The effectiveness of primary series CoronaVac vaccine in preventing COVID-19 illness: A prospective cohort study among healthcare workers in Azerbaijan, May-November 2021.

Background: Healthcare workers (HCWs) have suffered considerable morbidity and mortality during the COVID-19 pandemic. Few studies have evaluated the CoronaVac vaccine effectiveness (VE), particularly in Eastern Europe, where the vaccine has been widely used.

Methods: We conducted a prospective cohort study among HCWs in seven hospitals in Baku, Azerbaijan between May 17 and November 30, 2021, to evaluate primary series (two-dose) CoronaVac VE against symptomatic SARS-CoV-2 infection. Participants completed weekly symptom questionnaires, provided nasopharyngeal swabs for SARS-CoV-2 RT-PCR testing when symptomatic, and provided serology samples at enrollment that were tested for anti-spike and anti-nucleocapsid antibodies. We estimated VE as (1 - hazard ratio)*100 using a Cox proportional hazards model with vaccination status as a time-varying exposure, adjusting for hospital and previous SARS-CoV-2 infection status.

Results: We enrolled 1582 HCWs. At enrollment, 1040 (66%) had received two doses of CoronaVac; 421 (27%) were unvaccinated. During the study period, 72 PCR-positive SARS-CoV-2 infections occurred; 36/39 (92%) sequenced samples were classified as Delta variants. Primary series VE against COVID-19 illness was 29% (95% CI: -51%; 67%) for the entire analysis period. For the Delta-only period (July 1-November 30, 2021), primary series VE was 19% (95% CI: -81%; 64%). For the entire analysis period, primary series VE was 39% (95% CI: -40%; 73%) for HCWs vaccinated within 14-149 days and 19% (95% CI: -81%; 63%) for those vaccinated ≥150 days.

Conclusions: During a period in Azerbaijan characterized by mostly Delta circulation, VE point estimates suggested that primary series CoronaVac protected nearly 1 in 3 HCWs against COVID-19, but 95% confidence intervals were wide, with lower bounds that crossed zero, reflecting the limited precision of our VE estimates. Our findings underscore the need to consider booster doses for individuals who have received the primary series of CoronaVac.