LONDON — British scientists said Tuesday they will launch the world's first human challenge trials for covid-19, in which healthy volunteers will be deliberately infected with the coronavirus in hopes of further speeding the drive to a vaccine.

The United States is moving more cautiously, with leading government researchers saying human challenge trials might be too risky or unnecessary.

But the British scientists say the potential payoff is massive — that accelerating vaccine development by even three months could save hundreds of thousands of lives globally.

Andrew Catchpole, chief science officer for hVIVO, a commercial pharmaceutical company that will recruit the volunteers, manufacture the challenge strain of the virus and conduct the tests, said the human challenge study will now be reviewed by a specially convened ethics committee and Britain’s Medicines and Healthcare products Regulatory Agency, its version of the U.S.

He said possible candidates include those vaccines that have proven themselves in large-scale Phase 3 trials, or they may be earlier in their development but potentially more effective.

Hedging its bets, the British government has signed an agreement for 100 million doses of a vaccine being tested in trials by Oxford University and AstraZeneca, as well as inking deals with Pfizer, Sanofi and Valneva.

But the British government hopes challenge trials will help accelerate the study of vaccines in mid-stages of development.

Christopher Chiu, an immunologist at Imperial College who is one of the lead investigators in the study, said the British human challenge model would be able to tell within 10 weeks how effective a coronavirus vaccine is. .

In the modern era, challenge trials have been used extensively to study and find treatments for influenza, malaria, typhoid, dengue fever and cholera.

If a clinical subject became seriously ill or died in the British challenge trials, the scientists, pharmaceutical companies and the government could be blamed and condemned as reckless.

Peter Openshaw, an immunologist at Imperial College London and co-investigator on human challenge consortium, said that “deliberately infecting volunteers with a known human pathogen is never undertaken lightly.”.

But, “it is really vital that we move as fast as possible toward getting effective vaccines and other treatments for covid-19, and challenge studies have the potential to accelerate and de-risk the development of novel drugs and vaccines.”.

With that in mind, he said, the need for human challenge trials is clear.

In the United States, the government has been funding researchers at Colorado State University to develop a potential challenge strain of the coronavirus that could be used if such experiments become necessary?

vaccine trials projected to reach a decisive milestone that will show whether the vaccines are working by the end of this year or early next year, many scientists said the resource is not likely to be needed.