A panel of outside advisers to the U.S. Centers for Disease Control and Prevention was considering on Thursday whether to recommend that Americans choose to receive one of the two authorized messenger RNA (mRNA) COVID-19 vaccines over Johnson & Johnson's shot, due to rare but sometimes fatal cases of blood clotting.

J&J's vaccine uses a technology based on a modified version of an adenovirus to spur immunity in recipients, while the Moderna and Pfizer/BioNTech vaccines use mRNA technology.

The CDC's Advisory Committee on Immunization practices was expected to vote on the issue on Thursday, after a subcommittee said it supported making the "preferential recommendation."

Cases of thrombosis with thrombocytopenia syndrome, which involves blood clots accompanied by a low level of platelets, have previously been reported in recipients of the J&J vaccine.

The CDC said that the rate of such incidents is higher than previously estimated, both in women and men.

At least nine people have died following the blood clotting incidents in the United States, the CDC has said.