An unusual experiment to prevent nursing home staff members and residents from infection with the coronavirus has succeeded, the drug maker Eli Lilly announced on Thursday.

A drug containing monoclonal antibodies — laboratory-grown virus-fighters — prevented symptomatic infections in residents who were exposed to the virus, even the frail older people who are most vulnerable, according to preliminary results of a study conducted in partnership with the National Institutes of Health.

The researchers found an 80 percent reduction in infections among residents who got the drug, compared with those who got a placebo, and a 60 percent reduction among the staff, results that were highly statistically powerful, Eli Lilly said.