Coronavirus disease 2019 vaccination and reduced severity-related outcomes in high-risk groups in Thailand during the Omicron period

Abstract

Despite the coronavirus disease 2019 (COVID-19) booster vaccination, individuals with comorbidities continue to experience disproportionately severe COVID-19 disease and adverse outcomes. Evidence on associations between vaccination status and clinical outcomes among high-risk group during period of Omicron predominance in low- and middle-income countries remains scarce. This retrospective observational study was conducted at a tertiary care hospital in Bangkok, Thailand. It aimed to evaluate the clinical outcomes of COVID-19 infection in high-risk individuals and assess the association between vaccination status and severity-related outcomes, including hospitalization, intensive care unit (ICU) admission, need for oxygen therapy, and survival at discharge. Of the 1,379 patients, 1,241 (90.0%) received at least two vaccine doses, mainly AZD1222 for the first 1,105 (87.9%) and second 1,073 (86.5%) doses; BNT162b was the most frequently administered booster with 697 (76.7%) doses. Overall, 725 (52.6%) patients were hospitalized, and 585 (80.7%), 286 (39.4%), and 116 (16%) of whom developed pneumonia, required oxygen therapy, and necessitated ICU admission, respectively. Unvaccinated patients demonstrated higher odds of hospitalization (adjusted odds ratio [aOR] = 6.9; 95% confidence interval [CI]: 4.0–11.8; p < 0.001), oxygen therapy (aOR = 6.6; 95% CI: 4.2–10.5; p < 0.001), and ICU admission (aOR = 4.6; 95% CI: 2.5–8.4; p < 0.001). Similarly, patients with 1–2 doses exhibited increased odds of hospitalization (aOR = 3.2; 95% CI: 2.4–4·3; p < 0.001, oxygen therapy (aOR = 2.4; 95% CI: 1.6–3.4; p < 0.001), and ICU admission (aOR = 2.9; 95% CI: 1.7–4.8; p < 0.001) compared with those receiving three doses. Patients with chronic kidney disease, and cardiovascular disease were strongly associated with adverse outcomes and ICU admission. During the Omicron period predominance in Thailand, high-risk patients who received two doses of a viral vector vaccine followed by an mRNA booster remained vulnerable to severe COVID-19 infection; receipt of at least three COVID-19 vaccine doses was associated with significantly lower odds of severe outcomes. These findings highlight the significance of continued vaccination strategies, targeted prevention, and timely clinical management in medically vulnerable populations.