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.
The ACIP in June 2025 voted to no longer recommend the use of influenza vaccine from thimerosal-containing multidose vials for any recipient. In addition, a few states have legislation restricting its use in pregnancy. There is no scientific evidence that thimerosal in vaccines is a cause of adverse events unless the patient has a systemic allergy to thimerosal. More than 95% of influenza vaccine doses administered in the 2024–25 season came from single dose products without preservatives, so the new ACIP recommendation should have limited impact on access to influenza vaccination in the 2025–26 season.
No, these two antibody preparations should not interfere with each other. Administer nirsevimab (Beyfortus, Sanofi) or clesrovimab (Enflonsia, Merck) as recommended.
The patient should receive a 2025–2026 COVID-19 mRNA dose now, of the same manufacturer as the initial two doses (if feasible), in order to complete the 3-dose initial mRNA vaccine series recommended for people with moderate or severe immunocompromise. Advise the patient that he should receive an additional 2025–2026 Formula COVID-19 dose of any brand in 6 months (minimum interval 2 months if using Comirnaty, Spikevax, or Nuvaxovid; minimum interval 3 months if using mNexspike).
Because pneumococcal recommendations have changed over the years, providers should generally avoid assuming which pneumococcal vaccines a patient has received. Ideally, providers and patients should try to verify which vaccines were received, including by checking medical records and the jurisdiction’s immunization information system (immunization registry) where the patient was likely vaccinated.
Per the CDC “General Best Practices Guidelines for Immunization”, self-reported doses of influenza and PPSV23 are acceptable. All other vaccines must be documented with a written, dated record. This means that if a patient reasonably recalls receiving a PPSV23 after turning 65, you may accept that as a history of PPSV23 and administer a recommended pneumococcal conjugate vaccine option (PCV20, or PCV21).
Alternatively, if vaccination records cannot be obtained, and the patient is uncertain whether they received PCV13 or PPSV23, you may choose to classify the patient as having an unknown vaccination history and administer either PCV20 or PCV21 alone or PCV15 followed by PPSV23 one year later. When giving the PCV15 and PPSV23 series to a patient with an immunocompromising condition, cochlear implant, or cerebrospinal fluid leak, an 8-week minimum interval between PCV15 and PPSV23 may be considered.
The MenACWY booster dose should be given at 14 years (5 years after the primary series) and every 5 years thereafter. The every-5-year booster dose schedule for people with high-risk conditions takes precedence over the routine adolescent schedule.
Yes. The use of a 4-dose HepB schedule is acceptable when giving the monovalent HepB vaccine birth dose followed by the use of Pediarix (DTaP-HepB-IPV) or Vaxelis (DTaP-IPV-Hib-HepB). The use of a 4-dose HepB schedule, including schedules with a birth dose, has not increased vaccine reactogenicity and results in higher final antibody titers that should correlate with longer duration of detectable antibody. The federal Vaccines for Children (VFC) program provides up to four doses of HepB for VFC-eligible children. You may still use monovalent HepB in a 3-dose series.
No. Shingrix recombinant zoster vaccine contains only a small part of the varicella zoster virus that causes shingles. Shingrix does not contain any live varicella zoster virus.
Contraindications are the following:
HPV vaccines are not recommended for use during pregnancy. If a person is found to be pregnant after starting the vaccination series, the remainder of the 2- or 3-dose series (depending on the age of first HPV vaccination) should be delayed until completion of pregnancy. Pregnancy testing is not needed before vaccination. If a vaccine dose has been administered during pregnancy, no intervention is needed. You can find more information about HPV vaccine and pregnancy in the ACIP recommendations at: www.cdc.gov/mmwr/preview/mmwrhtml/rr6305a1.htm.
Your understanding of the general Td/Tdap recommendation is correct, and this is the schedule that should be followed for persons 7 years old and older who have never received tetanus-containing vaccine or who cannot provide documentation of prior vaccination. ACIP recommends that Tdap or Td may be used in situations when only Td was previously recommended. Be sure to document doses administered in your state’s immunization information system so other healthcare providers will have access to the record of immunization and a primary series will not need to be repeated in the future.
Absolutely not. No vaccines should ever be mixed in the same syringe unless the combination has been specifically approved by the FDA.
