Real-life effectiveness of COVID-19 vaccine during the Omicron variant-dominant pandemic: how many booster doses do we need?

The surge in coronavirus disease 2019 (COVID-19) caused by the Omicron variants of the severe acute respiratory syndrome coronavirus 2 necessitates researches to inform vaccine effectiveness (VE) and other preventive measures to halt the pandemic. A test-negative case-control study was conducted among adults (age ≥18 years) who were at-risk for COVID-19 and presented for nasopharyngeal real-time polymerase chain reaction testing during the Omicron variant-dominant period in Thailand (1 January 2022-15 June 2022). All participants were prospectively followed up for COVID-19 development for 14 days after the enrolment. Vaccine effectiveness was estimated and adjusted for characteristics associated with COVID-19. Of the 7971 included individuals, there were 3104 cases and 4867 controls. The adjusted VE among persons receiving 2-dose, 3-dose, and 4-dose vaccine regimens for preventing infection and preventing moderate-to-critical diseases were 33%, 48%, 62% and 60%, 74%, 76%, respectively. The VE were generally higher among those receiving the last dose of vaccine within 90 days compared to those receiving the last dose more than 90 days prior to the enrolment. The highest VE were observed in individuals receiving the 4-dose regimen, CoronaVac-CoronaVac-ChAdOx1 nCoV-19-BNT162b2 for both preventing infection (65%) and preventing moderate-to-critical diseases (82%). Our study demonstrated increased VE along with an increase in number of vaccine doses received. Current vaccination programmes should focus on reducing COVID-19 severity and mandate at least one booster dose. The heterologous boosters with viral vector and mRNA vaccines were highly effective and can be used in individuals who previously received the primary series of inactivated vaccine.

Comparing real-life effectiveness of various COVID-19 vaccine regimens during the delta variant-dominant pandemic: a test-negative case-control study.

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.