BNT162b2 vaccine effectiveness in preventing asymptomatic infection with SARS-CoV-2 virus: a nationwide historical cohort study

Galia Zacay, David Shasha, Ronen Bareket, Itai Kadim, Fabienne Hershkowitz-Sikron, Judith Tsamir, David Mossinson, Anthony Heymann,

Keywords: Covid-19, SARS-CoV-2, vaccine effectiveness, asymptomatic infection, observational study

There is strong evidence regarding the efficacy and effectiveness of BNT162b2 vaccine in preventing symptomatic infection with SARS-CoV-2 virus.
There is a relative paucity of data regarding effectiveness in prevention of asymptomatic infection.

Research questions:
What is the real-world effectiveness of BNT162b2 vaccine in preventing symptomatic and asymptomatic infection with SARS-CoV-2 virus?

In this real-world observational study, we identified a sub-population of individuals in a large health maintenance organization who were repeatedly tested for SARS-CoV-2 infection by PCR. We included these individuals in the study cohort, and compared those 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 January 1,2021 until February 11, 2021.

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 person-days was recorded, compared with a rate of 53 positive tests per 10,000 person-days for the unvaccinated group. The estimated vaccine effectiveness against infection with SARS-CoV-2 virus after two vaccine doses was 89% (95% confidence interval 82%-94%). The estimated effectiveness two weeks following the first vaccine dose was 61% (95% confidence interval 49%-71%).

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 infection rates.

Points for discussion: