Reactogenicity within 2 weeks after mRNA COVID-19 vaccines: Findings from the CDC v-safe surveillance system

Vaccine. 2021 Nov 26;39(48):7066-7073. doi: 10.1016/j.vaccine.2021.10.019. Epub 2021 Oct 16.


Background: Post-authorization monitoring of mRNA-based COVID-19 vaccines is needed to better characterize their reactogenicity. We assessed reactions reported during the 2 weeks after receipt of BNT162b2 (Pfizer-BioNTech) and mRNA-1273 (Moderna) vaccines.

Methods: We monitored persons who enrolled in v-safe after vaccination health checkerSM, a U.S. smartphone-based vaccine monitoring system, after receiving BNT162b2 or mRNA-1273. V-safe participants received text message prompts to complete web-based surveys. We analyzed responses from persons who received BNT162b2 or mRNA-1273 from December 14, 2020 through March 14, 2021 and completed at least one survey by March 28, 2021. We measured the proportion of participants reporting local and systemic reactions solicited in surveys completed days 0 through 7 post-vaccination. For day 14 surveys, participants described new or worsening symptoms in a free-text response. We assessed the proportion of participants reporting new or worsening local and systemic reactions.

Results: One-third of participants were aged <45 years, two-thirds were female, and approximately half received BNT162b2 vaccine. A total of 4,717,908 participants reported during the 7 days after dose 1 and 2,906,377 reported during the 7 days after dose 2. Most reported at least one injection-site reaction (68.5% after dose 1; 72.9% after dose 2) or at least one systemic reaction (50.6% after dose 1; 69.5% after dose 2). Reactogenicity was greater after dose 2 and among mRNA-1273 recipients, persons aged <45 years, and females. New or worsening local and systemic reactions were uncommon during week 2 after either dose; the most frequent were local reactions for dose 1 mRNA-1273 recipients (2.6%). These reactions were reported more often among females after dose 1 mRNA-1273 (3.6%).

Conclusions: During post-authorization monitoring among >4 million vaccinees, local and systemic reactions were commonly reported following mRNA-based vaccines. Reactions were most common during the first week following dose 2 and among persons aged <45 years, females, and mRNA-1273 recipients.

Keywords: COVID-19; COVID-19 vaccines; Reactogenicity; mRNA COVID-19 vaccines.

MeSH terms

  • BNT162 Vaccine
  • COVID-19 Vaccines*
  • COVID-19*
  • Centers for Disease Control and Prevention, U.S.
  • Female
  • Humans
  • RNA, Messenger
  • SARS-CoV-2
  • United States


  • COVID-19 Vaccines
  • RNA, Messenger
  • BNT162 Vaccine