A third dose of its vaccine candidate SCB-2019 resulted in a 19-fold increase in neutralising antibody levels against the Omicron BA.2 subvariant from pre-booster levels, Clover said in a filing to the Hong Kong stock exchange.

The booster shot also elicited a 12-fold rise in the neutralising antibody level against the BA.1 subvariant, the company said, citing preliminary analysis without providing detailed readings.

The results came from a cohort of participants, who had no natural infection of the coronavirus and showed waning antibodies before the booster, in a clinical trial that has enrolled 3,755 participants in Brazil, the Philippines and Columbia, Clover said.

SCB-2019, which is backed by the Coalition for Epidemic Preparedness Innovation (CEPI), has yet to be approved for commercial use.

Antibody-based readings differ from a vaccine's efficacy, a rate that reflects how well it could lower the risk of COVID disease or death.

Data from a large clinical trial conducted before the spread of Omicron showed SCB-2019 was 67% effective against symptomatic COVID-19 of any degree of severity and 79% against the Delta varian

Clover did not discuss cell-based responses elicited by the booster, another important part of human immune system different from antibody-based responses.

Clover is also testing a separate candidate SCB-2020S in an early stage trial and working on a bivalent vaccine candidate targeting variants of concern including Omicron.