The “General Best Practice Guidelines for Immunization” (see www.cdc.gov/vaccines/hcp/acip-recs/general-recs/timing.html) makes the generic recommendation that live parenterally or nasally administered vaccines not given on the same day should be separated by at least 28 days. The CDC travel health website recommends that yellow fever vaccine and other parenteral or nasal live vaccines should be separated by at least 30 days if possible. Either interval is acceptable.
In general, if the error is discovered on the same clinic day, you can administer the other “half” of the dose on that same day. If the error is discovered later, the dose should not be counted, and then the person should be recalled to the office and given a full age-appropriate repeat dose.
If you give more than an age-appropriate dose (for example, an adult dose of HepA vaccine given to a child), count the dose as valid and notify the patient/parent about the error. There may be an increased risk of a local adverse reaction when more than the recommended dose is given. If the error occurred with the first dose of the series the child should still receive the second dose on schedule. Giving a “double” dose for the first dose does not negate the need for a second dose.
Avoid such errors by checking the vaccine vial label 3 times.
Because the surgeon is immune, the child’s rash is not a problem and there is no need for the surgeon to restrict activity. In comparing a vaccine rash to wild-type chickenpox infection, transmission is less likely with a vaccine rash and, in general, there are fewer skin lesions.
No. CDC’s Advisory Committee on Immunization Practices (ACIP) does not recommend more than one dose of influenza vaccine per season, except for certain children being vaccinated for the first time.
Nirsevimab (Beyfortus, Sanofi) and clesrovimab (Enflonsia, Merck) come in manufacturer-filled syringes (MFS). In most states, anyone who can administer injections can administer them. If you have questions about a specific type of healthcare worker, check the scope of practice rules in your state.
In 2008, ACIP reviewed evidence indicating that asthma is an independent risk factor for pneumococcal disease among adults. ACIP also reviewed evidence demonstrating an increased risk of invasive pneumococcal disease among smokers. Consequently, ACIP includes both asthma and cigarette smoking as indications for pneumococcal vaccination among adults age 19 through 49 years. People with these conditions should receive either a single dose of PCV20 or PCV21 alone, or a dose of PCV15 followed one year later by PPSV23. If they have already received PPSV23, but have not had a conjugate vaccine, they should receive a single dose of a recommended pneumococcal conjugate vaccine (PCV15, PCV20, or PCV21) at least one year following their dose of PPSV23.
If the person cannot provide written documentation of the previous vaccination, you should assume they are unvaccinated and vaccinate accordingly.
The efficacy data from the clinical trials were based on age at time of vaccination, and not on the weight of the individual. Hence, the dosage recommendations reflect this age-based efficacy data. The same holds true for HepB vaccine. In addition, higher response rates are expected in younger people, even if their weights are above the norm.
According to subject matter experts at CDC, your electronic health record is correct. The CDC website states that HepB dose #4, if given, must be at 24 weeks of age or later, at least 16 weeks from dose #1, and at least 8 weeks from dose #2. There is no minimum interval requirement between dose #4 and the previous dose. This information is not published in any current ACIP statement but it can be found under “Hepatitis B” at www.cdc.gov/cocasa/hcp/reports/algorithm-reference.html.
The only contraindication is a severe allergic reaction to a vaccine component or following a prior dose.
Yes. A woman with evidence of present or past HPV infection identified through cervical screening may be vaccinated, and should be vaccinated if age 26 or younger. Infection with one type of HPV does not prevent infection with additional types. Vaccination can prevent infections with additional HPV types included in the vaccine. Recipients of HPV vaccinations should be counseled that the vaccine will not have a therapeutic effect on any existing HPV infections or cervical lesions. In other words, vaccination does not treat existing HPV infections or the lesions (warts, cancer, or pre-cancerous changes) caused by them.
No. ACIP recommends that people age 11 years and older who have not yet received Tdap receive a dose of Tdap now. ACIP specifies no waiting interval between administering Td and Tdap.
There has been only one published report of mother to child transmission of varicella vaccine virus. If the susceptible woman were to be infected with wild varicella virus, the risk of transmission to the infant would be much higher. Breastfeeding is not a contraindication or a precaution to varicella vaccination of the mother when vaccination is indicated.
