A clinical trial for a potential COVID-19 treatment has been paused due to a possible safety issue. According to Eli Lilly (LLY 2.66%),  enrollment in the phase 3 study for its antibody drug LY-CoV555, has been paused by the National Institutes of Health (NIH), which is sponsoring antibody research in a program called ACTIV-3.

"Out of an abundance of caution, the ACTIV-3 independent data safety monitoring board (DSMB) has recommended a pause in enrollment," said Lilly spokeswoman Molly McCully in an emailed statement quoted by CNBC. "Lilly is supportive of the decision by the independent DSMB to cautiously ensure the safety of the patients participating in this study."

No further details have yet been released by the NIH or the company about what type of adverse event led to the pause.

A medical professional using a microscope.

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Lilly is testing LY-CoV555 in combination with remdesivir, the Gilead Sciences antiviral drug that has been repurposed as a COVID-19 treatment. LY-CoV555 is a monoclonal antibody -- a synthetic protein that can target specific antigens (molecules that trigger an immune response in the body).

News of the Lilly clinical trial suspension comes on the heels of a stoppage in Johnson & Johnson's late-stage testing of its JNJ-78436735 COVID-19 vaccine candidate. This apparently occurred because a participant in the trial developed an unexplained illness; the details haven't been provided.

Additionally, last month AstraZeneca suspended the phase 3 trial of its coronavirus vaccine candidate, AZD1222, due to an adverse event experienced by a participant in the U.K. While the company restarted the trial in that country, it has yet to resume the study of AZD1222 being conducted in the U.S.

It should be noted that in large-scale vaccine and drug trials, it is not unusual for at least one participant to experience adverse events or possible side effects that may or may not be related to the treatment being tested. More investigation is needed to determine why and how these recent ones occurred.