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)

Treatment with antibiotics is not a valid reason to defer vaccination. If the child or adult is otherwise well, or has only a minor illness, vaccines should be administered. But if the person has a moderate or severe acute illness (regardless of antibiotic use) vaccination may be deferred until the person’s condition has improved.

Last reviewed: August 29, 2022

ACIP recommendations and package inserts do not always match. Occasionally, ACIP may use different data to formulate its recommendations, or try to add flexibility to its recommendations (as was the case in this situation), which results in a recommendation different than the package insert. Published recommendations of national advisory groups (such as ACIP or AAP’s Committee on Infectious Diseases) should be considered equally as authoritative as those on the package insert. You should consider 8 months 0 days as the maximum age for a dose of rotavirus vaccine.

Last reviewed: June 7, 2023

Yes. Death as a result of fulminant hepatic failure is rare, however, older age (over 40 years) and preexisting chronic liver disease increases the risk of severe disease and death from hepatitis A. The person-to-person U.S. multistate outbreaks that began in 2016 have disproportionately affected adults with chronic liver disease and other health problems related to drug use and unstable housing. From 2016 through June 2023, CDC received reports of nearly 45,000 cases of acute HAV infection. Of these, approximately 61% have been hospitalized and nearly 1% (more than 420 people) have died.

Last reviewed: June 25, 2023

If a child is behind on immunizations, the Advisory Committee on Immunization Practices (ACIP) recommends using the minimum intervals between each dose until the child is caught up. The minimum interval for a vaccine can be used as many times as necessary, until the child is back on schedule. See Table 2 of the CDC Recommended Child and Adolescent Immunization Schedule, available at www.cdc.gov/vaccines/schedules/hcp/index.html.

Last reviewed: June 6, 2023

Fainting is most commonly reported in adolescents, according to the Vaccine Adverse Event Reporting System. One study showed that 62% of all fainting reports to VAERS were among adolescents 11 to 18 years old; however, due to the limitations of VAERS, we cannot use this system to determine how frequently fainting occurs after vaccination.

Last reviewed: August 31, 2022

In June 2021, ACIP voted to recommend Dengvaxia for routine use in children age 9 through 16 years with laboratory-confirmed previous dengue virus infection and living in areas where CDC has classified dengue as endemic. The recommendation is only for persons with confirmed previous DENV infection because Dengvaxia is associated with an increased risk for severe dengue in those who experience their first natural infection (i.e., primary infection) after vaccination.

The vaccine is not approved for use in U.S. travelers who are visiting but not living in an area where dengue is common.

The most current ACIP recommendations for Dengvaxia are available at www.cdc.gov/mmwr/volumes/70/rr/pdfs/rr7006a1-H.pdf.

CDC has developed a pre-vaccination checklist for evaluating whether a child should receive Dengvaxia: www.cdc.gov/dengue/resources/DVBD_FS_Vaccination_Checklist-508.pdf.

Last reviewed: January 17, 2025

Shingrix was studied in immunocompetent adults in 2 pre-licensure clinical trials. Efficacy against shingles was 97% for people 50–59 years of age, 97% for people 60–69 years of age, and 91% for people 70 years and older. Among people 70 years and older vaccine efficacy was 85% four years after vaccination.

Vaccine effectiveness (VE) has been evaluated for a limited number of specific immunocompromising conditions. VE estimates vary depending upon the underlying cause of immunocompromise. Studies have estimated VE of 68.2% for autologous hematopoietic cell transplant recipients, and 87.2% and 90.5% for patients with hematologic malignancies and potential immune-mediated diseases, respectively.

Last reviewed: March 9, 2022

HBV is stable in the environment and remains viable for 7 or more days on environmental surfaces at room temperature. HBV can be transmitted despite the absence of visible blood. Any high level disinfectant that is tuberculocidal will inactivate HBV. The Environmental Protection Agency also registers disinfectants specifically approved for use against HIV and HBV; a current list is available at this website: www.epa.gov/pesticide-registration/list-d-epas-registered-antimicrobial-products-effective-against-human-hiv-1.

Last reviewed: January 17, 2025

ACIP recommends annual vaccination for all people ages 6 months and older who do not have a contraindication to influenza vaccination.

