UPDATE: On Wednesday, Eli Lilly gave more detail on why, for now, no new patients are being enrolled in a trial of its monoclonal antibody treatment. The pause, recommended by an independent review board, affects only a trial of patients in the hospital. The move does not affect a separate trial of the drug in patients with mild to moderate COVID-19; it is proceeding as planned.

President Donald Trump seems to have recovered from COVID-19 after being treated with a drug that contains potent immune-system proteins called monoclonal antibodies. But on Tuesday, an independent review board recommended pausing a trial of a similar drug, citing a potential safety concern.

Such pauses are not uncommon, and the drug, made by Eli Lilly, could turn out to be perfectly safe. The bigger question is whether it — along with the one that Trump got, made by Regeneron — will make much difference in fighting the pandemic.

The answer is far from clear, infectious disease experts say.

Both companies are seeking emergency approval to distribute their drugs more widely, citing promising outcomes in several hundred patients.

But the results were announced in news releases and have not yet been vetted for publication in medical journals, making any conclusions premature, said Paul A. Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia. There is no evidence yet that either drug saves lives, he said.

And in the case of Regeneron’s drug, it seemed to alleviate symptoms only in those patients whose immune systems had not yet made their own antibodies. Most patients infected with the coronavirus do produce their own antibodies, so they may not get much benefit from Regeneron’s synthetic version, Offit said.

“They cherry-picked data,” he said of company officials.

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A brief review: Antibodies are Y-shaped proteins that can recognize and latch onto the surface of a virus, preventing it from infecting a cell. But some do so better than others, and many have no anti-infective powers at all.

The U.S. Food and Drug Administration has granted emergency approval for doctors to treat COVID-19 patients with plasma — the antibody-laden, liquid portion of blood donated by those who have recovered from the disease. But plasma contains antibodies with a wide range of potency.

Monoclonal antibodies, in theory, are a better approach, said La Salle University biologist Brian DeHaven, who studies how the immune system responds to viruses. Scientists identify only the most potent varieties of these antiviral proteins and “clone,” or culture, them in large steel tanks. They are administered by intravenous infusion.

But timing is key, said Zachary Klase, an associate professor in the department of pharmacology and physiology at Drexel University College of Medicine. If such a treatment is administered after the start of symptoms, in most patients the immune system is already winning the battle — even though they still feel ill.

“With a lot of viruses, by the time you feel sick, the amount of virus in you has already peaked and is coming back down,” he said.

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The only viral disease for which monoclonal antibodies have been shown to save lives is Ebola. For most viruses, such drugs make more sense as a preventive measure, said Offit, the CHOP physician. One such drug, Synagis, is given to high-risk infants to prevent infection with a microbe called respiratory syncytial virus.

Nevertheless, monoclonal antibodies may yet prove useful against COVID-19, said John Mellors, who is developing such a treatment at the University of Pittsburgh and UPMC. For high-risk patients, the drugs might make sense as a treatment or as a preventive measure, he said.

“Let’s say you’re in a nursing home, and somebody has COVID, and there’s a whole wing of older people,” said Mellors, chair of infectious diseases at UPMC. “You’d give them the antibodies.”

But further study is needed, he said. And scientists must review the potential safety issue in the version made by Eli Lilly. Details were scarce Tuesday, but the Indianapolis-based company said the review board had recommended a pause in enrolling new patients “out of an abundance of caution.”

And the drugs are likely to be expensive. To judge from monoclonal antibodies that are used to treat cancer, they could cost several thousand dollars per dose. A vaccine for COVID-19 would be far less expensive, and therefore easier to make and deliver to millions of people, said DeHaven, the La Salle scientist.

Still, Trump is a convert, describing his Regeneron antibody treatment as a “cure.”

Not so fast. The president received a variety of treatments, and it is not clear which, if any, aided his recovery. Offit said:

“He may have had the same outcome if he wasn’t treated at all.”