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  • 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 COVID19, 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.


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