The CDC’s Advisory Committee on Immunization Practices focused on safety data from the company and CDC vaccine experts, including details about the cases in the six women, who were between the ages of 18 and 48. They developed symptoms, most often headaches, six to 13 days after vaccination. One vaccine recipient, a Virginia woman, died in March, and another is in critical condition, health officials have said.
Two of six patients have been discharged, and three remain in the hospital.
A CDC official and an executive from Johnson & Johnson described the six cases in the greatest detail yet. All of the women were White, and only one person was taking hormonal contraceptives that can cause blood clots, suggesting that was not the reason for the clots.
Tom Shimabukuro, of the vaccine safety team at CDC, explained that the rare, severe clots were especially alarming because they were accompanied by low levels of blood cells involved in clotting — a combination virtually unheard of among healthy, young people.
“We have a picture where we have clots forming in large [blood] vessels in the presence of low platelets, so it’s kind of a paradox here,” Shimabukuro said. “This is unusual — it usually doesn’t happen.”
The combination has also been seen, rarely, among people who received vaccine developed by AstraZeneca and University of Oxford. Four of those vaccine recipients were treated with heparin initially, an anticoagulant that is not recommended because the events closely resemble an immune-triggered reaction to the drug that could worsen the clots.
Some of the women had blood clots in other parts of their body, and Shimabukuro said that the agency would cast a wider net, in looking for clotting accompanied by low levels of platelets.
In a company presentation, Aran Maree, chief medical officer for Janssen, the division of Johnson & Johnson that developed the vaccine, also presented data on two cases of clots in people who received vaccine in the clinical trial, one of whom was a 25-year-old man with a hallmark of the symptoms.
“I’d like to reiterate that, based on the current data, Johnson & Johnson believes the overall benefit risk profile for a vaccine is positive across the population for which it is authorized,” Maree said.
It’s not clear what action the advisory committee will take. It could vote to continue the overall pause for a few weeks, or pause use for certain age groups or people.
The blood clots are similar to those reported by several European countries after the use of AstraZeneca’s coronavirus vaccine, which uses a similar technology. Several experts said it is necessary to understand whether the risks of the vaccines outweigh the benefits for certain groups of people. But they also said the risk of developing a clot after receiving the vaccine appears far lower than the chance of a clotting issue caused by a severe covid-19 infection or from using hormonal birth control, such as oral contraceptives.
The immediate practical effect of the recommended pause was limited because the overwhelming majority of the 192 million shots administered in the United States have been made by Pfizer-BioNTech and Moderna using a different technology.
FDA and CDC officials said they decided to recommend pausing the use of Johnson & Johnson’s one-shot vaccine — which is the United States’ third authorized coronavirus shot — because they were worried about the possible harm if clinicians did not know how to detect, manage and treat the complication. They want to make sure medical professionals are aware that blood clots potentially associated with a vaccine should not be treated with a conventional drug, heparin, which could exacerbate the clots.
Officials also want to “stimulate reporting” in the event that there are additional cases that could help clarify the syndrome or risk factors, Anne Schuchat, CDC’s principal deputy director, said in an interview Tuesday.
“We recognize that it came kind of quickly for the partners and the patients who had appointments scheduled. But once we had the information and pulled together the concern about the potential ongoing harm, if patients had this very rare, severe event and aren’t managed appropriately, it made us want to get the information out to the health-care community before additional vaccinations with this product continue,” Schuchat said.
In many ways, the scenario playing out in the United States echoes what happened in Europe last month as rare clotting events began to be recognized in early March among younger adults, predominantly women, who had recently received the vaccine developed by AstraZeneca and University of Oxford.
The rare but alarming clotting cases in Europe caused some countries to suspend vaccination altogether, and scientists began to study whether the clots were connected to vaccination. A societal debate about the risk tolerance of regulators and the possible effect of vaccination suspensions on hesitancy erupted as countries halted vaccinations in the middle of a pandemic. As time went on, more cases were recognized and studied.
Out of 34 million people given vaccinations in Europe, regulators there have identified 169 cases of blood brain clots and 53 cases of abdominal clots that raised suspicion.
After weeks of scientific work and meetings, European scientists and regulators concluded that the clotting events were possibly linked to the vaccine — and determined a diagnostic test and a treatment. Many countries have restricted use of the AstraZeneca-Oxford vaccine to older adults as a result of the safety signal.
The type of blood brain clots identified in the six U.S. cases result in a condition called cerebral venous sinus thrombosis. The normal rate of those blood clots in the general population is 2 to 14 per million people in a year. What stood out to scientists and physicians was that these clots were accompanied by very low levels of platelets, the blood cells involved in clotting. That combination of symptoms — a risk of bleeding and clotting at the same time — sounds counterintuitive but occurs in rare immune responses, when the body creates antibodies that bind to platelets, causing them to be activated and also consumed.
A similar syndrome occurs in some patients after exposure to the anticoagulant heparin, which triggered warnings not to use that drug when treating these patients.
Since the U.S. vaccination program began Dec. 14, safety experts from the CDC vaccine advisory committee have been monitoring data weekly. That vaccine safety subgroup, which reviews safety information from several surveillance systems, is scheduled to present data Wednesday.
Initial information about the six cases was detected in a vaccine-monitoring system run by the CDC and the FDA. That three-decade-old system, known as the Vaccine Adverse Event Reporting System, or VAERS, is an early-warning platform that also collects information about possible side effects or health problems after vaccination. It monitors unusual or unexpected patterns that require a closer look. Anyone can report a reaction or injury, including health-care providers, patients and patients’ representatives, such as caregivers or attorneys.
The recommendation to pause the Johnson & Johnson rollout resulted in swift action, with the U.S. military and many states announcing they were suspending the use of the single-shot. More than 7.1 million doses of Johnson & Johnson’s vaccine had been administered nationwide as of Tuesday. About 16 million doses have been delivered to states and territories, and through federal channels, since the beginning of March.
This is a developing story. It will be updated.