Ask the Experts: Vaccine Safety

Note: Information about the safety for specific vaccines are included in the Ask The Experts set for that vaccine.

Results (16)

Vaccine recommendations are determined after extensive studies in large clinical trials. They include studies on how vaccine recipients respond to multiple vaccines given simultaneously. The overall aim is to provide early protection for infants and children against vaccine-preventable diseases that could endanger their health and life.

No scientific evidence exists to support that delaying vaccinations or separating them into individual antigens is beneficial for children. Rather, this practice prolongs susceptibility to disease, which could result in a greater likelihood of the child becoming sick with a serious or life-threatening disease. There could also be added expense (e.g., multiple office visits), additional time off from work for parents, and increased likelihood that the child will fail to get all necessary vaccinations.

Last reviewed: August 31, 2022


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Last reviewed: March 31, 2023

Paul Offit, MD, and Charlotte Moser, BS, of the Vaccine Education Center (VEC) at the Children’s Hospital of Philadelphia, published an article, “The Problem with Dr. Bob’s Alternative Vaccine Schedule,” in the January 2009 issue of Pediatrics. It includes a section about aluminum. You can read it in its entirety at pediatrics.aappublications.org/cgi/content/full/123/1/e164.

Here is the link to the VEC information page about aluminum in vaccines, including materials for families in English and Spanish:  www.chop.edu/vaccine-education-center/vaccine-safety/vaccine-ingredients/aluminum.

Last reviewed: August 28, 2024

Thimerosal, a very effective preservative, has been used to prevent bacterial contamination in vaccine vials since the 1930s. It contains a type of mercury known as ethylmercury, which is different from the type of mercury found in fish and seafood (methylmercury). At very high levels, methylmercury can be toxic to people, especially to the neurological development of infants.

In recent years, several large scientific studies have determined that thimerosal in vaccines does not lead to neurologic problems, such as autism. Nonetheless, because we generally try to reduce people’s exposure to mercury when possible, vaccine manufacturers have voluntarily changed their production methods to produce vaccines that are now free of thimerosal or have only trace amounts. They have done this because it is possible to do, not because there was any evidence that the thimerosal was harmful.

Comprehensive information about thimerosal in vaccines is available from the U.S. Food and Drug Administration at www.fda.gov/BiologicsBloodVaccines/SafetyAvailability/VaccineSafety/ucm096228.htm. The Vaccine Education Center at Children’s Hospital of Philadelphia also has extensive information about thimerosal and other vaccine ingredients at www.chop.edu/centers-programs/vaccine-education-center.

Last reviewed: August 31, 2022

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

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

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

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

Last reviewed: August 31, 2022

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

Last reviewed: August 31, 2022

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

Last reviewed: August 31, 2022

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

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

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

No. In general, although it is not ideal, receiving extra doses of vaccine poses no medical problem. Receiving excessive doses of tetanus toxoid (e.g., DTaP, DT, Tdap, or Td) can increase the risk of a local adverse reaction, however. For details, see Extra Doses of Vaccine Antigens in the ACIP’s “Best Practice Guidelines for Immunization” at www.cdc.gov/vaccines/hcp/acip-recs/general-recs/timing.html.

Vaccination providers frequently encounter people who do not have adequate documentation of vaccinations. Providers should only accept written, dated records as evidence of vaccination. With the exception of influenza vaccine and pneumococcal polysaccharide vaccine, self-reported doses of vaccine without written documentation should not be accepted. An attempt to locate missing records should be made whenever possible by contacting previous healthcare providers, reviewing state or local immunization information systems, and searching for a personally held record.

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 receive age-appropriate vaccination. Serologic testing for immunity is an alternative to vaccination for certain antigens (e.g., measles, rubella, hepatitis A, diphtheria, and tetanus).

Last reviewed: August 31, 2022

As the name implies, VAERS is a nationwide system for monitoring adverse events following vaccination. VAERS is operated jointly by the FDA and the CDC since 1990.

Last reviewed: August 31, 2022

Reports can be submitted online through the VAERS website or by completing and uploading a writable PDF VAERS report form. Both options can be accessed on the VAERS website at vaers.hhs.gov/reportevent.html. If you need further assistance with reporting to VAERS, please email info@VAERS.org or call 1-800-822-7967.

Last reviewed: August 31, 2022

Yes. Any occurrence of medical significance warrants a VAERS report. VAERS looks for trends, so such information is helpful. You can obtain more information about VAERS at vaers.hhs.gov or by calling (800) 822-7967.

Last reviewed: August 31, 2022

All significant health events that may have been related to a dose of vaccine, particularly those that lead to hospitalization, disability, or death, should be reported to VAERS. The healthcare provider doesn’t need to be certain the event was vaccine related in order to report it. It is not necessary to report minor adverse reactions, such as local reactions or low-grade fever.

Last reviewed: August 31, 2022

The National Vaccine Injury Compensation Program (NVICP) includes payment only for injuries determined to have occurred following vaccination with a vaccine routinely recommended for children or pregnant women in the United States. The recipient can be of any age, but the vaccine must be routinely recommended for children through age 18 years or pregnant women. The vaccine must be subject to an excise tax to be included in the program. Shingrix and PPSV23 vaccines are not routinely recommended for children or pregnant women; Shingrix is only recommended for adults. COVID-19 vaccines will transition from injury compensation coverage under the Countermeasure Injury Compensation Program to NVICP once criteria are met for inclusion in NVICP. More information about the program and the covered vaccines is at www.hrsa.gov/vaccine-compensation/covered-vaccines/index.html.

Last reviewed: August 31, 2022

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