BEIJING (NYTIMES) - The offer to employees at the state-owned oil giant was compelling: Be among the first in China to take a coronavirus vaccine.

The employees at PetroChina could use one of two vaccines "for emergency use" to protect themselves when working overseas as part of China's ambitious infrastructure programme, according to a copy of the notice, which was reviewed by The New York Times. They would effectively be guinea pigs for testing the unproven vaccines outside of the official clinical trials.

The offer was backed by the government. It stressed that data from clinical trials showed that the products, both made by Sinopharm, were safe. It did not mention the possible side effects or warn against the false sense of security from taking a vaccine that regulators had not approved.

"I don't think this is right, ethically," said Ms Joan Shen, Shanghai-based chief executive officer of pharmaceutical firm I-Mab Biopharma.

The unorthodox move to test people separately from the normal regulatory approval process reflects the formidable challenge facing China as it races to develop the world's first coronavirus vaccine.

Eager to find a long-term solution to the outbreak and burnish their scientific credentials, Chinese companies are rushing to get as much data as possible on their vaccines to prove they are safe and effective.

In China, they are selectively testing their vaccines on small pools of people like the PetroChina employees - an approach that does not count toward the regulatory process but could bolster their own confidence in the vaccines. In Brazil and other countries, they are conducting clinical trials, going through the normal regulatory channels.

The dual strategy, though, is risking scientific setbacks and political backlash, potentially undercutting China's efforts.

Such "emergency use" is rare, and the taking of unapproved vaccines is typically reserved for healthcare professionals. Although the government has stressed that taking the vaccine is voluntary, the state-owned workers and soldiers could feel pressure to participate.

As Chinese companies also look beyond their borders to test the vaccine, they are running into mistrust and skepticism.

Health experts have questioned why the Canadian government is allowing CanSino Biologics, which has teamed up with the People's Liberation Army, to run human trials in the country. Rumours have spread about the authenticity of a Chinese-made vaccine that is being tested in Brazil as supporters of President Jair Bolsonaro cast doubt.

The strategy is born out of necessity.

Chinese companies cannot find enough candidates at home to conclusively determine whether their vaccines would prevent infections, a problem faced by research institutes and pharmaceutical-makers in countries that have largely tamed the coronavirus. Phase three trials, the final stage before approval, require vaccines to be tested in roughly tens of thousands of volunteers in places with large, active outbreaks.

Along with the testing at the oil company, Sinopharm, which has completed phase two trials for two products, has injected the vaccine into its chairman and other senior officials, according to the state-owned Assets Supervision and Administration Commission (SASAC), the government agency managing all employees at state-backed companies. The Chinese government has allowed the CanSino-military vaccine to be given to its armed forces, a first for the military of any country.

Dr Yang Zhanqiu, a virologist at Wuhan University, was skeptical about the decision to give vaccines to state-owned employees for business travel.

"It does not make much sense at all because the length of time that the employees take to travel is not the same, the locations may be different, and it is not easy to do tracking and monitoring," Dr Yang said. "It may just be a psychological comfort for employees."

A PetroChina employee based overseas confirmed that his colleagues in China had been invited to take the vaccines. PetroChina, SASAC and Sinopharm did not respond to requests for comment.

Such testing does not help the companies clear any regulatory hurdles, since it is not part of the official clinical trials. Mainly, Chinese companies could use it to give themselves extra reassurance that the vaccines are safe, presuming they do not discover any problems.

"If you are a regulatory body, if you play by the rules, if you are hard-nosed about it, you say this is very wrong," said Mr Ray Yip, former head of the Gates Foundation in China.

Mr Yip added that it would be useful for company executives to know that they had given the dose to "a couple of thousand people, but no one has dropped dead, so that's pretty good."

Mr Yip said the people taking the vaccines should read up on reports of the safety data and make an informed decision. He said he would be willing to take it.

"If you offer that to me saying it's safe and there's an 85 per cent chance that it works, would I take it today?" he said. "You know what - I probably will. Because then I don't have to worry."

In a post on its official WeChat account, a government agency reported that the "vaccine pretest" on Sinopharm employees showed that antibody levels were high enough in subjects to combat the coronavirus, indicating that it was safe and effective.

The agency did not make clear what vaccine the employees had taken. The Paper, a newspaper owned by the Shanghai government, separately said 180 employees had taken the vaccine.

Last month, the Beijing Institute of Biological Products, which is developing one of Sinopharm's vaccines, published its preclinical data in a peer-reviewed journal, Cell, saying the vaccine induced high levels of antibodies in macaques and protected against Sars-CoV-2, the virus that causes Covid-19.

The Wuhan Institute, which Sinopharm owns, said its vaccine had caused no adverse reactions among volunteers, according to Xinhua, China's official news agency. Volunteers achieved full antibodies after two doses in a 28-day programme.