We designed this observational study to emulate a target trial of the causal effect of the BNT162b2 vaccine on Covid-19 outcomes.4 (link) Eligibility criteria included an age of 16 years or older, not having a previously documented positive SARS-CoV-2 polymerase-chain-reaction (PCR) test, and being a member of the health care organization during the previous 12 months.
Population groups in which internal variability in the probability of exposure or the outcomes is high and controlling for the high variability is not feasible (e.g., high variability in infection risk among patient-facing health care workers in dedicated Covid-19 wards as compared with administrative staff) were excluded. Such population groups are persons not having a documented geostatistical living area, those who have had interactions with the health care system during the preceding 3 days that may indicate the start of symptomatic disease and may preclude vaccination, nursing home residents, persons medically confined to the home, or health care workers.
Each day during the period from December 20, 2020, to February 1, 2021, all newly vaccinated persons were matched in a 1:1 ratio to unvaccinated controls. For each person, follow-up ended at the earliest of the following events: occurrence of an outcome event, death unrelated to Covid-19, vaccination (for unvaccinated controls), vaccination of the matched control (for vaccinated persons), or the end of the study period. Newly vaccinated persons were eligible for inclusion in the study, even if they had previously been selected as a control.
We matched vaccine recipients and controls on variables associated with the probability of both vaccination and infection or severity of Covid-19: age, sex, sector (general Jewish, Arab, or ultra-Orthodox Jewish), neighborhood of residence (since disease activity and vaccination uptake vary greatly across defined geostatistical areas), history of influenza vaccination during the preceding 5 years (0, 1 or 2, 3 or 4, or ≥5 vaccinations), pregnancy (a potential risk factor for severe Covid-195 (link) and associated with the rate of vaccination owing to evolving vaccination guidelines for pregnant women), and the total number of coexisting conditions that had been identified by the Centers for Disease Control and Prevention (CDC) as risk factors for severe Covid-19 as of December 20, 2020.6 ,7 (See Supplementary Methods 3 for additional information about the matching process. Theprotocol and statistical analysis plan are available at NEJM.org.)
The five outcomes of interest were documented SARS-CoV-2 infection confirmed by positive PCR test, documented symptomatic Covid-19, hospital admission for Covid-19, severe Covid-19 (according to National Institutes of Health criteria)8 and death from Covid-19. Each of these outcomes includes the outcomes that follow it. In a supplementary analysis, we also evaluated an additional outcome, SARS-CoV-2 infection without documented symptoms, as an imperfect proxy for asymptomatic infection (since mild symptoms may not be documented).
Table S1 provides details on definitions of variables. Persons with missing data for smoking status or body-mass index (BMI) were dropped from the analysis.
Population groups in which internal variability in the probability of exposure or the outcomes is high and controlling for the high variability is not feasible (e.g., high variability in infection risk among patient-facing health care workers in dedicated Covid-19 wards as compared with administrative staff) were excluded. Such population groups are persons not having a documented geostatistical living area, those who have had interactions with the health care system during the preceding 3 days that may indicate the start of symptomatic disease and may preclude vaccination, nursing home residents, persons medically confined to the home, or health care workers.
Each day during the period from December 20, 2020, to February 1, 2021, all newly vaccinated persons were matched in a 1:1 ratio to unvaccinated controls. For each person, follow-up ended at the earliest of the following events: occurrence of an outcome event, death unrelated to Covid-19, vaccination (for unvaccinated controls), vaccination of the matched control (for vaccinated persons), or the end of the study period. Newly vaccinated persons were eligible for inclusion in the study, even if they had previously been selected as a control.
We matched vaccine recipients and controls on variables associated with the probability of both vaccination and infection or severity of Covid-19: age, sex, sector (general Jewish, Arab, or ultra-Orthodox Jewish), neighborhood of residence (since disease activity and vaccination uptake vary greatly across defined geostatistical areas), history of influenza vaccination during the preceding 5 years (0, 1 or 2, 3 or 4, or ≥5 vaccinations), pregnancy (a potential risk factor for severe Covid-195 (link) and associated with the rate of vaccination owing to evolving vaccination guidelines for pregnant women), and the total number of coexisting conditions that had been identified by the Centers for Disease Control and Prevention (CDC) as risk factors for severe Covid-19 as of December 20, 2020.6 ,7 (See Supplementary Methods 3 for additional information about the matching process. The
The five outcomes of interest were documented SARS-CoV-2 infection confirmed by positive PCR test, documented symptomatic Covid-19, hospital admission for Covid-19, severe Covid-19 (according to National Institutes of Health criteria)8 and death from Covid-19. Each of these outcomes includes the outcomes that follow it. In a supplementary analysis, we also evaluated an additional outcome, SARS-CoV-2 infection without documented symptoms, as an imperfect proxy for asymptomatic infection (since mild symptoms may not be documented).
Table S1 provides details on definitions of variables. Persons with missing data for smoking status or body-mass index (BMI) were dropped from the analysis.