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

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 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