WASHINGTON — Drugmakers Pfizer and BioNTech announced Tuesday they have formally submitted a request asking US officials for emergency authorization of their Covid-19 booster vaccine for people aged 18 and older.

The move follows research published by the companies in late October indicating a third shot is 95.6 percent effective against symptomatic infection, based on clinical trials carried out on 10,000 people.

The companies asked the US Food and Drug Administration to add the new population segment, amending emergency use authorization already granted in September for a third dose for everyone aged 65 and up, as well as people at high risk of developing severe Covid-19.

Pfizer’s initial proposal for approval of boosters to everyone aged 16 and over, which was backed by President Joe Biden’s administration, was rejected at that time.

Several countries have already approved Covid-19 booster shots to increase immunity in people who have already been vaccinated, but whose protection may have dipped after several months, per some studies.

The European Medicines Agency (EMA) approved a booster for over-18s at the beginning of October, allowing national regulators to decide which groups should be eligible first.