ACIP recommends that all HCP born during or after 1957 have adequate presumptive evidence of immunity to measles, mumps, and rubella, defined as documentation of two doses of measles and mumps vaccine and at least one dose of rubella vaccine, laboratory evidence of immunity, or laboratory confirmation of disease. Further, ACIP recommends that healthcare facilities should consider vaccination of all unvaccinated healthcare personnel who were born before 1957 and who lack laboratory evidence of measles, mumps, and/or rubella immunity or laboratory confirmation of disease. During an outbreak of measles or mumps, healthcare facilities should recommend 2 doses of MMR separated by at least 4 weeks for unvaccinated healthcare personnel regardless of birth year who lack laboratory evidence of measles or mumps immunity or laboratory confirmation of disease. During outbreaks of rubella, healthcare facilities should recommend 1 dose of MMR for unvaccinated personnel regardless of birth year who lack laboratory evidence of rubella immunity or laboratory confirmation of infection or disease.
When injectable vaccine volume is lost (patient moves, syringe leaks), it may be difficult to judge how much vaccine the patient actually received. Use your discretion to determine whether an adequate dose was given. In general, you should treat this as a nonstandard injectable dose and should not count it. If it was an inactivated vaccine, you should re-immunize the person as soon as possible. In the case of Shingrix (RZV; GSK), if the person is still in the office the dose can be repeated immediately; however, if the repeat Shingrix dose cannot be given on the same day, then CDC recommends that it should be given 4 weeks after the invalid dose.
If it was a live vaccine, you can give another dose if you detect the error on the same clinic day; otherwise, you should wait 28 days to give the next dose. However, if part of a dose of an oral vaccine (rotavirus) was spit out by an infant, count the dose and do not administer a second dose. If a person sneezes after live-attenuated influenza vaccine (Flumist; AstraZeneca), the dose can be counted as valid
A CDC-funded study found that people who had been vaccinated early in pregnancy with an influenza vaccine containing the pandemic H1N1 (H1N1pdm09) component and who also had been vaccinated the prior season with an H1N1pdm09-containing influenza vaccine had an increased risk of spontaneous abortion (miscarriage) in the 28 days after vaccination. This study did not quantify the risk of miscarriage and did not prove that influenza vaccine was the cause of the miscarriage. Earlier studies have not found a link between influenza vaccination and miscarriage. A larger follow-up study also funded by CDC which included 3 more years of data found no association between early miscarriage and influenza vaccination regardless of previous influenza season vaccination. These results are reassuring regarding the safety of influenza vaccination during pregnancy.
CDC, ACIP, and the American College of Obstetricians and Gynecologists (ACOG) all recommend influenza vaccination during any trimester of pregnancy. Influenza poses a danger during pregnancy and the vaccine can prevent influenza in pregnant people and their infants.
Only infants younger than age 8 months 0 days are routinely recommended to receive an RSV preventive antibody product during or before their first RSV season. The recommended use of nirsevimab in older infants and toddlers age 8 months through 19 months is narrowly limited to American Indian/Alaska Native children and children with specific conditions that put them at high risk of severe lower respiratory tract disease due to RSV.
All COVID-19 vaccines are administered intramuscularly. Preparation details and dose volume vary by product. Most 2025-2026 formula products are now available in manufacturer-filled syringes.
A previous history of chickenpox disease, even recent disease, is not known to interfere with the immune response to different vaccines. To review the true contraindications and precautions to vaccination, consult the appendix of the CDC Recommended Child and Adolescent Immunization Schedule (www.cdc.gov/vaccines/schedules/hcp/imz/child-adolescent.html).
CDC’s “General Best Practice Guidelines for Immunization” also contains the table of contraindications and precautions, in addition to a useful table titled “Conditions incorrectly perceived as contraindications or precautions to vaccination (i.e., vaccines may be given under these conditions)”. Both tables are available at www.cdc.gov/vaccines/hcp/imz-best-practices/contraindications-precautions.html, Tables 4-1 and 4-2.
All meningococcal conjugate vaccines (MenACWY, MenB, MenABCWY) should be administered by the intramuscular route.
