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A panel of the CDC’s outside vaccine advisers voted unanimously Thursday to back recommendations preferring Pfizer and Moderna’s COVID-19 vaccines over Johnson & Johnson’s, in light of new data presented to the panel about the risk of a rare blood clotting side effect linked to the Johnson & Johnson shot.

Nine deaths linked to the clotting issue have been identified following receipt of the Johnson & Johnson vaccine, out of more than 16 million people first vaccinated with that single-shot vaccine in the United States. Seven of the deaths have been in women and two were in men.

The advisers said Johnson & Johnson’s shots should be reserved for cases in which people could not receive, or did not want, the widely-available Pfizer and Moderna vaccines.

A total of 57 confirmed cases have been identified of the rare clotting side effect. Federal health officials told the CDC’s Advisory Committee on Immunization Practices (ACIP) that rates of the side effect remained rare, reported at a rate of 3.8 per million doses given. The highest risk appears to be in adult women under 50 years old. 

“I don’t feel comfortable, in the context of plenty of mRNA vaccines around, not being clear, not having the ACIP make a clear statement that reflects the recognition that we have of the concerns about these rare but oftentimes fatal side effects,” said Dr. Beth Bell, a member of the committee.

All confirmed cases have occurred within a month after vaccination, typically about nine days after. The agency told the panel that very rare but fatal cases of the side effect appeared to end in death quickly, despite the best efforts of doctors to treat it. 

“We’ve been struck on reviewing these cases by how rapidly patient status deteriorates, results in death,” said CDC’s Dr. Isaac See.

See told the panel that the agency was unsure what the exact “background rate” of these clotting events was in the general population, but unpublished data estimated that they typically occur at a rate that “is much, much lower than what we see here. A couple orders of magnitude.”

The meeting comes after the Food and Drug Administration moved on Tuesday to amend the J&J shot’s emergency use authorization, adding more detail to the warnings provided with the shot discussing the risk of the clotting events known as thrombosis with thrombocytopenia syndrome.

The FDA also added a contraindication for people who experienced the side effect after their first shot of Johnson & Johnson, or from other shots based on a similar adenovirus vector technology like the one developed by AstraZeneca, from getting a booster shot from the same type of vaccine. 

All recipients of Johnson & Johnson’s vaccine are recommended to receive a booster shot, two months after they were first vaccinated. Out of more than 2.7 million boosters of Johnson & Johnson’s vaccine administered in the U.S., most have gone to people who received a different brand for their first shot. 

No cases of the side effect have so far been reported among people who received Johnson & Johnson as a second dose.

While most communities were at one point offered Johnson & Johnson’s vaccines, federal health officials told the panel that jurisdictions had deployed the single-shot vaccine as a preferred option to offer vaccination in “more transient or transitional settings” like homeless shelters or airport clinics, or for people who preferred it over Pfizer or Moderna’s shots.

“We have patients who are coming into our public health clinics who are really saying, ‘I don’t want the mRNA vaccine for a variety of reasons, not contraindications but those reasons, and I want that [Johnson & Johnson] Janssen vaccine,'” National Association of County and City Health Officials’s Dr. Matthew Zahn told the committee. 

Federal health officials have grappled with the benefits and risks linked to Johnson & Johnson’s vaccine for months, especially since some studies have found it might offer less protection against COVID-19 infection compared to Pfizer and Moderna’s shots. 

The CDC’s advisers have also met previously to weigh the risks around the vaccine and Guillain-Barré syndrome, a rare nervous system disorder that most recover from but can occasionally result in paralysis. 

The Biden administration’s top doctors have urged caution around lab studies showing the other vaccines might provide stronger immunity than Johnson & Johnson. The drugmaker has also cited early research suggesting that its vaccine might offer better immunity as a booster shot for Pfizer and Moderna recipients to guard against new variants, as opposed to them seeking out a third mRNA dose.

However, the committee acknowledged the risks associated with the J&J vaccine appeared to have broadened since they last considered updating the vaccine’s recommendations. 

“I would not recommend a Janssen vaccine to my family members. On the other hand, I think we do have to recognize that different people make different choices and if they are appropriately informed, I don’t think we should remove that option from them,” said Bell.