Heterologous booster vaccines reduce severity and mortality in COVID-19 during BA.2 and BA.4/BA.5 omicron predominance in Thailand.

Background: The COVID-19 pandemic has evolved quickly, with variants of concern resulting in the need to offer booster vaccinations. Unfortunately, the booster uptake has been slow and vaccine response has shown to wane over time. Therefore, it's critical to evaluate the role of vaccinations on outcomes with newer sub-lineages of omicron.

Methods: Utilising a Hospital Information System established in Chiang Mai, Thailand, we conducted a cohort study by linking patient-level data of laboratory-confirmed COVID-19 cases to the national immunization records, during BA.2 and BA.4/BA.5 predominance.

Results: In adjusted cox-proportional hazard models, BA.4/BA.5 was not associated with more severe COVID-19 outcomes or deaths as compared to BA.2. Risk of severe outcomes and deaths were significantly reduced with third (87% and 95%) and fourth (88% and 95%) dose vaccination, while events were not observed with a fifth dose. Across the regimens, vaccination within 14-90 days prior showed the highest level of protection. All the vaccine types used for boosting in Thailand offered similar protection against severe COVID-19.

Conclusions: Boosters provide high level of protection against severe COVID-19 outcomes and deaths with newer omicron sub-lineages. Booster campaigns should focus on improving coverage utilising all available vaccines to ensure optimal protection.