BNT162b2 mRNA Vaccine Effectiveness Given Confirmed Exposure; Analysis of Household Members of COVID-19 Patients (preprint)

ABSTRACT

Importance While the mRNA BNT162b2 vaccine effectiveness was demonstrated in general population, the question of effectiveness given confirmed exposure has yet been answered, though it has policy implications, as the need for self-quarantine when exposed and protective measures for vaccinated in high-risk areas. Objective Assessing the BNT162b2 vaccine effectiveness in preventing SARS-CoV-2 infection given high-risk exposure, through analysis of household members of confirmed cases. Design Retrospective cohort study. Data of household members of confirmed SARS-CoV-2 cases between 20/12/2020 and 17/03/2021 were collected. Setting Nationally centralized database of Maccabi Healthcare Services (MHS), the second largest Healthcare Maintenance Organization in Israel. Participants 2.5 million MHS members were considered, of which we included only households with two adult members, given possible lower transmission and susceptibility among children. Households with no prior confirmed infections and a confirmed index case during the study period were included. Exposure Participants were classified into three vaccination groups in time of the index case (the confirmed exposure)- Unvaccinated; Fully Vaccinated(7 or more days post second dose) and a reference control group of Recently Vaccinated Once(0-7 days from the first dose, presumably still unprotected). Main Outcomes and Measures Assessing the probability of an additional SARS-CoV-2 infection in the household occurring within 10 days of an index case, calculated separately for the three vaccination groups. Main outcome was vaccine effectiveness given confirmed exposure. High testing rates among household members enabled us to estimate with a high degree of confidence effectiveness against asymptomatic SARS-CoV-2 infection as well. Results A total of 173,569 households were included, out of which 6,351 households had an index infection (mean [SD] age, 58.9 [13.5] years; 50% were women). Vaccine effectiveness of Fully Vaccinated compared to Unvaccinated participants was 80.0% [95% CI, 73.0-85.1] and 82.0% [95% CI, 75.5-86.7] compared to those Recently Vaccinated Once. Conclusion and Relevance The BNT162b2 vaccine is effective in a high-risk, real life, exposure scenario, but the protection rates afforded in these settings are lower than those previously described. Household members of COVID-19 patients and any individual with a confirmed exposure to COVID-19 are still at a considerable risk of being infected even if fully vaccinated.