ABSTRACT
Two-dose messenger RNA vaccines (BNT162b2/Pfizer and mRNA-1273/Moderna) against SARS-CoV-2 were rolled out in the US in December 2020, and provide protection against hospitalization and death from COVID-19 for at least six months. Breakthrough infections have increased with waning immunity and the spread of the B.1.617.2 (Delta) variant in summer 2021, prompting approval of boosters for all adults over 18. We measured anti-receptor binding domain (RBD) IgG and surrogate virus neutralization of the interaction between SARS-CoV-2 spike protein and the human angiotensin-converting enzyme (ACE2) receptor, before and after boosters in N=33 healthy adults. We document large antibody responses 6-10 days after booster, with antibody levels that exceed levels documented after natural infection with COVID-19, after two doses of vaccine, or after both natural infection and vaccination. Surrogate neutralization of B.1.617.2 is high but reduced in comparison with wild-type SARS-CoV-2. These data support the use of boosters to prevent breakthrough infections and suggest that antibody-mediated immunity may last longer than after the second vaccine dose.
