Safety of the COVID-19 Vaccine

Moderna and Pfizer

All data currently available indicate that the Moderna and Pfizer vaccines are safe. Thus far, no serious long-term side effects have occurred, and any side effects that do occur are not severe and occur within six weeks of vaccination. The FDA will continue to monitor for any rare safety issues.

While they may be new, the authorized COVID-19 vaccines have been tested in large clinical trials and the technology used (messenger RNA) has been in development for over a decade. Based upon data from these trials, the CDC and FDA, including an independent group of experts from the CDC’s Advisory Committee of Immunization Practices (ACIP), have found these vaccines to be safe and effective. While no medication or vaccine is without side effects, the COVID-19 vaccines are similar to other approved vaccines and side effects tend to be mild or moderate and last a couple of days. Millions of people in the United States have already received COVID-19 vaccines under the most intense safety monitoring in U.S. history.

Despite the large size of clinical trials, some rare or long-term side effects may not be seen. Fortunately, monitoring does not stop when the trials end. The United States has systems, such as VAERS, v-safe and MedWatch, to continuous monitor medications and vaccines for safety issues. If an issue is detected, experts take immediate action to determine if it is related to the vaccine and how best to proceed. Learn more about COVID-19 vaccine safety monitoring.

Johnson & Johnson Update

The Centers for Disease Control (CDC) and Food and Drug Administration (FDA) have paused the administration of the single-dose Johnson & Johnson (J&J) COVID-19 vaccine as they investigate six reported cases of an uncommon and severe type of blood clot. With more than 6.8 million doses of the J&J vaccine having been administered in the U.S., these adverse events are extremely rare and there are currently 6 reported cases. Pausing the use of the Johnson & Johnson vaccine while these reports are being assessed attests to the critical standards in place in the US to ensure vaccine safety. The Advisory Committee on Immunization Practices (ACIP) will convene Wednesday, April 14, to review the cases and provide further guidance.

The Yale COVID-19 Vaccine Program has not administered the J&J vaccine and Connecticut has also paused J&J vaccination. Until we know more, we are encouraging patients to to check with their vaccination site regarding rebooking if they are scheduled to receive J&J at another vaccination site. 

Should you have more questions, please contact a healthcare provider for more information.

Updated April 13, 2021.

Frequently Asked Questions

Should I be concerned about a severe reaction to the COVID-19 vaccine?

Severe adverse reactions were uncommon during COVID-19 vaccine trials. They occurred more frequently after the second dose. All recipients will be monitored for at least 15 minutes after vaccination for possible immediate hypersensitive reactions. Read more about possible side effects of the COVID-19 vaccine.