BNT162b2 Vaccine Effectiveness in Preventing Asymptomatic Infection with SARS-CoV-2 Virus: A Nationwide Historical Cohort Study

Background: There is strong evidence regarding the efficacy and effectiveness of BNT162b2 vaccine in preventing symptomatic infection with SARS-CoV-2 virus. There is a paucity of data regarding effectiveness in prevention of asymptomatic infection. Methods: In this real-world study, we identified a sub-population of patients in a large health maintenance organisation who were repeatedly tested for SARS-CoV-2 infection using a PCR test. We used these patients as the cohort for the study, and compared individuals who were vaccinated with BNT162b2 mRNA vaccine to the unvaccinated ones. A positive SARS-CoV-2 PCR test result was used as the outcome. Follow-up period was from Jan 1,2021 until Feb 11, 2021. Findings: 6,286 individuals were included in the cohort. Seven days following the second vaccine dose, a rate of six positive PCR tests per 10,000 patient-days was recorded, compared with a rate of 53 positive tests per 10,000 patient-days for the unvaccinated group. The estimated vaccine effectiveness against infection with SARS-CoV-2 virus after two vaccine doses was 89% (confidence interval 82%-94%). The estimated effectiveness two weeks following the first vaccine dose was 61% (confidence interval 49%-71%). Interpretation: In this study, vaccination with BNT162b2 reduced infection rates among individuals who underwent screening by frequent SARS-CoV-2 PCR testing. Using a cohort of frequently tested individuals reduced the indication bias for the PCR testing, which enabled estimation of asymptomatic infection rates. Funding: This study received no funding. Declaration of Interests: DS reports receiving personal fees from Pfizer, outside the submitted work (advisory board on Trumenba); and Consultation fees from GSK and Gilead. The other authors declare no competing interests. Ethics Approval Statement: This study was approved by MHMO’s institutional review board number 03-17-02- 21, and investigators were exempt from requesting informed consent.