A COVID-19 vaccine developed by a state-owned Chinese pharmaceutical firm has been administered to about a million people under the government's emergency use scheme, the company's chairman said.

"In terms of emergency use, the vaccines were applied to nearly a million people and there has not been a single case of a serious adverse event. People have had only mild symptoms," Liu Jingzhen, chairman of China National Pharmaceutical Group (Sinopharm), said.

"Until now, all our progress, from research to clinical trials to production and emergency use, we have been leading the world," Hong Kong-based South China Morning Post on Thursday quoted Liu as saying to a Chinese digital media publication.

His comments came amid criticism that Chinese firms have not publicised the clinical data of its vaccine studies.

Besides the recipients of the Sinopharm jabs, authorities in Zhejiang said they had made a COVID-19 vaccine developed by the privately owned pharmaceutical company Sinovac Biotec available to high-risk groups in the east China province under the emergency use scheme.

Meanwhile, the Lancet Infectious Diseases journal said that a Chinese inactivated COVID-19 vaccine candidate has been proved safe and tolerable and can induce a quick immune response.

The level of antibodies induced by the vaccine was lower than those in people who had recovered from COVID-19, whereas it was capable of protecting the human body from infections caused by the virus, state-run Xinhua reported, citing the Lancet findings.

The vaccine, CoronaVac, was developed by Chinese biopharmaceutical producer Sinovac Biotech.

It was tested in randomised, double-blind and placebo-controlled phase-1 and phase-2 clinical trials involving more than 700 healthy adults aged 18 to 59.

The vaccination is done with two doses taken 14 days apart and the vaccine candidate is effective, which makes it suitable for emergency use amid the pandemic, said Zhu Fengcai, one of the authors of the findings.

Zhu added that further research is needed to verify the duration of the immune response induced by the vaccination.

At present, the vaccine candidate is undergoing phase-3 clinical trials to confirm its effectiveness.