Despite the high efficacy of the BNT162b2 messenger RNA vaccine against severe
acute respiratory syndrome coronavirus 2 (SARS-CoV-2), rare breakthrough infections
have been reported, including infections among health care workers. Data
are needed to characterize these infections and define correlates of breakthrough
and infectivity.

At the largest medical center in Israel, we identified breakthrough infections by
performing extensive evaluations of health care workers who were symptomatic
(including mild symptoms) or had known infection exposure. These evaluations
included epidemiologic investigations, repeat reverse-transcriptase–polymerasechain-
reaction (RT-PCR) assays, antigen-detecting rapid diagnostic testing (Ag-
RDT), serologic assays, and genomic sequencing. Correlates of breakthrough infection
were assessed in a case–control analysis. We matched patients with
breakthrough infection who had antibody titers obtained within a week before
SARS-CoV-2 detection (peri-infection period) with four to five uninfected controls
and used generalized estimating equations to predict the geometric mean titers
among cases and controls and the ratio between the titers in the two groups. We
also assessed the correlation between neutralizing antibody titers and N gene cycle
threshold (Ct) values with respect to infectivity.