Abstract
Data on real-life vaccine effectiveness (VE) against the delta and omicron variant of COVID-19 infection among COVID-19 vaccine regimens are urgently needed to impede the COVID-19 pandemic. We conducted a test-negative case-control study to assess the VE of various vaccine regimens for preventing COVID-19 during the period when the delta variant and omicron was the dominant causative virus in Thailand. All individuals (age ≥18 years) at-risk for COVID-19 who presented for nasopharyngeal real-time polymerase chain reaction (RT-PCR) testing were prospectively enrolled and followed-up for disease development. Vaccine effectiveness was estimated with adjustment for individual demographic and clinical characteristics. Concerning the period of delta variant, there were 1,871 cases of the total of 4,589. The adjusted VE among persons receiving two-dose CoronaVac plus one BNT162b2 booster was highest (98%; 95% confidence interval [CI] 87 to 100), followed by those receiving two-dose CoronaVac plus one ChAdOx1 nCoV-19 booster (86%; 95% CI 74 to 93), two-dose ChAdOx1 nCoV-19 (83%; 95% CI 70 to 90), one CoronaVac dose and one ChAdOx1 nCoV-19 dose (74%; 95% CI 43 to 88) and two-dose CoronaVac (60%; 95% CI 49 to 69). Concerning the period of omicron variant, there were 471 cases of the total of 1,975. The diseases severity markly decreased while the vaccine effectiveness aso markly decreased, compared to the period of delta variant. Our study demonstrated the incremental VE with the increase in number of vaccine doses received. The two-dose CoronaVac plus one BNT162b2 or ChAdOx1 nCoV-19 booster regimens were highly effective in preventing COVID-19 during the rise of delta variant.
