A Covid-19 booster shot produced by China's Sinopharm had "significantly lower" neutralising activity against the Omicron variant, Chinese researchers said in a paper, although they added the vaccine's efficacy against Omicron remained unclear.

The study — conducted by researchers from Shanghai Jiao Tong University and a Shanghai-based lab specialising in respiratory infectious diseases — compared the activity of Sinopharm's booster vaccine against an older coronavirus strain from Wuhan.

The neutralising antibody activity of a Sinopharm BBIBP-CorV booster against Omicron showed a 20.1-fold reduction, compared with its activity against a Wuhan strain, according to the paper.

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