Regeneron’s antibody cocktail significantly reduces the risk of death among hospitalised Covid-19 patients whose own immune systems have failed to fight the virus, a large UK clinical trial has shown.

A combination of two antibodies called Regen-Cov, made by the US-based biotech company, cuts mortality by one-fifth among patients who did not mount an antibody response of their own, the Recovery study run by Oxford university found.

“It is the first time that any viral treatment has been shown to save lives in hospitalised Covid-19 patients,” said Martin Landray, Recovery’s joint chief investigator.

The findings, to be released later on Wednesday in a preprint paper, will provide a further boost for Regen-Cov, which is already well ahead of the competition in the corporate race to commercialise antiviral antibody treatments for Covid.

These are totally different to anti-inflammatory antibodies such as tocilizumab, which damp an overactive immune response that damages the patient’s own body rather than attacking the virus.

Global sales of Regen-Cov to the end of March amounted to $810m and future orders exceed $1bn, said Arsalan Azad, senior analyst at science analytics company Airfinity.

On Tuesday, one competitor, AstraZeneca, reported that its antibody cocktail failed to achieve its main target of preventing symptomatic disease among people exposed to the Sars-Cov-2 virus in a clinical trial conducted in the community, under quite different conditions to Recovery. Other companies in the Covid antibody field include Eli Lilly, GlaxoSmithKline and Celltrion.

The Recovery trial enrolled almost 10,000 patients in hospitals across the UK, whose blood was tested for antibodies against the virus.

One-third showed no trace of making their own antibodies. In this “seronegative” group, mortality was 30 per cent among those who received standard clinical care and just 24 per cent among those also given an intravenous infusion of Regen-Cov. The probability of that being a chance effect was less than one in a thousand.

“For every 100 seronegative patients treated with the antibody combination, there would be six fewer deaths,” said Landray. Regen-Cov also shortened the average hospital stay by four days. In contrast, the treatment had no effect on patients who had generated their own antibodies.

“Patients hospitalised with Covid-19 can be divided into two groups based on whether or not they have made antibodies to the virus,” said Fiona Watt, executive chair of the Medical Research Council which co-funded the study.

“If they do not have antibodies, then treatment with antibody-based drugs to the spike protein can reduce their risk of death and also time spent in hospital,” she said. “Patients who have made their own antibodies to the virus do not benefit from the new treatment, which is important information given the cost of drugs.”

Regen-Cov has emergency-use authorisation in the US but has not been licensed in the UK. Although the cost to the NHS is not yet known, it is likely to be in the £1,000 to £2,000 range per infusion, said Peter Horby, Recovery’s co-chief investigator.

An added cost would be of administering Sars-Cov-2 antibody tests to patients admitted to hospital with Covid, which is not yet standard procedure.