An evaluation of real–world COVID–19 vaccine effectiveness in four omicron–affected provinces in Thailand, January 2022

Abstract

Background: In January 2022, Thailand encountered a significant surge in the Omicron variant, which covered approximately 80% of the tested samples. Despite this increase, the proportion of individuals who received the third dose of the vaccine remained relatively low (32.3%). At the time of the study, it is uncertain whether infection with Omicron variant causes more severe symptoms compared to infections with other variants, such as Delta. Additionally, there is limited knowledge on the Omicron variant’s ability to evade the immunity which could potentially impact the effectiveness of public health measures, vaccines, and therapeutics. This study aims to assess the COVID–19 vaccine effectiveness in four provinces with reported cluster of Omicron variant including Kalasin, Chon Buri, Chiang Mai, and Ubon Ratchathani provinces.
Methods: We employed a Test–negative case–control study to investigate the population exposed to SARS–CoV–2 and tested for the agent across four provinces, namely Kalasin, Chonburi, Chiang Mai, and Ubon Ratchathani, during 1 to 31 January 2022. Secondary data from the Co–Lab database of the Department of Medical Sciences was linked with the MOPH–IC vaccination database of the Office of the Permanent Secretary for Public Health. An infected individual (case) was defined as an individual diagnosed with SARS–CoV–2 through Real–Time Polymerase Chain Reaction during the study period, with no history of infection in the preceding three months. While a control was defined as an individual with no history of SARS–CoV–2 detection since March 2021. Descriptive statistics, including numbers and percentages, were analyzed alongside inferential statistics, logistic regression analysis, to calculate the odds ratio (OR) and a 95% confidence interval by controlling for age group variables (0–11 years old, 12–17 years, 18–59 years, and 60–120 years). Then, the vaccine effectiveness was computed using the formula (1–OR) *100%.
Results: Of the total 4,127 population, 526 cases and 3,611 controls were identified. The age group and vaccination status of cases and controls was similar, with 18–59 years of age being the most common age group (89% and 95%, respectively), and receiving two doses of the vaccine (60% and 56%, respectively). Vaccine effectiveness revealed that two–dose vaccination conferred low effectiveness at 22% (2%–38%); however, three–dose and four–dose vaccination conferred high effectiveness of 72% (57%–82%) and 85% (60%–95%). A booster or third dose of any vaccine had comparable effectiveness, ranging from 64%–80%.
Conclusion: The Ministry of Public Health should continue to accelerate vaccination in all service forms to ensure timely administration of booster vaccines in response to the current situation.