An expert committee on Thursday recommended a booster dose of Moderna's anti-Covid vaccine in the United States for certain at-risk groups, a month after making a similar decision for the Pfizer shot.

The opinion submitted by the advisory committee of the Food and Drug Administration -- composed of researchers, epidemiologists and infectious disease experts -- is not binding, but it is rare for the FDA not to follow it. 

After a day of debate, the experts decided to authorize a booster dose of Moderna for three categories of people: the over-65s, people aged between 18 and 64 who are at a higher risk of developing a severe version of the coronavirus, and those whose work may involve frequent exposure to the virus. 

The same definitions were adopted for the Pfizer booster. 

The last category includes supermarket workers, health workers, prisoners and people in homeless shelters. 

The booster can be administered six months after the second injection of Moderna's vaccine. The dosage is 50 micrograms, which is half the dosage of the initial shots. 

A Moderna representative told the panel the booster would help combat the Delta variant, at a time when vaccine effectiveness has been observed to decline over time against infection and mild cases of the disease, even though it remains effective against severe cases. 

The committee will meet again on Friday to deliver its verdict on a booster dose for the Johnson & Johnson vaccine. 

It will also discuss the results of a study published this week that looked at the possibility of using a different vaccine for the booster shot than that used for the initial dose, which at the moment is not authorized in the United States.

The study suggests that people who have received the Johnson & Johnson vaccine may benefit from a booster dose of a different, messenger-RNA vaccine such as Pfizer or Moderna. 

Almost 15 million Americans have received a dose of the J&J vaccine, and nearly 70 million are fully vaccinated with Moderna. 

If the FDA officially allows booster shots for these two vaccines, an expert committee from the Centers for Disease Prevention and Control (CDC) will in turn have to deliver its own opinion next week, at a meeting scheduled for Wednesday and Thursday.

As a final step, the CDC will have to publish precise recommendations for these injections, intended for healthcare professionals administering them.