On Feb. 25, 2021, Pfizer and BioNTech announced they had begun an evaluation of the safety and immunogenicity of a third dose of the Pfizer-BioNTech COVID-19 vaccine (BNT162b2) to understand the effect of a booster on immunity against COVID-19 caused by the circulating and potential newly emerging SARS-CoV-2 variants.

The study drew upon participants from the Phase 1 study in the U.S. who were offered the opportunity to receive a 30 ﾵg booster of the current vaccine 6 to 12 months after receiving their initial two-dose regimen.