No. ACIP does not identify people who use smokeless tobacco products or vaping as being at increased risk for invasive pneumococcal disease or as being in a risk group recommended for vaccination.
Twinrix (GSK) is an inactivated combination vaccine containing both hepatitis A virus (HAV) and hepatitis B virus (HBV) antigens. The vaccine contains 720 EL.U. of hepatitis A antigen (half of the Havrix adult dose) and 20 mcg of hepatitis B antigen (the full Engerix-B adult dose).
In the U.S., Twinrix is licensed for use in people who are age 18 years or older. It can be administered to people who are at risk for both hepatitis A and hepatitis B, such as certain international travelers, people with HIV infection, people with chronic liver disease not caused by hepatitis B, men who have sex with men, people who use drugs, or to people who simply want to be immune to both diseases. Primary immunization consists of 3 doses given intramuscularly on a 0-, 1-, and 6-month schedule. In 2007, the FDA also approved a 4-dose schedule for Twinrix. It consists of 3 doses given within 4 weeks, followed by a booster dose at 12 months (0, 7 days, 21–30 days, and 12 months). The 4-dose schedule could benefit individuals needing rapid protection from hepatitis A and hepatitis B, such as people traveling to high-prevalence areas imminently.
Twinrix cannot be used for postexposure prophylaxis.
Yes. Poorer immune response rates are seen in infants who complete the vaccination series prior to age 6 months. Do not count dose #3, which you gave at age 4 months. Repeat dose #3 when the infant is at least 6 months of age (no earlier than age 24 weeks).
The only precaution is the presence of a moderate or severe acute illness, including having an active case of herpes zoster. If the patient has zoster, vaccination should be deferred until lesions have crusted and symptoms have abated.
There is currently no ACIP recommendation for Shingrix use in pregnancy; therefore, providers should consider delaying Shingrix until after pregnancy. There is no recommendation for pregnancy testing before vaccination.
As soon as possible, even if it is the same day.
No. The needle should be considered to be contaminated. The needle and syringe should be discarded. A new syringe, needle, and dose of vaccine should be used. Generally, a full repeat dose should be given, but you may use your clinical judgment to decide whether an adequate dose was administered before the patient pulled away.
Yes.
Yes. Healthcare personnel (HCP) with 2 documented doses of MMR vaccine are considered to be immune regardless of the results of a subsequent serologic test for measles, mumps, or rubella. Documented age-appropriate vaccination supersedes the results of subsequent serologic testing. In contrast, HCP who do not have documentation of MMR vaccination and whose serologic test is interpreted as “indeterminate” or “equivocal” should be considered not immune and should receive 2 doses of MMR vaccine (minimum interval 28 days). ACIP does not recommend serologic testing after vaccination. For more information, see ACIP’s recommendations on the use of MMR vaccine at www.cdc.gov/mmwr/pdf/rr/rr6204.pdf, page 22.
From the time influenza vaccines were first recommended for children until 2025, CDC, ACIP, and the American Academy of Pediatrics (AAP) expressed no preference for preservative-free vaccine for infants or any other group of vaccine recipients. In June 2025, a newly constituted group of 7 ACIP members voted to no longer recommend the use of preservative-containing formulations of influenza vaccine for any recipient. AAP continues to express no preference for preservative-containing or preservative-free products among available age-appropriate products in its guidance. See AAP 2025–26 influenza vaccination recommendations for children here: https://doi.org/10.1542/peds.2025-073620.
No scientific evidence exists that thimerosal in vaccines is a cause of adverse events unless the patient has a systemic allergy to thimerosal. However, some states have enacted legislation that restricts the use of thimerosal-containing vaccines. Check with your state immunization program to see if your state is one of them (www.immunize.org/official-guidance/state-policies/state-resources/).
The recipient should be informed of the error, and RSV vaccine should be administered as recommended. A 50-mg or 100-mg MFS dose of antibody is very small compared to the body weight of an adult and you should not assume the dose would have any protective effect. There is no defined waiting period after antibody administration for vaccine administration. Facilities that stock RSV vaccine and RSV preventive antibody products should put systems and procedures in place to prevent this type of error, including staff training and clear labeling and warnings in storage units.
