The effectiveness of primary series CoronaVac vaccine in preventing
COVID-19 illness: a prospective cohort study among health workers in
Azerbaijan, May–November, 2021
Abstract
Background Healthcare workers (HCWs) have suffered considerable
morbidity and mortality during the COVID-19 pandemic. Few studies have
evaluated 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 to December 1, 2021, to evaluate primary
series (two-dose) CoronaVac VE against symptomatic SARS-CoV-2 infection.
Participants completed weekly symptom questionnaires, provided nasal
swabs for SARS-CoV-2 RT-PCR testing when symptomatic, and provided
serology samples at enrolment 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 covariate. Results We enrolled 1582 HCWs. At enrolment,
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 variant. The adjusted primary series VE against COVID-19 illness
was 29% (95% CI:-51%;67%). For the delta-predominant period,
adjusted primary series VE was 19% (95% CI:-81%;64%). For the entire
analysis period, adjusted primary series VE was 39% (95%
CI:-40%;73%) for HCW vaccinated within 14–149 days, and 19%
(95%CI:-81;63) for those vaccinated ≥150 days. Conclusions During a
delta-predominant period in Azerbaijan, point estimates suggest that
primary series CoronaVac protected nearly 1 in 3 HCWs against COVID-19,
but this finding was not statistically significant. Our findings
underscore the need to consider booster doses in individuals who have
received primary series CoronaVac.