A government-sponsored clinical trial that is testing an antibody treatment for Covid-19 developed by the drugmaker Eli Lilly has been paused because of a “potential safety concern,” according to emails that government officials sent on Tuesday to researchers at testing sites. The company confirmed the pause.
The news comes just a day after Johnson & Johnson announced the pause of its coronavirus vaccine trial because of a sick volunteer, and a month after AstraZeneca’s vaccine trial was halted over concerns about two participants who had fallen ill after getting the company’s vaccine.
The Eli Lilly trial was designed to test the benefits of the antibody therapy on hundreds of people hospitalized with Covid-19, compared with a placebo. All of the study participants also received another experimental drug, remdesivir, which has become commonly used to treat coronavirus patients. It is unclear how many volunteers were sick, and what the details of their illnesses were.
In large clinical trials, pauses are not unusual, and illnesses in volunteers are not necessarily the result of the experimental drug or vaccine. Such halts are meant to allow an independent board of scientific experts to review the data and determine whether the event may have been related to the treatment or occurred by chance.
“This is why clinical trials are essential,” said Marion Pepper, an immunologist at the University of Washington. “The safety of the product has to be empirically proven.”
Without more details on the nature of the illnesses that may have prompted the pause, however, the seriousness of the event remains unclear, Dr. Pepper added.
Enrollment for the Eli Lilly trial, which was sponsored by several branches of the National Institutes of Health and the Department of Veterans Affairs, among other organizations, had been continuing. But on Tuesday, multiple officials sent emails to researchers telling them to stop adding volunteers to the study out of an “abundance of caution.”
Several experts praised the trial’s sponsors for halting the trial to address the safety of their product, as AstraZeneca and Johnson & Johnson have done with their vaccines. “They are doing things by the book,” said Dr. Maricar Malinis, an infectious disease physician at Yale.
In a statement sent over email, Molly McCully, a spokeswoman for Eli Lilly, confirmed the pause. “Safety is of the utmost importance to Lilly,” she said. “Lilly is supportive of the decision by the independent D.S.M.B. to cautiously ensure the safety of the patients participating in this study,” she added, referring to the independent panel of experts, or the data and safety monitoring board.
The N.I.H. and the V.A. did not immediately respond to requests for comment.
Eli Lilly is one of several companies pursuing experimental treatments for Covid-19 that use monoclonal antibodies — mass-produced mimics of immune molecules that the human body produces in reaction to the coronavirus.
Eli Lilly’s product is similar to a treatment designed by the drug company Regeneron, which developed an antibody therapy given to President Trump after he tested positive for the coronavirus this month. Mr. Trump has promoted such treatments, without evidence, as a “cure” for his condition, and has suggested that their approval and widespread distribution could be imminent.
The week after the president was treated, both companies applied for emergency clearance for their products from the Food and Drug Administration. (Eli Lilly has applied for authorization of its drug for mild or moderate cases of Covid-19, not for use in hospitalized patients like those enrolled in the halted trial.)
The company is also trialing its antibody treatment in people who have recently tested positive for the coronavirus, and in nursing home residents and workers who may have been exposed.
Antibodies can block the coronavirus from infecting cells, and preliminary data from Eli Lilly and Regeneron have hinted that the immunity-mimicking treatments may be able to tamp down the amount of virus in infected people and reduce their symptoms. Eli Lilly also hopes to collect data to figure out whether antibodies can protect certain people from developing Covid-19 after encountering the virus.
Still, if monoclonal antibodies end up being linked to an unexpected side effect — which has not yet been conclusively shown — it will be crucial to figure out how and why these immune molecules are sickening people, said Akiko Iwasaki, an immunologist at Yale.
The news of the trial’s pause prompted a small decline in Eli Lilly’s stock on Tuesday afternoon.
Katie Thomas contributed reporting.