last reviewed: July 23, 2023

No. Pediarix is licensed for doses 1, 2, and 3 of the DTaP primary series through age 6 years.

Last reviewed: July 15, 2023

No. Pregnancy tests prior to vaccination are not routinely recommended. However, patients of childbearing age should be asked about the possibility of their being pregnant prior to being given any vaccine for which pregnancy is a contraindication or precaution. The patient’s answer should be documented in the medical record. If the patient responds that they believe they may be pregnant, a test should be performed before administering vaccines not recommended or contraindicated in pregnancy.

Last reviewed: August 29, 2022

Healthcare providers should refer to the catch-up schedule which is approved and published each year by the ACIP, AAP, and AAFP (available at www.cdc.gov/vaccines/hcp/imz-schedules/child-adolescent-catch-up.html). The catch-up schedule will help determine the number of additional doses needed and the minimum intervals between doses. However, if a healthy child receives a dose of Hib vaccine at age 15 months or older, he or she does not need any further doses regardless of the number of doses received before age 15 months. Some high-risk children between the age of 15 months and 59 months will be recommended for two doses of Hib vaccine based on previous history of incomplete vaccination.

Last reviewed: July 31, 2022

CDC recommends using separate refrigerator and freezer units for vaccine storage, but still allows use of a combination refrigerator/freezer if you only use the refrigerator portion for storing vaccines (as you are doing). CDC also recommends that you store food and beverages in a separate storage unit from vaccines, which you are technically doing, but there may still be an impact on the refrigerator temperature by the opening and closing of the freezer door by staff. (In most two-compartment units, cold air from the freezer is circulated for cooling the refrigerator.)

The best situation would be to get a stand-alone pharmaceutical/purpose-built refrigerator unit for your vaccines (consider one that meets the voluntary NSF/ANSI 456 certification for vaccine storage), and use your refrigerator/freezer combination unit for your food and drinks. For more information about storage unit features and recommendations, refer to pages 9 and 31 of the CDC Vaccine Storage and Handling Toolkit available at www.cdc.gov/vaccines/hcp/admin/storage/toolkit/storage-handling-toolkit.pdf.

Last reviewed: July 26, 2023

With rare exceptions*, all vaccines can be administered at the same visit. There is no upper limit for the number of vaccines that can be administered during one visit. ACIP and AAP consistently recommend that all needed vaccines be administered during an office visit. Vaccination should not be deferred because multiple vaccines are needed. All live vaccines (MMR, varicella, live attenuated influenza, yellow fever, and oral typhoid) can be given at the same visit if indicated. If live vaccines are not administered during the same visit, they should be separated by 4 weeks or more.

When giving several injections at a single visit, separate IM vaccines by at least 1 inch in the body of the muscle if possible to reduce the likelihood of local reactions overlapping. 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 older children: www.aimtoolkit.org/docs/Giving_all_the_doses_12mths.pdf

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

For details see ACIP’s “General Best Practices Guidelines for Immunization”, available at www.cdc.gov/vaccines/hcp/acip-recs/general-recs/administration.html.

*There are 3 exceptions to this general rule: 1) if both pneumococcal conjugate vaccine (PCV13, Prevnar 13, Pfizer) and pneumococcal polysaccharide vaccine (PPSV23, Pneumovax 23, Merck) are indicated for a high-risk patient, these vaccines should not be given at the same visit. The PCV13 should be given first followed by PPSV23 at least 8 weeks later. If PPSV23 has already been given, wait 8 weeks (for a child) or 1 year (for an adult age 19 years or older) before giving PCV13 to avoid interference between the two vaccines. 2) A person with anatomic or functional asplenia or HIV should receive both PCV13 and meningococcal ACWY (MenACWY) vaccines. If Menactra brand (Sanofi) MenACWY is used, the person should first receive all recommended doses of PCV13 followed by Menactra at least 4 weeks later. Menveo (GSK) or MenQuadfi (Sanofi) MenACWY brands can be given at the same time or at any time before or after PCV13. 3) Cholera vaccine should be administered before TY21a vaccine, and 8 hours should separate cholera vaccine and the first dose of TY21a.

