BEIJING - The Chinese effort for a vaccine is heating up, as scientists continue to administer experimental doses of treatment in thousands of people in China daily.

Their effort is being challenged most recently by American pharmaceutical firm Pfizer, which said early data on its Covid-19 vaccine candidate suggests 90 per cent effectiveness in phase three clinical trials.

Chinese pharmaceutical companies Sinovac Biotech, Sinopharm and CanSino Biologics have been carrying out phase three clinical trials outside the country, but have been giving out doses of the experimental treatment in the country.

This despite persistent concerns about the efficacy of the vaccines and the autonomy of those receiving the treatment in China.

Most of them are workers in state-owned companies and students planning to go overseas.

According to Chinese state media, shots of four experimental vaccines - made by the three companies and in urgent use - have been administered to the people.

"Several of us were anxious to get back to our project abroad so my supervisor helped to rush the doctors to give us the shots earlier," said an engineer who works at a state-owned enterprise who declined to be named because he is not authorised to speak to the media.

"Many of us are worried about the side effects because this is something we don't know anything about, but it felt incorrect to question it at the company... and all of us wanted to get back to work as soon as possible," he added.

He is usually based in a Middle Eastern country but has had to stay put in Beijing since the pandemic broke out early this year.

Employees at state-linked enterprises said they are given a cocktail of two separate injections, administered several weeks apart.

So far, about 100,000 people have been inoculated with the Sinopharm vaccine.

None have reacted badly to the treatment, said its chairman Liu Jingzhen last Friday.

About 56,000 of them have travelled abroad and none have contracted the virus, he added, noting that they included employees of state-owned enterprises China National Petroleum Corporation, China Petrochemical Corporation and technology giant Huawei.

Besides their own country, the Chinese companies are also doing clinical testing in dozens of other countries, such as Indonesia, Brazil and Bahrain, because there are too few coronavirus cases in China for a large study.

Brazil health regulator Anvisa on Wednesday allowed resumption of late-stage clinical trials for China's Sinovac Covid-19 vaccine, which had been suspended due to the death of a study subject that was registered in Sao Paulo as a suicide, Reuters reported.

Brazil's President Jair Bolsonaro, a longtime China sceptic who has baselessly dismissed the Sinovac vaccine as lacking in credibility, had hailed Monday's suspension as a personal victory.

The decision was severely criticised by the trial organizers, who said the move had taken them by surprise and that there had been no need to stop the study as the death had no relation to the vaccine.

The suspension further inflamed tensions between Mr Bolsonaro and Sao Paulo Governor Joao Doria, who has pinned his political ambitions on the Chinese vaccine that he aims to roll out in his state as early as January, with or without federal assistance.

Anvisa, in its statement on Wednesday, said the initial information it received about the case, which led to the suspension, had been incomplete and lacked the cause of the "severe adverse event".

It has strongly dismissed suggestions the decision could have been politically motivated.

"After evaluating the new data presented by the sponsor... Anvisa understands that it has sufficient reasons to allow the resumption of vaccination," the agency said.

"It is important to clarify that a suspension does not necessarily mean that the product under investigation does not offer quality, safety or efficacy," Anvisa added, according to Reuters.

China on Wednesday (Nov 11) reported two new cases, bringing the number of active cases to 600. In all, the country's official tally is 92,304 cases.

Phase three clinical trials are important in vaccines because adverse side effects usually show up at this point in experiments.

Meanwhile, in eastern China's Zhejiang province, thousands have paid 400 yuan (S$82) for an experimental dose.

Access to the vaccine will be formally expanded nationwide to high-risk groups like medical workers as early as December, China's Centre for Disease Control and Prevention's chief biosafety scientist Wu Guizhen told state television last month.

Chinese authorities have cautioned that supply is limited. Priority will be given to doctors and nurses, delivery workers, overseas workers and others with specified employment needs.

State media has reported that vaccine production is likely to hit 600 million doses by year's end.

Beijing also said it will join Covax, a World Health Organisation effort to ensure even distribution of the vaccine worldwide. China has pledged at least two billion doses of Chinese-made vaccines to the effort.