The Food and Drug Administration authorized the first injectable monoclonal antibody cocktail for long-term prevention of Covid-19 among people with weakened immune systems before they have been exposed to the coronavirus.

The FDA issued an emergency use authorization Wednesday for AstraZeneca’s antibody cocktail, Evusheld, for what is known as pre-exposure prophylaxis, or PrEP, against Covid-19.

To date, such laboratory-produced antibodies have been authorized 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.

Evusheld can be used as PrEP by people ages 12 and older who are moderately to severely immunocompromised and may not get adequate immune responses from a Covid vaccine. The therapy is also an option for the rare people who have histories of severe adverse reactions to a Covid vaccine or its components.

The AstraZeneca therapy involves getting preventive injections as often as every six months. According to a large placebo-controlled clinical trial, the cocktail is about 83 percent effective at preventing symptomatic disease during such an interval.