The COVID-19 pandemic has evolved quickly, with different variants of concern resulting in the need for countries to offer booster vaccinations. While studies have assessed homologous schedules in detail, the effectiveness of heterologous booster vaccine schedules against severity and mortality with newer variants, remains to be explored fully.


Utilising a Hospital Information System for COVID-19 established in Chiang Mai, Thailand, we conducted a cohort study by linking patient-level data on laboratory-confirmed COVID-19 cases to the national immunization records, during delta-predominant and omicron predominant periods.


Compared to omicron, COVID-19 cases during delta period were ten times more likely to have severe outcomes and in-hospital deaths. During omicron, a third vaccine dose had 89% reduced risk of both severe COVID-19 and deaths. Third dose received 14-90 days prior to the date of positive test showed highest protection (93%). Severe outcomes were not observed with third dose during delta, and fourth dose during omicron period. All the vaccine types used for boosting in Thailand offered similar protection against severe COVID-19.


Booster doses provided very high level of protection against severe COVID-19 outcomes and deaths. Booster campaigns should focus on improving coverage utilising all available vaccines to ensure optimal protection.

Fuente: International Journal of Infectious Diseases

Published: 11 November 2022