Drugmaker Eli Lilly said that it is pausing its late-stage trial for an antibody treatment for COVID-19 due to a "potential safety concern," but declined to state the nature of the issue.

"Safety is of the utmost importance to Lilly," a spokesperson said to CNN.

The news comes one day after Johnson & Johnson vaccine trial had to be halted due to an "unexplained illness" in one of the volunteers. Last month, AstraZeneca’s COVID-19 vaccine trial was paused to determine if a recipient’s illness was a side effect of the shot.

Temporary stoppages of large medical studies are relatively common. Few are made public in typical drug trials, but the work to make a coronavirus vaccine has raised the stakes on these kinds of complications.

Companies are required to investigate any serious or unexpected reaction that occurs during drug testing. Given that such tests are done on tens of thousands of people, some medical problems are a coincidence. In fact, one of the first steps the company said it will take is to determine if the person received the vaccine or a placebo.

"The trial, evaluating Lilly's investigational neutralizing antibody as a treatment for COVID-19 in hospitalized patients, is sponsored by the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health (NIH). Lilly is supportive of the decision by the independent DSMB (Data Safety Monitoring Board) to cautiously ensure the safety of the patients participating in this study," the company said in the statement.

The New York Times reports that Lilly's product is similar to the Regeneron compound that was given to President Trump after his diagnosis with COVID-19.

Shares of the drugmaker were down more than 3% on the news Tuesday.