BEIJING (REUTERS) - China's CanSino Biologics said the efficacy rate for its single-dose Covid-19 vaccine may fall over time although it should still have a rate of 50 per cent or more five to six months after inoculation.

A second shot given to trial participants six months after their first injection could offer substantial protection, CanSinoBIO's chief scientific officer Zhu Tao, said in an online presentation late on Wednesday (March 31).

"A booster shot six months later led to a seven times to 10 times increase in neutralising antibody levels, so we expect in this case efficacy could reach over 90 per cent," Mr Zhu said, though he cautioned more clinical trial data was needed for more precise estimates.

The company in February reported interim data that showed that the shot was 68.83 per cent effective at preventing symptomatic Covid-19 disease two weeks after vaccination but the rate fell to 65.28 per cent after four weeks.

The February data reflected a shorter time span after the inoculation, Mr Zhu said.

The vaccine has been approved in China, Pakistan, Hungary and Mexico.

CanSinoBIO is also planning a clinical trial in China for an inhaled version of the vaccine.