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Decreased infectivity following BNT162b2 vaccination: A prospective cohort study in Israel

Our take —

A cohort study among healthcare workers at a hospital in Israel was conducted to understand the impact of the BNT162b2 vaccine (Pfizer) on susceptibility to infection, regardless of symptoms, and infectiousness through measurement of viral shedding. It was found that those who were vaccinated were less likely to become infected and, among those who did, experienced a significant reduction in viral shedding compared with their unvaccinated counterparts. This study suggests that the vaccine is effective in this population, but that: 1) breakthrough infections do happen; and 2) infectiousness among those with breakthrough infections is reduced. These data were collected before the Delta variant was in high circulation, and implications may differ in contexts of high Delta circulation or with subsequent variants.

Study design

Prospective Cohort

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.

Study Strengths

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.

Limitations

The study sample was made up of primarily younger women, and the results presented here may not be generalizable to other populations.

Value added

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