Doctors, scientists and public health experts are looking to Europe for clues, where a similar vaccine made by AstraZeneca – not yet licensed in the United States – has also been linked to a number of rare blood clots .
“It’s hard to say if it’s the same problem,” said Dr Richard Kuhn, Ph.D., a virology expert at Purdue University, “but it appears that the vaccine triggers an antibody response that activates platelets, leading to clots. “
While many experts were hesitant to say for sure whether there is a link, Schaffner said there was a growing consensus in the scientific community after none of these rare clots were linked to Pfizer vaccines or Moderna, which use mRNA, a different technology.
“I think we shouldn’t be shy about this anymore,” Schaffner said, adding that maybe it’s time “to come to terms with the fact that these are vaccine-induced but very rare events.”
But experts have warned that even though there is a link, current evidence suggests that the risk of developing a blood clot after receiving the Johnson & Johnson vaccine is extremely low – lower, in fact, than being struck by lightning.
In response to a report on one of six clot cases published in the New England Journal of Medicine, Johnson & Johnson wrote a response on Friday, insisting that a clear link has yet to be made.
“At present, there is insufficient evidence to establish a cause and effect relationship between these events and the [Johnson & Johnson] Janssen scientists said, adding that the vectors used in their vaccine and those of AstraZeneca are “substantially different” and that these differences could result in “quite different biological effects”. The researchers added that “more evidence is needed” to further clarify the cause of this clotting, associated with a low platelet count in people receiving the COVID-19 vaccine.
Next week, the Centers for Disease Control and Prevention’s independent advisory committee will review all the evidence and make a recommendation on resuming use of J&J vaccines.
Still with more questions than answers, scientists are exploring different theories as to why this type of vaccine – called a viral vector vaccine – could cause rare bleeding problems.
Vaccines work by causing our immune system to develop antibodies against a virus, and a dominant theory is that viral vector vaccines somehow trigger an abnormal immune response, leading to blood clots.
German scientists have identified a specific antibody in many people who developed clots after receiving the AstraZeneca vaccine – and US researchers subsequently identified the same antibody in people who developed similar clots after receiving a vaccine. NOT A WORD. In these people, the body’s immune system has formed antibodies that attach themselves to platelets, the specialized blood cells that come together to form clots.
“It’s not the vaccine that’s the cause – it’s the body’s immune response to the vaccine,” said Dr Alex Spyropoulos, a blood clot specialist and professor at the Feinstein Medical Research Institutes.
Experts say the disease, cerebral venous sinus thrombosis, or CVST, is extremely rare.
The Food and Drug Administration and the CDC said that in the six women who had a clot, the problem manifested itself six to 13 days after receiving the hit, a period of time following an immune response, Spyropoulos said. .
It’s still unclear why a vaccine would trigger this cascade of events, but researchers said it mimics another well-documented reaction to heparin, a common blood thinner. For this reason, the CDC and the FDA have warned against using heparin to treat anyone recently vaccinated with the Johnson & Johnson vaccine.
Existing data in the United States and Europe suggests that women may be at higher risk – perhaps in part because women are already more likely than men to develop CVST, based on previous data.
“We have to be careful with the assumptions we make,” Spryopoulos warned, “especially given the rarity of these events. “
It is possible that similar cases will develop in men, but if the CDC’s advisory committee concludes the risk is higher in women, the FDA could move to a black box warning – a label for drugs and devices. with potentially serious side effects – on the Johnson & Johnson Vaccine for some women.
While abruptly stopping an already authorized vaccine may cause concern and hesitation around the vaccine, experts said it was a good sign that the drugs were being properly controlled for their safety.
“I really want to stress to the public that they need to remain confident in our concepts and the times regarding vaccines in general,” said Dr. Jason Goldman, medical intern representing the American College of Physicians on the group. experts, adding that members of the general public should “not let this decision spoil your decision to get vaccinated in general.”
“We are confident in the process,” Goldman added. “And we will make the right decision regarding public safety. “
Amanda Benarroch, MD, resident in psychiatry at the Mayo Clinic in Rochester, Minnesota, is a contributor to the ABC News medical unit. ABC News’ Sasha Pezenik and Sony Salzman contributed to this report.