AstraZeneca has requested emergency use authorisation from US regulators for its new treatment to prevent Covid-19 for people who respond poorly to vaccines because of a weakened immune system.

In a statement on Tuesday, the Anglo-Swedish drugmaker said it included data in its filing with the Food and Drug Administration from a late-stage trial that showed the drug reduced the risk of people developing any Covid-19 symptoms by 77 percent.

The antibody therapy called AZD7442 could protect people who do not have a strong enough immune response to Covid-19 vaccines or to supplement a vaccination course for those, such as military personnel, who need to booster their protection further, AstraZeneca has said.

While vaccines rely on an intact immune system to develop targeted antibodies and infection-fighting cells, AZD7442 contains lab-made antibodies designed to linger in the body for months to contain the virus in case of an infection.

A US authorisation for AZD7442 - based on two antibodies discovered by Vanderbilt University Medical Center in the United States - could be a major win for AstraZeneca, whose widely used Covid-19 vaccine has yet to be approved by US authorities.

Talks regarding supply agreements for AZD7442 are ongoing with the United States and other governments, AstraZeneca said.

Covid-19 therapies based on the same class of monoclonal antibodies are being developed by rivals Regeneron, Eli Lilly and GlaxoSmithKline with partner Vir, competing for a role in Covid-19 treatment and prevention. But Astra's filing has cemented its lead in prevention.

That contrasts with delays in Astra's quest for approval for its Covid-19 vaccine Vaxzevria in the United States, where the vast majority of those willing to get immunised have received shots from the Pfizer-BioNTech alliance, Moderna or Johnson & Johnson.

Astra said in July it expected to seek US approval for the vaccine in the second half of this year.

Trial results on the AZD7442 therapy, first published in August, were taken three months after injection but the company hopes it can tout the shot as a year-long shield as trial investigators will follow up with participants as far out as 15 months.