Washington, Dec 9 (IANS): The US Food and Drug Administration (FDA) has granted emergency use authorisation to AstraZeneca's monoclonal antibody treatment for Covid-19 among people with weakened immune systems.

AstraZeneca's Evusheld -- an injectable monoclonal antibody cocktail -- has been approved for pre-exposure prophylaxis, or PrEP, against Covid-19 among individuals aged 12 and older and who are not currently infected with the SARS-CoV-2 virus and who have not recently been exposed to an individual infected with SARS-CoV-2.

Until now, such laboratory-produced antibodies have been authorised only as early treatment of Covid-19 or as preventive therapy for high-risk people immediately after close contact with someone who has tested positive.

"Vaccines have proven to be the best defence available against Covid-19. However, there are certain immune compromised individuals who may not mount an adequate immune response to Covid-19 vaccination, or those who have a history of severe adverse reactions to a Covid-19 vaccine and therefore cannot receive one and need an alternative prevention option," said Patrizia Cavazzoni, director of the FDA's Centre for Drug Evaluation and Research, in a statement.

"Today's action authorises the use of the combination of two monoclonal antibodies to reduce the risk of developing Covid-19 in these individuals," Cavazzoni said.

One dose of Evusheld, administered as two separate consecutive intramuscular injections (one injection per monoclonal antibody, given in immediate succession), may be effective for pre-exposure prevention for six months.

The therapy is also an option for the rare people who have histories of severe adverse reactions to a Covid vaccine or its components. However, pre-exposure prevention with Evusheld is not a substitute for vaccination in individuals for whom Covid-19 vaccination is recommended, the FDA said.

The cocktail is made up of Tixagevimab and cilgavimab -- long-acting monoclonal antibodies that are specifically directed against the spike protein of SARS-CoV-2. It is designed to block the virus' attachment and entry into human cells. Tixagevimab and cilgavimab bind to different, non-overlapping sites on the spike protein of the virus, the FDA said.

A recent large placebo-controlled clinical trial showed that the cocktail is about 83 per cent effective at preventing symptomatic disease during such an interval. The therapy involves getting preventive injections as often as every six months.

However, it is not clear how the super mutant Omicron variant of the coronavirus might affect the efficacy of the monoclonal antibody therapy.

Meanwhile, AstraZeneca said it is testing Evusheld against the new variant, that has so far spread to 57 countries.