According to Nikkei Asia, the University of Hong Kong (HKU) and the Chinese University of Hong Kong conducted the study, finding that a third shot of what is one of the world's most widely used Covid-19 vaccines does not produce enough levels of virus-neutralising antibodies.

The third dose of a vaccine produced by Pfizer and BioNTech, however, significantly improved protection for those who had previously taken two doses of Sinovac, also known as CoronaVac, the study indicated.

A Pfizer booster will "achieve optimal protection against (the) Omicron variant," Malik Peiris, the HKU professor who led the research was quoted as saying.

Sinovac last week said its own study had found that a third dose was "effective in improving the serum neutralising against the omicron strain".

It said that 94 per cent of those who had received three Sinovac doses generated enough neutralising antibodies. The company did not elaborate on what kind of antibody levels three shots produced.

Sinovac did not respond to requests for comment.

CoronaVac is one of two Chinese vaccines approved by the World Health Organisation (WHO) for emergency use.

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