Merck & Co Inc has said it plans a large study of what could become the first pill to target the coronavirus in people at risk of severe Covid-19, but will no longer pursue the use of the experimental antiviral drug in hospitalised patients.

The company said it would study the drug molnupiravir in a phase 3 trial among Covid-19 patients out of hospitals who have at least one risk factor for poor outcomes, such as advanced age, obesity, or diabetes.

Trial participants can have had symptoms for no more than five days, shortened from seven days in a prior study.

“We had some encouraging signals in the outpatient trial. That was not the case in the inpatient trial, but that was not unexpected,” Roy Baynes, Merck's chief medical officer, told Reuters.