Usually, clinical trials are paused because a volunteer has suffered a side effect or become ill, but the company did not say what happened.
"Safety is of the (utmost) importance to Lilly," a spokesperson told CNN by email.
It said the trial's Data Safety Monitoring Board (DSMB), an independent group of medical experts who monitor clinical trials, recommended the pause.
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"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 to cautiously ensure the safety of the patients participating in this study," the company said in the statement.
Lilly is testing a combination of two lab-engineered immune system proteins called monoclonal antibodies to treat severely ill patients with coronavirus. It is similar to the treatment made by Regeneron that was given to President Trump earlier this month.
The idea behind monoclonal antibody treatments is to give the immune system a head start on fighting the virus. The treatments use antibodies demonstrated to hone in on the coronavirus and neutralize it the most effectively. They are infused and patients can have reactions to the infusions.