Last reviewed: December 28, 2022

As of 2021, every state has an IIS that is capable of accepting and storing immunization records on individuals of all ages. To find out the status of the IIS in your state you should contact your state IIS program. CDC maintains a listing of contact information for all state IIS programs at www.cdc.gov/iis/contacts-locate-records/.

Last reviewed: February 9, 2024

The ACIP recommends that routine HPV vaccination be initiated for all children at age 11 or 12 years. ACIP notes that vaccination may be started at age 9 years, if preferred, and should start at age 9 for any child that the provider at risk of exposure to HPV due to suspected abuse. There is no downside to beginning the series at age 9, and this option is often easier for families and clinics because it gives more time to complete the 2-dose series before the 13th birthday. Vaccination is also recommended for all people age 13 through 26 years who have not been vaccinated previously or who have not completed the vaccination series.

Last reviewed: March 2, 2024

MenB vaccines were approved based on the serologic response to the vaccine. No data are available on vaccine effectiveness against clinical disease or duration of protection against clinical disease. Short term protection refers to the known duration of the antibody response. Available data indicate that protective antibody levels wane in most recipients within 1–2 years of completion of the primary series. Antibody levels rise sharply within 1-2 weeks of a booster dose.

Last reviewed: March 24, 2024

Effective December 14, 2009, the CDC revised the vaccination criteria for U.S. immigration. CDC will use these criteria for vaccines recommended by the ACIP to decide which vaccines will be required for U.S. immigration. The criteria will be used at regular periods, as needed, by CDC. The new criteria are:

  • The vaccine must be age-appropriate* for the immigrant applicant, AND at least one of the following:
  • The vaccine must protect against a disease that has the potential to cause an outbreak, OR
  • The vaccine must protect against a disease that has been eliminated or is in the process of being eliminated in the United States.

*ACIP recommends vaccines for a certain age range in the general U.S. public. These ACIP recommendations will be used to decide which vaccines are age-appropriate for the general immigrant population.

Current immigration law requires that immigrants have proof of vaccination against mumps, measles, rubella, tetanus, diphtheria, pertussis, meningococcal disease, pneumococcal disease, Haemophilus influenzae type B, rotavirus, varicella, influenza, hepatitis A, hepatitis B, and polio. Human papillomavirus (HPV) and zoster vaccines are not required for immigrants. Additional information is available on the CDC website at www.cdc.gov/immigrant-refugee-health/hcp/panel-physicians/vaccination.html.

Children adopted from outside the U.S. and political refugees are recommended to receive age-appropriate vaccination, with catch-up vaccination as appropriate, per the guidance in ACIP’s “General Best Practice Guidelines for Immunization” (see www.cdc.gov/vaccines/hcp/acip-recs/general-recs/special-situations.html). People entering the U.S. as visitors are not required to provide proof of vaccination regardless of the length of stay.

Last reviewed: August 22, 2020

The 2024-25 vaccines are all trivalent (containing two influenza A and one influenza B strains). The B/Yamagata virus vaccine antigens are no longer included in influenza vaccines because B/Yamagata viruses have not been detected globally since March 2020.

The 2024-25 vaccines include a new influenza A(H3N2) component.

Egg-based influenza vaccines include:

  • an A/Victoria/4897/2022 (H1N1)pdm09-like virus;
  • an A/Thailand/8/2022 (H3N2)-like virus; and,
  • a B/Austria/1359417/2021 (B/Victoria lineage)-like virus.

Cell culture-based or recombinant vaccines include:

  • an A/Wisconsin/67/2022 (H1N1)pdm09-like virus;
  • an A/Massachusetts/18/2022 (H3N2)-like virus; and,
  • a B/Austria/1359417/2021 (B/Victoria lineage)-like virus.
Last reviewed: August 11, 2024

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