Data on real-life
vaccine effectiveness (VE), against the delta variant (B.1.617.2) of the severe acute respiratory syndrome coronavirus 2 among various
coronavirus disease 2019 (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 was the dominant causative virus (≥ 95%) in Thailand (25 July 2021-23 Oct 2021). All individuals (age ≥18 years) at-risk for
COVID-19, 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. Of 3353 included individuals, there were 1118 cases and 2235 controls. The adjusted VE among persons receiving two-dose
CoronaVac plus one
BNT162b2 booster was highest (98%; 95% confidence interval [CI] 87-100), followed by those receiving two-dose
CoronaVac plus one
ChAdOx1 nCoV-19 booster (86%; 95% CI 74-93), two-dose
ChAdOx1 nCoV-19 (83%; 95% CI 70-90), one
CoronaVac dose and one
ChAdOx1 nCoV-19 dose (74%; 95% CI 43-88) and two-dose
CoronaVac (60%; 95% CI 49-69). One dose of
CoronaVac or
ChAdOx1 nCoV-19 had a VE of less than 50%. Our study demonstrated the incremental VE with the increase in the number of
vaccine doses received. The two-dose
CoronaVac plus one
BNT162b2 or
ChAdOx1 nCoV-19 booster regimens was highly effective in preventing
COVID-19 during the rise of delta variant.