Effectiveness of the CoronaVac Vaccine on Symptomatic COVID-19 Infection, Severe Disease, ICU/semi-ICU Admission, and Mortality in Samut Sakhon Province: a Test-Negative Case-Control Study

A mass vaccination campaign with the inactivated severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccine, CoronaVac, was implemented in Thailand during the early months of the COVID-19 pandemic. As the Delta variant became the dominant strain in the country, we aimed to evaluate the real-world effectiveness of this particular vaccine among adults in Samut Sakhon Province, Thailand. A test-negative case-control study was conducted from 1 Jun to 31 Jul 2021 to evaluate the effectiveness of CoronaVac against symptomatic COVID-19 infection, severe disease, admission to intensive care unit (ICU)/semi-ICU and mortality. We estimated odds ratios using multiple logistic regression. Among 11,371 participants included in the study, 3,116 (27.4%) tested positive for COVID-19 and 3,333 (29.3%) completed two doses of vaccine. The adjusted vaccine effectiveness of two-dose CoronaVac was 65.8% (95% confidence interval (CI) 61.9 – 69.3) for the prevention of symptomatic infection, 71.8% (95% CI 58.5 – 81.6) for severe disease, 72.7% (95% CI 56.6 – 83.9) for ICU/semi-ICU admission and 86.7% (95% CI 34.8 – 99.3) for mortality. Results of this study demonstrate that CoronaVac had moderate effectiveness against symptomatic COVID-19 infection, while it was effective in preventing severe disease, ICU/semi-ICU admission and COVID-19 related deaths in a setting where the two variants were circulating.