RESUMO
BackgroundMethodologically rigorous studies on Covid-19 vaccine effectiveness (VE) in preventing SARS-CoV-2 infection are critically needed to inform national and global policy on Covid-19 vaccine use. In Israel, healthcare personnel (HCP) were initially prioritized for Covid-19 vaccination, creating an ideal setting to evaluate real-world VE in a closely monitored population. MethodsWe conducted a prospective study among HCP in 6 hospitals to estimate the effectiveness of the BNT162b2 mRNA Covid-19 vaccine in preventing SARS-CoV-2 infection. Participants filled out weekly symptom questionnaires, provided weekly nasal specimens, and three serology samples - at enrollment, 30 days and 90 days. We estimated VE against PCR-confirmed SARS-CoV-2 infection using the Cox Proportional Hazards model and against a combined PCR/serology endpoint using Fishers exact test. FindingsOf the 1,567 HCP enrolled between December 27, 2020 and February 15, 2021, 1,250 previously uninfected participants were included in the primary analysis; 998 (79.8%) were vaccinated with their first dose prior to or at enrollment, all with Pfizer BNT162b2 mRNA vaccine. There were four PCR-positive events among vaccinated participants, and nine among unvaccinated participants. Adjusted two-dose VE against any PCR- confirmed infection was 94.5% (95% CI: 82.6%-98.2%); adjusted two-dose VE against a combined endpoint of PCR and seroconversion for a 60-day follow-up period was 94.5% (95% CI: 63.0%-99.0%). Five PCR-positive samples from study participants were sequenced; all were alpha variant. InterpretationOur prospective VE study of HCP in Israel with rigorous weekly surveillance found very high VE for two doses of Pfizer BNT162b2 mRNA vaccine against SARS-CoV-2 during a period of predominant alpha variant circulation. FundingClalit Health Services
RESUMO
The SARS-CoV-2 pandemic has been raging for over a year, creating global detrimental impact. The BNT162b2 mRNA vaccine has demonstrated high protection levels, yet apprehension exists that several variants of concerns (VOCs) can surmount the immune defenses generated by the vaccines. Neutralization assays have revealed some reduction in neutralization of VOCs B.1.1.7 and B.1.351, but the relevance of these assays in real life remains unclear. We performed a case-control study that examined the distribution of SARS-CoV-2 variants observed in infections of vaccinated individuals ("breakthrough cases") and matched infections of unvaccinated individuals. We hypothesized that if there is lower vaccine effectiveness against one of the VOCs, its proportion among the breakthrough cases should be higher than among unvaccinated cases. Our results show that vaccinees that tested positive at least a week after the second dose were indeed disproportionally infected with B.1.351, as compared with unvaccinated individuals (odds ratio of 8:1). Those who tested positive between two weeks after the first dose and one week after the second dose, were disproportionally infected by B.1.1.7 (odds ratio of 26:10), suggesting reduced vaccine effectiveness against both VOCs at particular time windows following vaccination. Nevertheless, the B.1.351 incidence in Israel to-date remains low and vaccine effectiveness remains high among those fully vaccinated. These results overall suggest that vaccine breakthrough infection may be more frequent with both VOCs, yet a combination of mass-vaccination with two doses coupled with non-pharmaceutical interventions control and contain their spread.
