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

ABSTRACT

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.

KEYWORDS:

• COVID-19
• real-life
• vaccine
• effectiveness
• Omicron variant
• Thailand

Acknowledgements

We would like to thank Mrs Siriphan Hongsuppinyo, and the personnel of Information Technology section, Ms Kanokporn Thongphubeth, R.N. and the personnel of Infectious Control unit, Ms Uayporn Pinitchai, R.N. and the personnel of COVID-19 inpatient units, the personnel of Screening & Admissions unit, Thammasat University Hospital for their contribution in patient screening and identification, data collection and IT support, the personnel of IBOTNOI Company Limited who developed the Chatbot Software for use in this study and the Research Group in Pediatrics, Faculty of Medicine, Thammasat University and Thammasat Postdoctoral Fellowship, Thammasat University for the study support.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This study was funded by Health Systems Research Institute Grant [number 64-119] and Thammasat University Research Fund [Contract No. TUFT 44/2565].