Regeneron Pharmaceuticals Inc's Covid-19 antibody cocktail reduces deaths in hospitalised patients who have not mounted their own antibody response, a large British study has found.

The therapy, REGEN-COV, has been granted emergency use authorisation for people with mild-to-moderate Covid-19 in the US, but results from the recovery trial provide the clearest evidence of its effectiveness among hospitalised patients.

It has found that the antibody therapy reduces by a fifth the 28-day mortality of people admitted to hospital with Covid-19 whose immune system has not mounted an antibody response, known as seronegative.

The result translates into six fewer deaths for every 100 seronegative patients treated with the therapy, researchers say, Reuters reports.