CDC strongly encourages reporting of this medication error and any suspected adverse events following the error to the Vaccine Adverse Event Reporting System (VAERS) at https://vaers.hhs.gov if the antibody was administered at the same visit as vaccines. If antibody was administered alone, report the incident to MedWatch online (www.fda.gov/medwatch), by fax, by mail, or by contacting FDA at 1-800-FDA-1088.
Yes. CDC published an appendix to its interim clinical considerations for the use of COVID-19 vaccines to address a wide range of errors in vaccine administration. It includes a detailed table outlining actions to take after an error has occurred: www.cdc.gov/covid/hcp/vaccine-considerations/appendices.html.
Categories of errors covered in the CDC table include:
Ask the Experts refers our readers to this CDC table for the most current and comprehensive guidance on COVID-19 vaccine administration errors and how to manage them. For all vaccine administration errors the following steps are recommended: inform the patient of the error, report the error to VAERS (https://vaers.hhs.gov) unless CDC’s guidance states that the error does not need to be reported, evaluate why the error occurred, and implement strategies to prevent future errors.
No, unless chronic lung disease is present, which puts them at increased risk of pneumococcal disease. PCV20 or PCV21 alone or PCV15 followed one year later by PPSV23 is recommended for current smokers of cigarettes age 19 through 49 years (see www.cdc.gov/mmwr/volumes/71/wr/pdfs/mm7104a1-H.pdf).
Breastfeeding is NOT a precaution to vaccination with Shingrix (recombinant zoster vaccine, RZV). Recombinant vaccines pose no known risk to mothers who are breastfeeding or to their infants. Clinicians should consider vaccination without regard to breastfeeding status if Shingrix is otherwise indicated.
Vaccination of the parents against pertussis after the baby is born is not optimal, but it may be helpful and should be done if the parents have not previously received Tdap, regardless of when they last received Td vaccination. It takes about 2 weeks after Tdap receipt for the parents to have protection against pertussis. Once the parents have protection, they are is less likely to transmit pertussis to the infant. However, the newborn remains at risk of contracting pertussis from others, including siblings, grandparents, and other caregivers. They should be counseled about the importance of Tdap vaccination of the mother during future pregnancies. See CDC’s web page for more information: www.cdc.gov/pertussis/hcp/vaccine-recommendations/vaccinating-pregnant-patients.html.
Empty or expired vaccine vials are considered medical waste and should be disposed of according to state regulations.
Approximately 5 to 15% of susceptible people who receive MMR vaccine will develop a low-grade fever and/or mild rash 7 to 12 days after vaccination. However, the person is not infectious, and no special precautions ( such as exclusion from work) need to be taken.
There is no known risk associated with MMR or varicella vaccination in someone with selective IgA or IgM deficiency. It is possible that the immune response may be weaker, but the vaccines are likely effective.
No. Even a woman found to be infected with a strain of HPV that is present in the vaccine could receive protection from the other strains in the vaccine.
If an infant received an adult formulation of HepB (adult formulations contains more antigen than pediatric formulations), the dose may be counted as valid and does not need to be repeated. The next age-appropriate dose should be given on the usual schedule. Heplisav-B is an exception because it is not FDA-approved for use in children: if a dose of Heplisav-B is administered more than 4 days before the 18th birthday, the dose should not be considered valid and should be repeated using an age-appropriate formulation. Hepatitis B vaccines are very safe vaccines and no unusual adverse events would be expected because of this administration error.
The liquid vaccine component (the diluent) of Menveo contains the C, W-135, and Y serogroups, and the lyophilized vaccine component (the freeze-dried powder) contains serogroup A. Because the patient received only the diluent, he or she is not protected against invasive meningococcal disease caused by N. meningitidis serogroup A.
Invasive disease with N. meningitidis serogroup A is very rare in the United States but is more common in some other countries. If the recipient (of the C-Y-W135 “diluent” only) is certain not to travel outside the United States, then the dose does not need to be repeated. However, if the recipient plans to travel outside the United States the dose should be repeated with correctly reconstituted Menveo, the one-vial formulation of Menveo that does not require reconstitution, or with a dose of another brand of MenACWY. There is no minimum interval between the incorrect dose and the repeat dose.
