A group of medical experts convened by the US Centers for Disease Control and Prevention (CDC) was set to hold a meeting on the J&J COVID vaccine on Wednesday, and might recommend new restrictions on its use.
During the live-streamed event, the experts are expected to provide an update on the safety of the shot, which has so far been administered to 7.2 million Americans.
Health authorities announced a pause on Tuesday after six women aged 18 to 48 developed a rare disorder characterized by blood clots in the brain and low platelets.
One person has so far died and another is in critical condition.
CDC director Rochelle Walensky told reporters on Wednesday that two of the women also had abdominal clots, and the symptoms are consistent with rare side effects from the AstraZeneca vaccine seen in Europe.
“I want to share with you my confidence in the system that we have in place,” she said.
“Jointly, CDC and FDA (Food and Drug Administration) were able to identify these rare events and act quickly to alert health care providers, as well as the public.”
People who received the J&J vaccine are being urged to report if they experience symptoms including severe headache, abdominal pain, leg pain or shortness of breath.
Doctors meanwhile have been told not to treat such clots with the common thinner heparin, because this might make the condition worse, based on the leading biological hypothesis for what is driving the effect.
Both the J&J and AstraZeneca vaccines are based on adenovirus vector technology, which is now under scrutiny.
Other COVID adenovirus vector vaccines include Russia’s Sputnik V and China’s CanSino.
Cases involving brain or abdomen clots together with low platelets have not been linked to the Pfizer nor Moderna vaccines, which are based on messenger RNA technology.
The meeting of the Advisory Committee on Immunization Practices (ACIP) begins at 1:30 pm (1730 GMT) and the participants are scheduled to hold a vote on “updated recommendations for use” sometime after 3:30 pm (1930 GMT).
Restrictions might include recommending that only people beyond a certain age receive it.
The reasoning is that the older people get, the more they risk developing a serious form of COVID, and the more it is in their interest to be vaccinated despite potential side effects.
But US officials have also emphasized that they have more than enough supply lined up of the Moderna and Pfizer vaccines to cover the adult population by the end of July, which further complicates the picture.