A panel of outside advisers to the U.S. Centers for Disease Control on Thursday recommended Americans receive either the Pfizer or Moderna vaccines over Johnson & Johnson's dose, due to rare but sometimes fatal cases of blood clotting that have been reported in recipients of J&J’s shot.
The CDC itself still needs to sign off on the guidance.
But the agency did say incidents of blood clots are higher in both men and women than has been previously reported – with the highest rates in women under 50.
The CDC has said at least nine people in the U.S. have died following blood clotting incidents.
Members of the advisory panel also said J&J's vaccine is less effective in preventing COVID-19 than the other two authorized vaccines.
In a presentation to the panel, a leading J&J scientist said the vaccine generates a strong and long-lasting immune response with just a single shot.
J&J's vaccine uses a modified version of a virus to spur immunity in recipients, while Pfizer and Moderna use messenger RNA – or mRNA - technology that makes proteins to trigger an immune response.
J&J's one-dose vaccine received emergency use authorization in March. In April, U.S. regulators paused administering the vaccine for 10 days in order to investigate the blood clotting.
A CDC scientist on Thursday said that the rate of deaths from the clots did not decrease after the pause in April.
According to CDC data, around 16 million Americans received J&J's vaccine out of more than 200 million fully vaccinated people.