Study population and setting
A cohort study of healthcare workers at Sheba Medical Center in Israel was conducted between December 19, 2020 and March 14, 2021 to understand the effectiveness of vaccination (BNT162b2 – Pfizer-BioNTech) in reducing infectiousness among SARS-CoV-2 cases. The start of the study corresponded to the start of vaccine roll-out, and during the study period 7794/9347 (83%) of eligible HCW received at least one dose, and 7324/9347 (78%) received two doses. Those who had previously been infected with SARS-CoV-2 were not eligible for vaccination. The authors estimated prevalence of SARS-CoV-2 in exposed individuals using RT-PCR (susceptibility to infection) and viral shedding using (cycle threshold) Ct values. HCWs experiencing an exposure (at work, home, etc.) was required to undergo PCR testing.
Summary of Main Findings
Fully vaccinated healthcare workers experienced a reduced prevalence of SARS-CoV-2 infection compared with those who were unvaccinated (1.8% vs. 5.2%). Mean Ct values for unvaccinated versus vaccinated healthcare workers were 22.2 +/- 1.0 and 27.3 +/- 2.2, suggesting a significant mean difference (5.09, 95% CI: 2.8-7.4) or a significant reduction in viral shedding among those vaccinated.
This study estimates prevalence of infection among those who were exposed, regardless of symptoms, providing an estimate of the vaccine’s impact on susceptibility to infection.
The study sample was made up of primarily younger women, and the results presented here may not be generalizable to other populations.
This study shows that in a real-world setting the vaccine was effective at preventing infections following exposure, and that Ct values were significantly higher (i.e., which is an imperfect proxy measure for viral load being lower) in people who were vaccinated compared with those who were unvaccinated.
This review was posted on: 9 August 2021