An antibody drug helped prevent symptomatic COVID-19 among nursing home residents and staff treated in a clinical trial run by the medicine’s maker, Eli Lilly, and federal government researchers.

Study results, which were announced Thursday by Lilly, showed participants given the drug developed coronavirus disease at a significantly lower rate than those who received placebo. In a sub-group of nearly 300 residents enrolled in the study, for example, treatment appeared to reduce the risk of contracting COVID-19 by up to 80%, Lilly said.

The data are an important finding as elderly residents in nursing homes or other assisted living facilities are particularly vulnerable to infection and at higher risk of poor outcomes from COVID-19.

Lilly’s drug, called bamlanivimab, is currently authorized by the Food and Drug Administration to treat mild or moderate COVID-19 in groups considered high risk, such as adults older than 65 or who have underlying conditions like diabetes. Thursday’s results are the first evidence preventive treatment with an antibody drug like bamlanivimab can protect against COVID-19.

Lilly is discussing the study data with the FDA and plans to soon ask the agency to broaden bamlanivimab’s authorization to cover preventive use in nursing homes, Janelle Sabo, head of Lilly’s COVID-19 therapeutics platform, confirmed to BioPharma Dive.

But as states across the U.S. prioritize immunizing nursing home residents with the two available coronavirus vaccines, the window for using antibody drugs like bamlanivimab may be shrinking.

“The horse is a little bit out of the barn at this point,” said Thomas Russo, an infectious disease physician and division chief at the University of Buffalo’s Jacobs School of Medicine. “The space for monoclonal antibodies in treating the most vulnerable is shrinking.”

Vaccinations take time, however, as the two doses required for both Pfizer’s and Moderna’s shots are spaced several weeks apart. And the initial U.S. rollout, while gathering pace in recent weeks, has gone poorly, with only about half of all doses distributed actually administered. Just under two million doses had been administered to residents of long-term care facilities through Jan. 20, according to data from the Centers for Disease Control and Prevention.

“We know that the rollout of vaccines has been slower than we’d all like,” said Lilly’s Sabo. Bamlanivimab, she added, could play a role in the interim or for vulnerable people who can’t or won’t be vaccinated.

Proving a preventive benefit to antibody drugs in a broader adult population would also help, although Lilly does not have a trial ongoing testing prophylactic treatment outside of nursing homes. Regeneron, which developed an antibody drug that’s also authorized in the U.S. for treatment of mild to moderate COVID-19, is studying preventive use of its treatment among household contacts of people infected with SARS-CoV-2.

Rollout of Lilly’s and Regeneron’s antibody drugs has also faltered, hamstrung by difficulties in administering treatment, which must be infused in a healthcare setting and given soon after symptoms start.

Government officials have pushed for broader uptake of the drugs in recent weeks. But many of the logistical challenges have yet to be solved. Some 550,000 doses of bamlanivimab and Regeneron’s treatment have been delivered to states through Jan. 19, according to the federal government.

“We need creative ideas on how to get [antibody drugs] to people,” said Preeti Malani, an infectious disease physician at the University of Michigan, in an interview. She noted, for instance, efforts to treat eligible people in their own homes, rather than have them come into an infusion clinic.

Lilly’s study could also be an example. The unusual trial used retrofitted recreational vehicles, or RVs, as mobile infusion clinics to reach residents and staff of nursing homes where a coronavirus infection had recently been confirmed.

Participants were treated with a 4200 milligram dose of bamlanivimab and tested weekly for SARS-CoV-2 infection. After 8 weeks, there were significantly fewer cases of symptomatic COVID-19 among residents and staff given the drug than those who received placebo.

Lilly did not provide detailed data in its statement, but said full results would be submitted for publication in a peer-reviewed medical journal. No deaths attributed to COVID-19 were recorded among participants given Lilly’s drug, Lilly said.

While the study focused on preventive use, some 130 volunteers enrolled in the trial were already infected with SARS-CoV-2 and formed an exploratory cohort assessing bamlanivimab treatment.

There, the data were similar to the interim results from an earlier trial that led to the drug’s emergency authorization as a treatment for mild-to-moderate COVID-19, according to Sabo.

Complete results from that trial, called BLAZE-1, were published in JAMA Thursday. The study findings also included new data from an experimental combination pairing bamlanivimab with another antibody drug developed by Lilly and partner Junshi Biosciences.