Researchers in England are deliberately exposing volunteers to the coronavirus that causes COVID-19. The goal is to speed up the development of new vaccines and treatments.

But exposing people to a potentially fatal disease with no particularly effective therapy strikes some as unnecessary, if not unethical.

Human challenge experiments differ from other studies of COVID-19 in a very important respect. "The main difference is the control," says Christopher Chiu, an infectious disease researcher at Imperial College London and lead scientist for the challenge study.

He says with a challenge study, you know exactly when a person was exposed to the virus, and exactly how much virus they were exposed to.

Without knowing these things, you have to wait until people are exposed to the virus by chance.

"You'll end up having to recruit a lot more people, give a lot more people your candidate vaccine, before you'll see a result," Chiu says.

Indeed, it took many months and tens of thousands of volunteers to show that the Pfizer, Moderna and Johnson & Johnson vaccines worked. A challenge study could, in theory, have been able to show that with a fraction of that number and in far less time.

But it does mean exposing perfectly healthy people to a dangerous virus.

Chiu says he and his colleagues are acutely aware of that risk, and that's why they only plan to include certain kinds of people in their studies.

"We know from over a year of the pandemic now that young healthy adults are at very low risk of getting severe COVID," he says.

Chiu says in addition to involving only healthy young adults, the experiments will be carried out at the Royal Free Hospital in London, a facility with ample experience caring for COVID-19 patients. Volunteers will be treated at the first sign of illness, a time when therapies seem to be most effective.

But studying only young adults is one of the flaws people see in challenge studies to test the efficacy of an experimental vaccine, for example.

"The data you get from a challenge study about whether that works is not really going to give you the information you want," says Seema Shah, a lawyer and medical ethicist at Northwestern University Feinberg School of Medicine.

Shah says it's important to know if a vaccine works in older people, unhealthy people, or people from different ethnic backgrounds. You don't get that information from a challenge study.

"When this all started early in the pandemic there was one big reason for doing these challenge studies, and that was to accelerate vaccine testing," Shah says.

But now, multiple COVID-19 vaccines have been shown to work, so the original impetus has passed.

Another issue challenge studies must deal with is viral variants. Researchers would have to collect anew basic data, such as the minimum infectious dose for each new viral variant they wanted to study, a time-consuming process. Still, such studies may be useful for testing vaccines in the future.

For example, if coronavirus cases become less common, it will take far more than 30,000 or 40,000 volunteers to tell if a new a new vaccine is significantly better than the ones presently available. And it will make comparing vaccines head to head much easier.

"So in those circumstances a human challenge infection model could be very helpful," says Kanta Subbarao a virologist and director of the World Health Organization Collaborating Centre for Reference and Research on Influenza in Melbourne, Australia.

But she's concerned that not enough is known about the risks of being exposed to SARS-CoV-2, the virus that causes COVID-19, even for health young adults.

"We talked early on about whether we should even consider SARS-CoV-2, and said we would not," Subbarao says. "But we are exploring one of the common cold coronaviruses."

She thinks that work might provide clues to a universal coronavirus vaccine, one that would prevent both COVID-19 and the common cold.

Right now, Imperial College's Chiu and his colleagues are trying to determine the minimum amount of virus it takes to make someone sick with COVID-19. After that, they can turn their attention to testing new treatments and vaccines.

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