A protein-based Covid-19 vaccine made by Sinopharm, when given as a booster after two doses of an earlier shot from the Chinese firm, elicited a stronger antibody response against the Omicron variant than a third dose of the original, a study showed.

The study, which was published on Tuesday and had not been peer reviewed, came amid concerns over the effectiveness of Sinopharm's BBIBP-CorV shot, one of the two leading Covid-19 vaccines exported by China, against the Omicron variant.