Shares of the Indianapolis-based company were rising nearly 1% to $202.91.
Eli Lilly said the trial of the drug, bamlanivimab, involved 965 participants - 299 nursing home residents and 666 staff members -who tested negative for the coronavirus. They were given either an intravenous infusion of the drug or a placebo.
After eight weeks, the participants who had received bamlanivimab were 57% less likely to develop symptomatic Covid-19 than those who had received the placebo.
Lilly said nursing home residents have an 80% lower risk of contracting Covid-19 compared with those who had received the placebo.
The company said there were four deaths among the residents attributed to Covid-19 and none of them were attributed to the group that had received bamlanivimab.
Eli Lilly said bamlanivimab was given emergency use authorization (by the U.S. Food and Drug Administration last October at a dose of 700 mg for non-hospitalized Covid patients.
"We are exceptionally pleased with these positive results, which showed bamlanivimab was able to help prevent Covid-19, substantially reducing symptomatic disease among nursing home residents, some of the most vulnerable members of our society," Daniel Skovronsky, Lilly's chief scientific officer, said in a statement.
The U.S. death toll from the virus is 406,196, according to Johns Hopkins University, the most in the world. Nursing home residents represent about a quarter of those fatalities, Bloomberg reported, citing data from the Centers for Medicare & Medicaid Services.
The number of people infected in the U.S. was 24,440,100.
As of Wednesday, nearly 36 million doses of Covid-19 vaccine from Pfizer (PFE) - Get Report and Moderna (MRNA) - Get Report had been distributed and under 2 million shots had been administered at nursing homes, according to the U.S. Centers for Disease Control and Prevention.