Safety Monitoring of an Additional Dose of COVID-19 Vaccine - United States, August 12-September 19, 2021

MMWR Morb Mortal Wkly Rep. 2021 Oct 1;70(39):1379-1384. doi: 10.15585/mmwr.mm7039e4.


On August 12, 2021, the Food and Drug Administration (FDA) amended Emergency Use Authorizations (EUAs) for the Pfizer-BioNTech and Moderna COVID-19 vaccines to authorize administration of an additional dose after completion of a primary vaccination series to eligible persons with moderate to severe immunocompromising conditions (1,2). On September 22, 2021, FDA authorized an additional dose of Pfizer-BioNTech vaccine ≥6 months after completion of the primary series among persons aged ≥65 years, at high risk for severe COVID-19, or whose occupational or institutional exposure puts them at high risk for COVID-19 (1). Results from a phase 3 clinical trial conducted by Pfizer-BioNTech that included 306 persons aged 18-55 years showed that adverse reactions after receipt of a third dose administered 5-8 months after completion of a 2-dose primary mRNA vaccination series were similar to those reported after receipt of dose 2; these adverse reactions included mild to moderate injection site and systemic reactions (3). CDC developed v-safe, a voluntary, smartphone-based safety surveillance system, to provide information on adverse reactions after COVID-19 vaccination. Coincident with authorization of an additional dose for persons with immunocompromising conditions, the v-safe platform was updated to allow registrants to enter information about additional doses of COVID-19 vaccine received. During August 12-September 19, 2021, a total of 22,191 v-safe registrants reported receipt of an additional dose of COVID-19 vaccine. Most (97.6%) reported a primary 2-dose mRNA vaccination series followed by a third dose of the same vaccine. Among those who completed a health check-in survey for all 3 doses (12,591; 58.1%), 79.4% and 74.1% reported local or systemic reactions, respectively, after dose 3, compared with 77.6% and 76.5% who reported local or systemic reactions, respectively, after dose 2. These initial findings indicate no unexpected patterns of adverse reactions after an additional dose of COVID-19 vaccine; most of these adverse reactions were mild or moderate. CDC will continue to monitor vaccine safety, including the safety of additional doses of COVID-19 vaccine, and provide data to guide vaccine recommendations and protect public health.

MeSH terms

  • Adolescent
  • Adult
  • Adverse Drug Reaction Reporting Systems
  • Aged
  • Aged, 80 and over
  • COVID-19 / epidemiology
  • COVID-19 / prevention & control
  • COVID-19 Vaccines / administration & dosage*
  • COVID-19 Vaccines / adverse effects*
  • Centers for Disease Control and Prevention, U.S.
  • Child
  • Child, Preschool
  • Drug-Related Side Effects and Adverse Reactions / epidemiology*
  • Female
  • Humans
  • Infant
  • Infant, Newborn
  • Male
  • Middle Aged
  • Product Surveillance, Postmarketing*
  • United States / epidemiology
  • Vaccines, Synthetic / administration & dosage
  • Vaccines, Synthetic / adverse effects
  • Young Adult
  • mRNA Vaccines


  • COVID-19 Vaccines
  • Vaccines, Synthetic