Evaluation of COVID-19 vaccine effectiveness in Health Region 4, Thailand, January–April 2022

Abstract

Background: Thailand has mandated COVID-19 vaccination to attain herd immunity, reduce outbreaks, and minimize severe cases and deaths. This study assesses vaccine effectiveness in Health Region 4, Thailand, from January to April 2022.
Methods: The study employed a test-negative case-control design, utilizing data from diverse sources, including the Co-lab database of the Department of Medical Sciences, which contains information on individuals who have been tested for COVID-19, data on the hospitalization status of COVID-19 patients from the Department of Medical Services. Additionally, data on COVID-19 mortality from the Department of Disease Control, and information on COVID-19 vaccination from the Office of the Permanent Secretary, Ministry of Public Health. We analyzed data using descriptive and inferential statistics, and we employed multiple logistic regression methods adjusting for age group and test rationales. We calculated the adjusted odds ratio (adjusted OR) and 95% confidence interval (CI). Vaccine effectiveness was calculated using (1–adjusted OR) *100%.
Results: Among 247,046 individuals, 101,239 were infected, 28.8% experienced severe symptoms, and 10.0% died, while 145,807 were non-infected. The majority of severe cases and deaths occurred in the unvaccinated group, constituting 47.3% and 51.0%, respectively. In addition, primarily were among the elderly. We found that receiving four doses of vaccine had low effectiveness in preventing COVID-19 infection, at 35.3% (95% CI: 31.8–38.6). However, the vaccine effectiveness against severe symptoms was high, increasing with the number of vaccine doses administered. Two doses and three doses effectiveness were 82.0% (95% CI: 77.0–86.0) and 96.0% (95% CI: 94.0–98.0), respectively, in preventing severe symptoms. Vaccine effectiveness in the age group of 18–59 years and ≥ 60 years showed similarly high effectiveness in preventing severe symptoms and death, with two doses and three doses of the vaccine being effective at 68.0–92.0% and 88.0–99.0%, respectively. The vaccine effectiveness of two and three doses against death were around 76.0–92.0% and 81.0–99.0%, respectively. Regarding the effectiveness of three-dose vaccine regimen in preventing severe symptoms, the vaccine effectiveness is equally high regardless of the specific brand used.
Conclusion: Although the effectiveness of the COVID-19 vaccine against infection was low, receiving two or more doses of the vaccine was still highly effective in preventing severe symptoms and death, especially with three doses. Therefore, people, especially the elderly, should be vaccinated with at least two doses and should consider obtaining booster doses to increase the effectiveness. Furthermore, as the level of immunity and effectiveness of the vaccine may decrease over time, and with the emergence of new COVID-19 mutations, it is crucial to regularly evaluate the vaccine’s effectiveness to ensure its continued effectiveness in preventing severe symptoms and death.