By Peter Loftus

Eli Lilly & Co. has started a study exploring whether its experimental Covid-19 drug can prevent infections among vulnerable residents and staff at nursing homes and other long-term care facilities.

Indianapolis-based Lilly said Monday that it is testing its antibody-based drug in senior homes that have had a recently diagnosed case of Covid-19, putting residents and staff at high risk of exposure.

The study, which aims to enroll up to 2,400 subjects, will track whether Lilly's drug reduces the rate of infection and disease in the weeks after dosing.

Lilly has been exploring whether the drug, code-named LY-CoV555, could treat other kinds of Covid-19 patients. Studies already under way are testing whether the drug is safe for hospitalized Covid-19 patients, and whether it can clear viral loads and keep patients with milder disease out of the hospital.

The company has said that if testing is successful, its drug could get government approval by the end of the year.

Nursing homes and other senior-care facilities have been hit hard by the new coronavirus, accounting for a large percentage of deaths during the pandemic. In recent weeks, the resurgence of the virus in Sunbelt states, which initially spread among younger people, has shown signs of reaching the elderly in care homes.

Lilly's experimental therapy belongs to a class of agents known as antibody drugs that are under development for treatment and perhaps prevention of Covid-19. Antibodies are proteins deployed by the immune system to fight foreign invaders such as viruses.

Public-health officials say antibody drugs, if successful in testing, could yield available doses sooner than vaccines and serve as a bridge to curbing the pandemic until any successful vaccines are produced at large scale.

Antibody drugs might also help protect older adults who may not get a sufficient immune boost from a vaccine.

Lilly's drug, developed in collaboration with Canadian biotechnology company AbCellera Biologics Inc., was derived from the blood sample of one of the first Americans to recover from Covid-19 earlier this year.

Researchers isolated antibodies in the blood sample that they thought helped the patient fight off the new coronavirus. The researchers then essentially cloned the antibodies to test whether they treat or prevent disease.

The new study in nursing homes is a final stage, or Phase 3, trial intended to generate data to support obtaining regulatory approval. Lilly is running the study in partnership with the National Institute of Allergy and Infectious Diseases.

To run the study, Lilly is using customized recreational vehicles to take the antibody and personnel to U.S. nursing homes where a resident or staff member has been infected. Researchers will give the antibody by intravenous infusion to others at the nursing home who consent.

"The idea behind it is to enable high-quality conduct of a clinical study at the point of care in a nursing home" without interrupting the day-to-day care of residents, said Janelle Sabo, Lilly's head of clinical innovation, systems and clinical supply chain.

Ms. Sabo said many nursing homes don't have the capacity to manage an experimental drug product or run an IV infusion center on site.

Lilly is also testing other experimental antibodies and will evaluate whether combinations work better than single antibodies. Regeneron Pharmaceuticals Inc. is also testing an experimental antibody to treat or prevent Covid-19.

Write to Peter Loftus at peter.loftus@wsj.com