U.S. researchers will create a strain of the coronavirus that could be used in possible vaccine trials called human challenge experiments, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said in an interview Friday.

The United States isn’t committed to embarking on such ethically fraught trials but has begun the process to create a stock of coronavirus strain that could be used to infect people, in case such trials become necessary, Fauci said. He called it a “Plan C or Plan D,” a preliminary step being taken because creating a strain that meets exacting regulatory standards will take months. Large, 30,000-person trials that are testing the effectiveness of experimental vaccines are likely to yield results sooner and provide much-needed safety data.

“You generally do [human challenge trials] if you don’t have enough infections in the community at any given time to get a signal from the vaccine,” Fauci said. “Unfortunately for us, we don’t have that problem — we have a lot of infections.”

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Typically, researchers test vaccines by inoculating people with the experimental drug or a placebo and waiting to see if those who got the real vaccine develop fewer cases or less-severe illness after being exposed to a virus in daily life. To tell if a coronavirus vaccine is 60% effective, Fauci has estimated that 150 to 160 people within the 30,000 participating in each trial need to become sick, for example.

In challenge trials, people would be exposed to the virus after an experimental vaccination. The idea has gained popular support through an organization called 1Day Sooner, which has signed up more than 30,000 volunteers eager to assume personal risk to speed up the scientific race for a vaccine. Reuters first reported Friday that the United States would begin taking steps toward preparing for such trials.

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Fauci and others have repeatedly argued that challenge experiments are likely to be slower, more ethically fraught and harder to scientifically interpret than many people appreciate. It takes months to create the coronavirus strains that could be used in such trials to infect people. Fauci said a viral strain would not be ready before the end of the year.

Challenge trials are routinely used against diseases such as malaria, which have rescue medications to prevent or moderate illness. There aren’t yet similar treatments for COVID-19, the disease caused by the coronavirus. There is also concern that the potential risks of a coronavirus challenge trial aren’t yet completely known, because doctors are still learning about the long-term effects of infection. Scientists and companies have said such trials won’t be seriously considered until better therapies exist.

Because challenge trials would probably select from people unlikely to develop severe illness and die of the disease, it also may be difficult to extrapolate results to understand whether the vaccine would protect people truly at risk from the virus.

William Phillips, 71, a Nobel laureate in physics, said he has signed up to be considered for challenge trials, even though he knows there is almost no chance he would be selected because of his age.

“As a scientist, what I want is data. And it just seemed to me that challenge trials are a way of getting reliable data more quickly, and more unambiguously, than a standard trial,” Phillips said. “From a dispassionate point of view, if you ask yourself: What is the risk-benefit ratio of doing something like this? Sure, you might kill a few people, to put it kind of bluntly, but think of how many people you save if you get a vaccine into circulation earlier.”

Even if a challenge trial were pursued, it would not replace other trials. The safety data gathered from the large trials will be essential. But important scientific questions could be answered by human challenge studies if the ethical, scientific and logistical concerns could be satisfied.

Fauci pointed out that scientists could ask targeted questions, learning whether a vaccine completely protects people against infection in their upper nasal passages, for example. Such studies might also help scientists understand how many viral particles are necessary for someone to become sick.

“We are in the process of getting the system going for developing a challenge stock [of coronavirus], even though we really don’t consider that something we really will have to operationalize,” Fauci said.