Background: COVID-19 vaccine trials and post-implementation data suggest vaccination decreases SARS-CoV-2 infections.
Objective: Estimate COVID-19 vaccination's impact on SARS-CoV-2 case rates and viral diversity among healthcare workers (HCW) during a high community prevalence period.
Design setting participants: A prospective cohort study from Boston Medical Center (BMC)'s HCW vaccination program, where staff received two doses of BNT162b2 or mRNA-1273.
Measurements: PCR-confirmed SARS-CoV-2 cases among HCWs from December 09, 2020 to February 23, 2021. Weekly SARS-CoV-2 rates per 100,000 person-day overall and by time from first injection (1-14 and >14 days) were compared with surrounding community rates. Viral genome sequences from SARS CoV-2 positive samples.
Results: SARS-CoV-2 cases occurred in 1.4% (96/7109) of HCWs given at least a first dose and 0.3% (17/5913) of HCWs given both vaccine doses. Adjusted SARS-CoV-2 infection rate ratios were 0.73 (95% CI 0.53-1.00) 1-14 days and 0.18 (0.10-0.32) >14 days from first dose. HCW SARS-CoV-2 cases >14 days from initial dose compared to within 14 days were more often older (46 versus 38 years, p=0.007), Latinx (10% versus 8%, p=0.03), and asymptomatic (48% versus 11%, p=0.0002). SARS-CoV-2 rates among HCWs fell below those of the surrounding community, with a 18% versus 11% weekly decrease respectively (p=0.14). Comparison of 48 SARS-CoV-2 genomes sequenced from post-first dose cases did not indicate selection pressure towards known spike-antibody escape mutations.
Limitations: Unable to adjust for infection risk outside of the workplace. Lack follow up on symptoms post SARS-CoV-2 diagnosis. Small number of vaccinated HCW cases.
Conclusion: Our results indicate a positive impact of COVID-19 vaccines on SARS-CoV-2 case rates. Post-vaccination isolates did not show unusual genetic diversity or selection for mutations of concern.