Last reviewed: August 11, 2024

The Advisory Committee on Immunization Practices (ACIP) considers evidence of immunity to varicella to be:

  • Written documentation of 2 doses of varicella vaccine given no earlier than age 12 months with at least 4 weeks between doses.
  • U.S.-born before 1980*
  • A healthcare provider’s diagnosis of varicella or verification of history of varicella disease
  • History of herpes zoster, based on healthcare provider diagnosis or verification of disease history
  • Laboratory evidence of immunity or laboratory confirmation of disease

    *Note: Although there is only a very small chance of susceptibility, due to the potential for severe consequences from varicella infection, year of birth is not accepted as evidence of varicella immunity for healthcare personnel, immunosuppressed people, and pregnant people.

Last reviewed: May 16, 2023

Yes, although the package inserts states that Pediarix should only be given to infants born to mothers who are HBsAg-negative, ACIP voted in 2003 to expand its recommendations for use to include infants born to women whose HBsAg status is positive or unknown beginning no earlier than age 6 weeks.

Last reviewed: January 27, 2025

In April 2018, the Advisory Committee on Immunization Practices (ACIP) published a compilation of all previous recommendations for the prevention of pertussis, tetanus, and diphtheria (MMWR 2018;678 [RR-2]:1-31). The document can be accessed on the CDC website at www.cdc.gov/mmwr/volumes/67/rr/pdfs/rr6702a1-H.pdf.

In January 2020, ACIP published updated Tdap recommendations, stating that either Td or Tdap may be used in situations where Td only was previously recommended. The document can be accessed on the CDC website at www.cdc.gov/mmwr/volumes/69/wr/pdfs/mm6903a5-H.pdf.

Last reviewed: March 31, 2022

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: March 16, 2025

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

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

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

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

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: January 17, 2025

Yes, for the primary series. If either ActHIB (PRP-T), Hiberix (PRP-T), Vaxelis (DTaP-IPV-Hib-HepB), or Pentacel (DTaP-IPV/Hib) is used for a routine primary series dose, a complete routine primary series consists of three doses. PedvaxHIB (PRP-OMP) requires a 2-dose primary series, but if administering a mixed-product primary series including only one dose of PedvaxHIB, a total of 3 doses is needed to complete the primary series.

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 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: January 21, 2025

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 relevant subsection of CDC’s General Best Practices for Immunization, available at  www.cdc.gov/vaccines/hcp/imz-best-practices/special-situations.html#cdc_report_pub_study_section_7-persons-vaccinated-outside-the-united-states.

Last reviewed: January 20, 2025

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
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: January 17, 2025

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 17 cases in 2022.
  • 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: November 15, 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 mRNA COVID-19 series or 2-dose primary Novavax protein vaccine 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: November 16, 2024

There is no federal law requiring a refusal form. 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: February 16, 2025

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

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

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: February 27, 2025

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

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

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: January 27, 2025

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

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 (e.g., 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 (e.g., 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. If the vaccine given was the live virus vaccine, the dose is considered valid and does not need to be repeated.

* 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: March 16, 2025


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Last reviewed: May 24, 2023

ACIP has recommended that all adults age 65 years and older receive a second dose of the 2024–2025 Formula COVID-19 vaccine 6 months after their first 2024–2025 Formula COVID-19 vaccine dose (minimum interval of 2 months). However, ACIP has not made any standing COVID-19 vaccination recommendations that go beyond the 2024–2025 Formula COVID-19 vaccines. 

Last reviewed: November 16, 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.

Any person who initiated a COVID-19 vaccination primary series (e.g., a child 6 months through 4 years or a person with moderate or severe immunocompromise) with a previous formulation should follow the age-appropriate recommended schedule for completion of their primary series with the 2024–2025 formula of the same brand, if feasible. 

Last reviewed: November 16, 2024

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

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

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

All three RSV vaccines are currently licensed and recommended as a one-time dose for any person. Data are not available at this time to make recommendations for revaccination. At this time, a pregnant person who receives Abrysvo (Pfizer) during one pregnancy is not recommended to receive Abrysvo during a subsequent pregnancy: in subsequent pregnancies, the baby should receive nirsevimab (Beyfortus, Sanofi) after delivery for RSV protection. ACIP will make decisions concerning RSV revaccination as more data become available over time.

Last reviewed: August 25, 2024

Healthcare providers should refer to the catch-up schedule published as part of the CDC Recommended Child and Adolescent Immunization Schedule (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 single 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; these special situations are reviewed in the Hib notes section of the schedule: www.cdc.gov/vaccines/hcp/imz-schedules/child-adolescent-notes.html.

Last reviewed: January 21, 2025

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.

