Breakthrough COVID-19 infection rate
Breakthrough infection among fully vaccinated individuals was defined according to the US Centers for Disease Control and Prevention, as infection occurring ≥14 days after the second dose in a two-dose series or ≥14 days after a single-dose vaccine. Participants were queried for evidence of breakthrough COVID-19 infection following each vaccine dose and electronic medical records were reviewed for the evidence of infection 6 months after each vaccination, based on the results of positive SARS-CoV-2 PCR tests. Follow-up was 1 year after the first vaccine dose.