Antibodies triggered by a third dose of Sinopharm's Covid-19 shot given to those who completed its primary two-dose regimen dropped sharply after six months, and a fourth shot did not significantly boost them against Omicron, a Chinese study showed.

The study, published on Monday before peer-preview, said repeated immunisation using inactivated vaccines such as the Sinopharm shot as a fourth booster may not be ideal to further increase antibody response against Omicron.

Certain protein-based vaccines...