Statistical analysis
death, SARS-CoV-2 infection
SARS-COV-2 INFECTION, EVENT, MAY, REGRESSION, EVENTS
The baseline characteristics of individuals who received the BNT162b2 vaccine were compared with individuals who received the ChAdOx1 vaccine using standardized difference. For the primary outcome, person-months of follow-up for each individual were calculated as the amount of time from the index date to the first of the following events: confirmed diagnoses of SARS-CoV-2 infection, death, disenrollment from a GP practice participating in IMRD, 10 months follow-up, receiving a different COVID-19 vaccine of the second dose, or the end of the study (October 31, 2021). We calculated the rate of SARS-CoV-2 infection and plotted cumulative incidence curves of SARS-CoV-2 infection for individuals who received the BNT162b2 vaccine and individuals who received the ChAdOx1 vaccine, respectively. We estimated the absolute rate difference (RD) of SARS-CoV-2 infection between the two comparison groups. We obtained the hazard ratio (HR) of SARS-CoV-2 infection for the individuals who received the BNT162b2 vaccine vs. those who received the ChAdOx1 vaccine using Cox proportional hazard model accounting for competing event (i.e., death). We tested the proportional hazard assumption by plotting the cumulative incidence curve of each outcome. We then conducted a weighted cox regression to obtain a non-proportional HR if the proportional hazard assumption was violated [We performed four sensitivity analyses to assess the robustness of the study findings. First, we conducted subgroup analyses to assess the comparative effectiveness of these two vaccines according to sex (male vs. female), age (≥65 vs. <65 years), and study period (i.e., SARS-CoV-2 alpha-variant predominance period from January 1 to May 16, 2021 vs. delta-variant predominance period from May 17 to October 31, 2021 [All 