Real-World Effectiveness of COVID-19 Vaccines against Severe Outcomes during the Period of Omicron Predominance in Thailand: A Test-Negative Nationwide Case-Control Study

Abstract

Due to the widespread Omicron variant of SARS-CoV-2 in Thailand, the effectiveness of COVID-19 vaccines has become a major issue. The primary objective of this study is to examine the real-world effectiveness of COVID-19 vaccines based on secondary data acquired under normal circumstances in a real-world setting, to protect against treatment with invasive ventilation of pneumonia during January to April 2022, a period when Omicron was predominant. We conducted a nationwide test-negative case-control study. The case and control were matched with a ratio of 1:4 in terms of age, date of specimen collection, and hospital collection specimen and the odds ratio was calculated using conditional logistic regression. Overall, there was neither a distinction between mix-and-match regimens and homologous mRNA regimens against severe symptoms, nor was there a decline of the protective effect over the study period. The third and fourth dose boosters with ChAdOx1 nCoV-19 or mRNA vaccines provided high levels of protection against severe outcomes, approximately 87% to 100%, whereas two doses provided a moderate degree (70%). Thus, this study concludes that current national vaccine strategies provide favourable protective benefits against the Omicron variant. All Thais should receive at least two doses, while high-risk or vulnerable groups should be administered at least three doses.

Keywords: COVID-19; Omicron; SARS-CoV-2; Thailand; pneumonia; vaccine effectiveness.

Figure 1

Vaccine effectiveness against SARS-CoV-2 infection, pneumonia requiring invasive ventilation, and death stratified by number of doses received, regardless of vaccine brand. Note: N/A referred to the fact that estimated value and its 95% confidence interval (CI) were in the negative form.

Figure 2

Vaccine effectiveness against pneumonia requiring invasive ventilation stratified by age group and number of doses received, regardless of vaccine brand. Note: N/A referred to the situation where the estimated value was insignificant, as the 95% confidence interval (CI) covered the null value.

Figure 3

Vaccine effectiveness against pneumonia requiring invasive ventilation stratified by time interval between sample collection date and vaccination date, regardless of vaccine brand. Note: The maximum time interval (days) of cases with pneumonia requiring invasive ventilation and the compared group were 323 and 391 for one dose; 289 and 398 for two doses; 196 and 288 for three doses; and 42 and 255 for four doses. Only one pneumonia case requiring invasive ventilation had received four doses of vaccine. CI: confidence interval.