Hugh Montgomery, professor of intensive care medicine at University College London and the AZD7442 principal investigator, said: “These compelling results give me confidence that this long-acting antibody combination can provide my vulnerable patients with the long-lasting protection they urgently need to finally return to their everyday lives. 

“Importantly, six months of protection was maintained despite the surge of the delta variant among these high-risk participants who may not respond adequately to vaccination.”

A separate trial shows the drug also reduces the risk of severe Covid and death by 88 per cent when given within three days of the onset of symptoms.

The injection could be a breakthrough in treatment for elderly and immunocompromised people who cannot mount an adequate immune response against the virus even after vaccination. 

The research of both trials has been submitted for peer review and is expected to be published in the coming weeks. 

About one in 50 people does not make an adequate response to the vaccine, and the treatment could also be useful to helping protect at-risk groups such as elderly people living in care homes. 

Groups who could benefit include people with cancer who are being treated with chemotherapy, patients on dialysis, or those taking medication to suppress their immune systems such as people undergoing organ transplants, or with arthritis or multiple sclerosis. 

‘Potential to make a significant difference’

The drug, which only needs to be administered in a single dose, is a combination of two antibodies derived from immune B-cells donated by convalescent patients who had recovered from the virus. The antibodies bind to sites on the spike protein which the virus uses to enter cells. 

AstraZenca said it had been designed to last for three times as long as conventional antibodies and a single dose could give protection for up to 12 months. It is also currently in trials as a potential treatment for hospitalised Covid patients. 

Mene Pangalos, the executive vice president of biopharmaceuticals research and development at AstraZeneca, said: “AZD7442 is the only long-acting antibody with phase three data to demonstrate benefit in both pre-exposure prophylaxis and treatment of Covid-19 with one dose. 

“These new data add to the growing body of evidence supporting AZD7442’s potential to make a significant difference in the prevention and treatment of Covid-19.”

The company has now submitted the findings to regulators, who are deciding whether to grant approval to the new treatment. AstraZeneca has already agreed to supply the US government with 700,000 doses of AZD7442 if granted an emergency use authorization by the Food and Drug Administration, and has agreements to supply to other countries.