The effectiveness of the COVID-19 vaccine during the outbreaks of the Delta and Omicron variants in the population of Trang Province

Abstract

Background: Thailand has implemented a policy to vaccinate its population against Coronavirus Disease 2019 (COVID-19), which has played a crucial role in reducing the incidence of severe cases and deaths from the virus in Trang Province. Despite ongoing infections in certain population groups, particularly during the period of viral mutations in 2021–2022, vaccination remains a key tool in controlling the outbreak in the region. This study was conducted with the aim of assessing the effectiveness of COVID-19 vaccines in preventing infections during the outbreaks of the Delta and Omicron variants in the population of Trang Province.

Methods: This study utilized an analytical test-negative case-control design involving residents of Trang Province who sought COVID-19 testing services in healthcare facilities between September and October 2021, during the Delta variant outbreak, and between March and April 2022. The study utilized central database records, including SARS-CoV-2 testing results from real-time polymerase chain reaction (RT-PCR) and antigen test kit (ATK) methods, managed by the Department of Medical Sciences, as well as COVID-19 vaccination data from the MOPH-IC database under the Ministry of Public Health. Descriptive statistics were used to summarize demographic characteristics by vaccination status, and multiple logistic regression analysis was applied to estimate the adjusted odds ratio (adjusted OR) with a 95% confidence interval (CI), controlling for age and the reason for testing. Vaccine effectiveness was calculated using the formula (1–adjusted odds ratio) ×100%.

Results: The study found that most COVID-19 cases had not received vaccination. The effectiveness of two vaccine doses was 31.1% for the Delta variant and 48.7% for the Omicron variant, while three doses provided an effectiveness of 87.8% and 41.4%, respectively. The most effective vaccine regimen for the Delta variant was CoronaVac (2 doses) + BNT162b2 (1 dose), while for the Omicron variant, it was CoronaVac (4 doses) + BNT162b2 (2 doses). Among the uninfected group, most individuals had not been vaccinated, but those who received at least one dose had a higher proportion. The highest vaccination rates were observed in the elderly and the 18-59 age group. A period of ≥14-89 days after receiving the vaccine showed that the third dose provided the best protection, with the CoronaVac (4 doses) + BNT162b2 (2 doses) regimen offering the highest effectiveness across all variants.

Discussion: The study on vaccine effectiveness in Trang Province during the Delta and Omicron variant outbreaks showed that a significant portion of the population had not received the full vaccination regimen, which resulted in higher infection rates among those partially vaccinated. Two doses of the vaccine were more effective during the Delta period (31.1-87.8%) but showed a decline during the Omicron period (41.4-54.3%), as the Omicron variant is more capable of evading immune responses. The vaccine coverage in Trang remained below target due to slow vaccine distribution and public hesitancy. Mixed vaccine regimens, such as CoronaVac + BNT162b2, showed higher effectiveness. The study highlights that booster doses improve immunity, but regional factors and the nature of viral mutations should be considered.