BEIJING (REUTERS) - A coronavirus vaccine candidate China is developing may not be ready for sale until at least 2021, as researchers struggle to move into large-scale human trials in the country because of a lack of new infections, a senior company executive said.

More than 10 experimental vaccines are being tested in humans globally as scientists race to protect against the novel coronavirus that has killed more than 450,000 people.

But none of them has yet passed late-stage phase three trials that require thousands of participants to determine a vaccine candidate's effectiveness.

China, where the virus was first detected last year, saw less than 10 new local cases reported daily on average in May, making it less favourable for a late-stage clinical trial.

"We hope we can launch more international cooperations and conduct a multiple-centre phase three clinical study to help bring the vaccine to the market," China National Biotec Group's (CNBG) vice president Zhang Yutao told state media China News Service.

"The vaccine won't be on the market until at least next year based on current plans," he said in the interview broadcast late on Thursday (June 18).

A new outbreak in the capital Beijing city last week has infected more than 180 people, but Yang said the number of new patients compared with the population was still too low to make it an ideal trial site.

CNBG is an affiliate of state-backed China National Pharmaceutical Group (Sinopharm). One of the two experimental vaccines developed by its units showed some positive signs in early-stage human trials.

State media reported last week that China was offering its two vaccine candidates to employees at state-owned firms travelling overseas.