On Jun. 16, 2021, the University of Oxford announced the Randomised Evaluation of COVID-19 Therapy (RECOVERY) trial demonstrated that the investigational antibody combination developed by Regeneron reduced the risk of death when given to patients hospitalised with severe COVID-19 who have not mounted a natural antibody response of their own.

The treatment uses a combination of two monoclonal antibodies (casirivimab and imdevimab, known as REGEN-COV in the US) that bind specifically to two different sites on the coronavirus spike protein, neutralising the ability of the virus to infect cells.

Previous studies in non-hospitalised COVID-19 patients have shown that the treatment reduces viral load, shortens the time to resolution of symptoms, and significantly reduces the risk of hospitalisation or death. In a small trial in hospitalised patients, preliminary evidence suggested a clinical benefit in patients who had not mounted a natural antibody response of their own when they entered the trial (were seronegative). RECOVERY is the first trial large enough to determine definitively whether this treatment reduces mortality in patients hospitalised with severe COVID-19.

Between 18 September 2020 and 22 May 2021, 9785 patients hospitalised with COVID-19 were randomly allocated to receive usual care plus the antibody combination treatment (casirivimab 4g with imdevimab 4g by intravenous infusion) or usual care alone as part of the RECOVERY trial. Of these, about one-third were seronegative at baseline (ie they had not mounted a natural antibody response of their own), one-half were seropositive (ie they had already developed natural antibodies), and one-sixth had unknown serostatus. Among patients who received usual care alone, 28-day mortality was twice as high in those who were seronegative (30%) vs. those who were seropositive (15%) at study entry. Follow-up is complete for 99% of participants and preliminary results are announced today.

Among patients who were seronegative at baseline (the primary analysis population for this comparison), the antibody combination significantly reduced the primary outcome of 28-day mortality by one-fifth compared with usual care alone (24% of patients in the antibody combination group died vs 30% of patients in the usual care group; rate ratio 0·80; 95% confidence interval 0·70–0·91; p=0·001). Thus, for every 100 such patients treated with the antibody combination, there would be six fewer deaths.