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...