The administration of Covaxin as a booster dose enhances vaccine effectiveness against Covid-19's Delta variant and gives protection against Omicron variants BA.1.1 and BA.2, a study by ICMR and Bharat Biotech has found.

The protective efficacy of Bharat Biotech's Covaxin following two and three-dose immunizations against the Delta variant and the efficacy of the Covaxin against Omicron variants were studied in a Syrian hamster model (animal model to study human-associated diseases), it said.

The antibody response, clinical observations, viral load reduction and lung disease severity after virus challenge were observed, it added.

The findings of the study have been published on Tuesday on bioRxiv, a pre-print server and have not been peer reviewed.

"In the Delta infection study, where we compared the protective response between the two and three-dose regimens, we could observe the advantage of the booster dose vaccination in the protection. Although the neutralising antibody levels were comparable among the groups, lung disease severity was found more reduced after the three dose vaccination.

"The virus shedding and viral organ load were considerably reduced in both the two-dose and three-dose immunised animals indicating the vaccine efficacy against Delta variant," the study by Indian Council of Medical Research and Bharat Biotech said.

In the second study in which the protective response was assessed against Omicron variants i.e. BA.1 and BA.2, following three-dose vaccinations, lesser virus shedding, lung viral load and lung disease severity were observed in the immunised groups in comparison to the placebo groups.

"The evidence from the present study shows that Covaxin booster immunisation tends to broaden the protective immune response and reduces disease severity against the Delta and Omicron variant infection," it further said.