Last reviewed: January 17, 2025

The following information is taken from the CDC web page with information on the vaccination of U.S.-bound refugees and V-93 applicants (www.cdc.gov/immigrant-refugee-health/hcp/panel-physicians/vaccination.html):

CDC has specific criteria to determine which vaccines applicants for a United States Immigrant Visa are required to show proof of having received.

The criteria are as follows:

The vaccine must be an age-appropriate vaccine, as recommended by the Advisory Committee on Immunization Practices (ACIP) for the general U.S. population AND at least one of the following:

  • The vaccine must protect against a disease that has the potential to cause an outbreak. An outbreak is defined as the occurrence of more cases of disease than expected in a given area or among a specific group of people, over a given period of time. For endemic diseases, an outbreak occurs when incidence rises above the normally expected level. For diseases with seasonal variation, the average incidence rates over particular weeks or months of previous years, or average high or low levels over a period of years, may be used as baselines.
  • The vaccine must protect against a disease that has been eliminated in the United States or is in the process of being eliminated in the United States.

Therefore, the vaccines required for applicants do not include all the vaccines recommended by the ACIP and CDC for routine U.S. domestic use, and are limited to vaccination for the following diseases:

  • Diphtheria
  • Tetanus
  • Pertussis
  • Polio
  • Measles
  • Mumps
  • Rubella
  • Rotavirus
  • Haemophilus influenzae type b (Hib)
  • Hepatitis A
  • Hepatitis B
  • Meningococcal disease
  • Varicella
  • Pneumococcal disease
  • Influenza
  • COVID-19

ACIP recommends vaccines for a certain age range in the general U.S. public. ACIP recommendations are used to decide which vaccines are age-appropriate for the general immigrant population. Refer to the CDC table of vaccines required for immigrant visa applicants by age: www.cdc.gov/immigrant-refugee-health/media/pdfs/Vaccine-Requirements-According-to-Applicant-Age-panel-physicians-p.pdf.

Children adopted from outside the U.S. and political refugees are recommended to receive age-appropriate vaccination, with catch-up vaccination as appropriate. More information is available in the “Special Situations” section of CDC’s General Best Practices for Immunization, available at  www.cdc.gov/vaccines/hcp/imz-best-practices/special-situations.html#cdc_report_pub_study_section_7-persons-vaccinated-outside-the-united-states. People entering the U.S. as visitors are not required to provide proof of vaccination regardless of the length of stay.

Last reviewed: January 20, 2025
Table 2
Tests Results Interpretation Vaccinate?
HBsAg negative susceptible vaccinate if indicated
anti-HBc negative
anti-HBs negative
HBsAg negative immune due to vaccination (or may represent passive transfer of antibodies from receipt of HBIG) no vaccination necessary
anti-HBc negative
anti-HBs positive with >10mIU/mL*
HBsAg negative immune due to natural infection no vaccination necessary
anti-HBc positive
IgM anti-HBc negative
anti-HBs positive
HBsAg negative acute resolving infection no vaccination necessary
anti-HBc positive
IgM anti-HBc positive
anti-HBs positive
HBsAg positive acutely infected no vaccination necessary
anti-HBc positive
IgM anti-HBc positive
anti-HBs negative
HBsAg positive chronically infected no vaccination necessary (may need treatment)
anti-HBc positive
IgM anti-HBc negative
anti-HBs negative
HBsAg negative four interpretations possible† use clinical judgment
anti-HBc positive
anti-HBs negative

* 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: January 17, 2025

MenB vaccines were approved based on the serologic response to the vaccine. Because meningococcal B disease is so rare, 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: November 15, 2024

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 injectable or intranasal 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 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.

*There are 2 exceptions to this general rule:

  1. If the 15-valent pneumococcal conjugate vaccine (PCV15, Vaxneuvance, Merck) and pneumococcal polysaccharide vaccine (PPSV23, Pneumovax, Merck) are to be given to a high-risk patient, these vaccines should not be given at the same visit. The PCV15 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 PCV15 to avoid interference between the two vaccines.
  2. Oral cholera vaccine should be administered before oral Ty21a vaccine (Vivotif, Bavarian Nordic): ACIP recommends at least 8 hours should separate oral cholera vaccine and the first dose of oral Ty21a to avoid any interference by the oral cholera vaccine buffer solution with the enteric-coated Ty21a vaccine dose.
Last reviewed: February 27, 2025

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 or your state immunization program. CDC maintains a listing of contact information for all state IIS programs at www.cdc.gov/iis/contacts-locate-records/.

Last reviewed: February 16, 2025

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

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

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