Even though young, healthy participants are unlikely to be seriously sickened or killed, the virus is unpredictable and the long-term consequences of an infection are unknown, with the pandemic having started only months ago.

Beyond the ethical reservations, scientists also posed practical questions about the London researchers’ plan to compare vaccines by inoculating people and then dripping virus into their noses.

For one thing, scientists stressed that several vaccine makers had already distanced themselves from the idea, known as a human challenge trial. The researchers - working with Imperial College London and hVivo, a company specializing in such studies, with backing from the British government - have not said what vaccines they will test.

There are also steep limits to what scientists can learn about real-world transmission from exposing people in secure isolation units. Does a vaccine that protects healthy, younger volunteers - the only group eligible for deliberate infection - also help the people most endangered by the coronavirus, older adults or people with pre-existing conditions?

And is dripping virus into a volunteer’s nose anything like the exposure people receive at work or in their homes?

“Is it breathed out, sneezed out, do you sniff it all in one fell chunk of virus coming at you?” said John Moore, a professor of microbiology and immunology at Weill Cornell Medical College. “No one really knows. It’s so hard to model.”

For now, scientists overseeing the trial, scheduled to begin in January, said they would use the antiviral medicine remdesivir to treat volunteers before the onset of symptoms. The British government, under fire for its pandemic response, hopes the trial will accelerate the development of vaccines. But scientists questioned whether the fierce race for immunization had unduly influenced plans for a human challenge trial.

“It’s a race for money and glory,” Prof. Moore said. “That’s the reality of it.”