Effectiveness of a third dose of BNT162b2 or mRNA-1273 vaccine for preventing post-vaccination COVID-19 infection: an observational study
Background BNT162b2 and mRNA-1273 vaccines are highly protective against COVID-19. Concern about waning immunity and reduced effectiveness against SARS-COV-2 variants led to use of a third dose six months after completing the primary series. We used data from the Veterans Health Administration to evaluate the effectiveness of a third dose of BNT162b2 or mRNA-1273 compared to the primary series in preventing post-vaccination COVID-19. Methods During January 1 - November 22, 2020, third dose recipients were matched (1:1) to demographically similar controls who did not receive a third dose. Eligible participants had completed the primary series at least six months (180 days) before recruitment date. Long-term care residents were excluded. Primary outcomes were documented SARS-CoV-2 infection and COVID-19 hospitalization. Effectiveness was estimated as 1-incidence rate ratio. Findings Following matching, the BNT162b2 group included 99,856 pairs and the mRNA-1273 group included 74,116 pairs. In BNT162b2 and mRNA-1273 groups, median age was 72 (interquartile range [IQR]: 65-75) and 72 (IQR: 67-76) years, 94,990 (95.1%) and 71,213 (96.1%) were male, and 61,261 (61.3%) and 52,170 (70.4%) were non-Hispanic White, respectively. Effectiveness of a third dose of BNT162b2 or mRNA-1273 compared to the primary series was 49.4% (95% confidence interval [CI]: 41.2-56.5%) and 46.0% (95% CI: 33.5-56.2%) for documented SARS-CoV-2 infection and 52.3% (95% CI: 33.8-65.6%) and 44.7% (95% CI: 10.7-65.7%) for COVID-19 hospitalization, respectively. Interpretation A third dose of BNT162b2 or mRNA-1273 is moderately effective against post-vaccination COVID-19 infection compared to the primary series.