BEIJING (NYTIMES) - Mr Ethan Zhang needed to get back to work. Work was in Ivory Coast, however, and since January the global coronavirus outbreak had stranded the 26-year-old translator in mainland China.

Then friends told Mr Zhang of a way he could get his hands on what might be the world's most coveted prize: a coronavirus vaccine.

Though China's vaccine candidates have not formally been proved safe or effective, officials have been injecting them into thousands of people across the country, ostensibly under an emergency-use policy. One such campaign, his friends said, was underway in the city of Yiwu in eastern China.

Mr Zhang took a plane to Yiwu from Beijing that night. He stood in line for four hours outside a hospital. He paid US$30 (S$40.29). He got his shot.

And he expressed little worry that the substance that had been injected into his arm is still in the testing phase, an attitude that is stirring worry among global health experts.

"I feel more relieved now that I have protection," Mr Zhang said. "Since they've started using it on some people on an emergency-use basis, it shows that there's a certain guarantee."

China has made its unproven candidates widely available to demonstrate their safety and effectiveness to a country that has long been skeptical of vaccines after a spate of quality scandals. Government officials and top pharmaceutical executives speak proudly of being inoculated.

The campaign has succeeded perhaps too well. Yiwu's 500 doses were consumed within hours. Other cities are limiting doses or asking people to show proof that they are travelling. The overwhelming demand has inspired a cottage industry of scalpers - called "yellow cows" in China, the people who usually score the newest iPhones or hot railway tickets - charging as much as US$1,500 for an appointment.

Those users could be taking big risks. People who have taken ineffective vaccines might believe they are safe and engage in risky behaviour. They can be barred from taking another, better vaccine because they have already been injected. In a few cases in the past, unproven vaccines have caused health risks.

The potential problems often go undiscussed. Copies of the vaccination consent forms for one candidate that were reviewed by The New York Times did not specify that the product was still in testing.

"These kinds of risks have not been clearly revealed," said Mr Yanzhong Huang, a senior fellow for global health at the Council on Foreign Relations and an expert on health care in China.

Any reports of deaths or illness could reignite mistrust in vaccines. China spent years vowing to clean up its vaccine industry after scandals.

"We risk losing confidence in people if indeed adverse effects occur," said Dr Kristine Macartney, director of the National Centre for Immunisation Research and Surveillance in Sydney.

It is unclear how many people have already received a vaccine candidate. China has made three of its four candidates in late-stage human testing, called Phase 3 trials, available to tens of thousands of employees at state-owned businesses, government officials and company executives since July.

Once Phase 3 trials are complete, the companies would submit results to the regulators of the countries that they want to sell their vaccines in. Authorities would review and assess them for approval.

Local governments have indicated that they plan to make the current vaccines available to more people. Beijing says it is keeping tabs on those who have been given the vaccines but has not disclosed any details.

Chinese officials have defended making vaccine candidates available.

Mr Zheng Zhongwei, a top official at China's National Health Commission, said last month that the move was a "very necessary means of protecting peoples' lives and health," given outbreaks abroad.

Last week, Sinopharm's chairman, Mr Liu Jingzhen, announced that some 100,000 people have taken the company's vaccine and none have shown any adverse reactions so far. He said that 56,000 of them had travelled abroad after taking the vaccine and none had been infected.

China's drive has taken nationalistic overtones, with many celebrating the fact that the country has candidates in late-stage trials.

The early releases have helped highlight one potential problem: deploying an approved vaccine. Demand is so high that the government and companies could struggle with distribution both at home and in other countries that Beijing has promised the treatment to.

Ms Wendy Zhang, 26, a medical worker from the eastern city of Jinan, said she had to wait 57 days to get her second shot of a vaccine in October because the candidates had run out. She said she had felt relieved after getting it.

"There has been no adverse reaction after the vaccination, indicating that the safety of the vaccine developed by China is beyond doubt," Ms Zhang said.