People with a metabolic disease, including diabetes, should receive annual influenza vaccination with an age-appropriate inactivated or recombinant influenza vaccine.
Vaccination will have the most benefit if administered in late summer or early fall, just before the RSV season. In most of the continental United States, this corresponds to vaccination during August–October.
If you have an opportunity to vaccinate an eligible patient and are concerned that there will not be an opportunity to vaccinate during an ideal time of year, you may administer RSV vaccine at any time of year. A meaningful degree of protection after vaccination should last at least two years.
No. CDC does not recommend repeating the dose of any COVID-19 vaccine in circumstances where the dose is administered in an incorrect route or an incorrect site (i.e., not in the deltoid or anterolateral thigh). In the case of a subcutaneous injection, the patient should be advised of the possibility of self-limited local or systemic side effects.
Providing a vaccine recipient with a COVID-19 vaccine VIS is recommended by CDC but not required. The requirement to provide the VIS to a patient is present only when a vaccine is included in the federal Vaccine Injury Compensation Program (VICP). COVID-19 vaccines are not included in the VICP at this time. The current official VIS and all available translations are available from Immunize.org: www.immunize.org/vaccines/vis/covid-19/.
No. ACIP does not designate people who smoke marijuana, but not cigarettes, as being in a risk group for vaccination. ACIP has not been presented evidence of an increased risk of pneumococcal disease among regular marijuana smokers.
Yes. MenACWY and MenB vaccines can be given at the same visit or at any time before or after the other. The pentavalent MenABCWY vaccine Penbraya (Pfizer) may be administered as an option for people age 10 or older who need both MenB-FHbp (Trumenba, Pfizer) and MenACWY vaccination at the same visit. For people age 10 years or older at increased risk of meningococcal disease, Penbraya may be used for additional MenACWY and MenB doses (including booster doses) if both would be given on the same clinic day and at least 6 months have elapsed since most recent Penbraya dose.
No. Twinrix contains 50% less hepatitis A antigen component than Havrix, GSK’s monovalent hepatitis A vaccine [720 vs. 1440 El. U.], so the patient would not receive the recommended dose of hepatitis A vaccine antigen. For this reason, 3 doses of Twinrix must comprise the series.
The minimum age for the last dose of HepB is age 24 weeks (the minimum age is the youngest age that is acceptable for giving a vaccine and having it “count” as a valid dose.) This allows healthcare providers more flexibility in administering HepB should a parent bring an infant in for a well-baby check before the infant reaches a full 6 months of age. If the third dose is given prior to age 24 weeks the dose should not be counted. Poorer response rates are seen in infants who complete the vaccination series prior to age 24 weeks. The third dose should be repeated when the infant is at least age 24 weeks.
Once they are no longer acutely ill, they can be vaccinated with Shingrix. There is no evidence that vaccine will have therapeutic effect for a person with existing zoster or postherpetic neuralgia.
In clinical trials of 9vHPV (Gardasil 9, Merck) involving more than 15,000 vaccine recipients, most adverse events were mild or moderate injection site-related pain, swelling, and redness. Up to 40% reported one of these injection site reactions after vaccination, and they were more common among females compared to males. Injection site reactions also were more likely following the second or third dose compared to the first dose. Fewer than 10% of recipients reported fever.
Yes. Tdap can be administered with all other vaccines that are indicated (e.g., meningococcal conjugate vaccine, hepatitis B vaccine, MMR). Each vaccine should be administered at a different anatomic site using a separate syringe.
Varicella vaccine is very safe. About 20% of vaccine recipients will have minor injection site complaints, such as pain, swelling, or redness. Fewer than 5% of recipients develop a localized or generalized varicella-like rash 5 to 26 days after vaccination. These rashes have an average of 2 to 5 lesions, and may be maculopapular rather than vesicular. Fever following varicella vaccine is uncommon.
Yes, as a healthcare professional, this person should get a second dose of MMR to ensure she is immune to rubella. There is no harm in providing MMR to a person who is already immune to one or more of the components. If she developed measles only one day after getting her first MMR, she must have been exposed to the disease prior to vaccination.
Yes. People with multiple sclerosis should be vaccinated against influenza. Multiple sclerosis is not a contraindication to any vaccine, including influenza vaccines.
