IMPORTANCE The BNT162b2 two-dose vaccine (BioNTech/Pfizer) has high effectiveness that
wanes within several months. The third dose is effective in mounting a significant immune response,
but its durability is unknown.

OBJECTIVE To compare antibody waning after second and third doses and estimate the association
of antibody kinetics with susceptibility to infection with the Omicron variant of SARS-CoV-2.

DESIGN, SETTING, AND PARTICIPANTS In a prospective longitudinal cohort study in a tertiary
medical center in Israel, health care workers who received the BNT162b2 vaccine were followed up
monthly for IgG and neutralizing antibody levels. Linear mixed models were used to compare
antibody titer waning of second and third doses and to assess whether antibody dynamics were
associated with Omicron transmission. Avidity, T cell activation, and microneutralization of sera
against different variants of concern were assessed for a subgroup.