Statistical model
VE was estimated as (1 – hazard ratio)*100. Hazard ratios comparing vaccinated and unvaccinated were estimated using Cox proportional hazards models with vaccination as a time-varying exposure; the vaccination status of some individuals changed over time from unvaccinated to vaccinated, and therefore the same participant could contribute person-time to both exposure categories. Study time was used as the underlying time scale in the Cox regression model. Participants were considered fully vaccinated 14 days after they received their second vaccine dose.
We calculated unadjusted and adjusted VE estimates. We categorized hospitals into two groups based on geographical location; three hospitals in central Baku were considered “central,” while four hospitals located on the outskirts of Baku were considered “peripheral.” Both unadjusted and adjusted VE estimates included hospital group and prior infection as fixed effects. We assessed other potential confounders (e.g., month, age, sex, occupation, hands-on care, smoking, household size, any chronic condition, BMI) if they changed the VE estimate by more than 5% using step-wise backward selection.
We defined previous infection as a PCR-confirmed infection prior to enrolment documented in either the Etabib database or the MoH/Mandatory Health insurance database. Participants who were unvaccinated at enrolment and had PCR-confirmed SARS-CoV-2 infection prior to enrolment began to contribute person-time on the date they became eligible for vaccination (six months after their last positive test). Participants who were vaccinated at enrolment and who had PCR-confirmed COVID-19 infection prior to enrolment were included in the analysis at the time point they were considered “at risk” of reinfection, which we defined as 90 days after their most recent positive PCR test.
Participants contributed person-time from enrolment, or, for individuals with prior PCR-confirmed SARS-CoV-2 infection, from the start of time at risk, until the earliest of outcome or exit from the study. Person-time ended at whichever came first of the following outcomes: 1) the day of the first SARS-CoV-2 infection, 2) the day of receipt of a second COVID-19 vaccination if it occurred before the recommended interval between the first and second dose 3) the day of receipt of a third vaccine dose, or 4) the day of the last weekly questionnaire before complete loss to follow-up, withdrawal from the study, transfer or retirement from their hospital of employment, death, or censor date for the analysis period (1 December 2021). Participants were also censored from the primary analysis upon receiving a dose of any COVID-19 vaccine other than CoronaVac.