US limits use of J&J vaccine due to risk of rare blood clotsThe Food and Drug Administration (FDA)

The Food and Drug Administration (FDA) on Thursday limited the use of Johnson & Johnson's single-dose covid-19 vaccine in the United States after reviewing the risk of rare but serious blood clots.

In a statement, the federal agency said the emergency authorization of J&J's serum will now be restricted to those over 18 years of age for whom no other vaccines are available or medically appropriate, or who do not want to receive another vaccine.

The change is due to the risk of developing a type of rare and dangerous blood clots, called thrombosis with thrombocytopenia syndrome (TTS), after the administration of the vaccine.

"We have been closely monitoring the vaccine and the appearance of TTS after inoculation and used updated information from our security services to review the emergency authorization," said Dr. Peter Marks, director of the FDA's Center for Biological Evaluation Research.

According to official data, about 18 million doses of the J&J vaccine have been administered in the US, which represents about 7.7% of citizens who consider themselves fully vaccinated in the country.

The FDA has, however, highlighted the rarity of this dangerous condition, with 3.23 cases per million doses of the vaccine administered and nine confirmed deaths.

In December of last year, the US health authorities had already recommended the use of Pfizer/BioNRech and Moderna vaccines as preferred over that of Johnson & Johnson.

The US, the country hardest hit by the covid-19 pandemic with nearly a million deaths, has 66% of the population vaccinated with the full schedule.