LONDON — In the coming weeks, a small, carefully selected group of volunteers is expected to arrive on the 11th floor of a London hospital to be given what the rest of the world’s 7.8 billion people have been trying to avoid: a coronavirus infection.

They will be administered tiny droplets of the virus into their nostrils as part of a plan authorized by British regulators on Wednesday to deliberately infect unvaccinated volunteers with the coronavirus.

The scientists hope to eventually expose vaccinated people to the virus as a way of comparing the effectiveness of different vaccines. But before that, the project’s backers have to expose unvaccinated volunteers in order to determine the lowest dose of the virus that will reliably infect them.

As many as 90 people could be part of the trial, but the number could be lower if researchers can determine the right dose with fewer volunteers.

By controlling the amount of the virus people are subjected to and monitoring them from the moment they are infected, scientists are hoping to discover things about how the immune system responds to the coronavirus that would be impossible outside a lab — and to develop ways of directly comparing the efficacy of treatments and vaccines.

“We are going to learn an awful lot about the immunology of the virus,” Peter Openshaw, an Imperial College London professor involved in the study, said on Wednesday. He added that the study would be able “to accelerate not only understanding of diseases caused by infection, but also to accelerate the discovery of new treatments and of vaccines.”

The idea of such a study, called a human challenge trial, has been hotly debated since the early months of the pandemic.

In the past, scientists have deliberately exposed volunteers to diseases like typhoid and cholera to test vaccines. But infected people could be cured of those diseases; Covid-19 has no known cure, putting the scientists in charge of the British study in largely uncharted ethical territory.

In order to try to ensure that participants do not become seriously ill, the British study will be restricted to young, healthy volunteers, in an 18 to 30 age range.

But there have been severe Covid-19 cases even in those types of patients, and the long-term consequences of an infection are also largely unknown. The age restrictions also may make it difficult to translate the findings to older adults or people with pre-existing conditions, whose immune responses might be different and who are the target group for treatments and vaccines.

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What You Need to Know About the Vaccine Rollout

• • Providers in the U.S. are administering about 1.3 million doses of Covid-19 vaccines per day, on average. Almost 30 million people have received at least one dose, and about 7 million have been fully vaccinated. How many people have been vaccinated in your state?
  • The U.S. is far behind several other countries in getting its population vaccinated.
  • In the near future, travel may require digital documentation showing that passengers have been vaccinated or tested for the coronavirus.
  • When can you get the vaccine? What are the vaccine’s side effects? Is it safe to take during pregnancy? We’ve have answers to many of your questions.

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