Frequently Asked Questions About Vaccine Safety

Vaccines are held to the highest standard of safety. The United States currently has the safest, most effective vaccine supply in history. Years of testing are required by law before a vaccine can be licensed. Once in use, vaccines are continually monitored for safety and effectiveness. However, like any medication, vaccines can cause side effects.

Are Vaccines Tested and Monitored for Safety?

Yes. Before vaccines are licensed, the FDA requires testing to ensure safety. This process can take 10 years or longer. Once a vaccine is in use, the CDC and FDA monitor its adverse events (health problems after vaccination) through the Vaccine Adverse Event Reporting System (VAERS). Any hint of a problem with a vaccine prompts further investigations by the CDC and FDA. If researchers find a vaccine may be causing a side effect, the CDC and FDA will initiate appropriate action that may include the changing of vaccine labels or packaging, distributing safety alerts, inspecting manufacturers' facilities and records, withdrawing recommendations for the use of the vaccine, or revoking the vaccine's license.

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Who Should Not Be Vaccinated?

Some people should not get certain vaccines or should wait to get them. For instance, children with compromised immune systems, as occurs with cancer patients, often need to wait to be vaccinated. Similarly, if a person has had a severe allergic reaction to a vaccine, a following dose is not recommended. However, a person with a mild, common illness, such as a cold with a low-grade fever, does not have to wait to be vaccinated. For more information, visit Who Should Not be Vaccinated with These Vaccines?

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What Should I Do if Someone Has a Reaction to a Vaccine?

• Call a doctor. If the reaction is severe, take the person to a doctor immediately.
• Tell your doctor what happened, when it happened, and when the vaccination was given.
• Ask your doctor, nurse, or health department to file a VAERS form, or call VAERS yourself at 1-800-822-7967.

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What are Possible Side Effects of Vaccination?

Each person is unique and may react differently to vaccination.

• Occasionally, people who receive a vaccine do not respond to it and may still get the illness the vaccine was meant to protect them against.
• In most cases, vaccines are effective and cause no side effects, or only mild reactions such as fever or soreness at the injection site.
• Very rarely, people experience more serious side effects, like allergic reactions. Be sure to tell your health care provider if you have health problems or known allergies to medications or food.
• Severe reactions to vaccines occur so rarely that the risk is difficult to calculate.

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For more information, visit Possible Side Effects from Vaccines.