With the Johnson & Johnson COVID-19 vaccine still on hiatus in the US after reports of a rare but serious blood clotting disorder in a small number of the roughly 7 million people who received the injection, health experts are now focusing on what could be behind those unusual side effects.
Doctors, scientists and public health experts are turning to Europe for clues, where a similar vaccine made by AstraZeneca – not yet approved in the US – has also been linked to some rare blood clots.
“AstraZeneca and Johnson & Johnson vaccines are made in a similar way,” said Dr. William Schaffner, an infectious disease physician at Vanderbilt University with expertise in preventive medicine and health policy. “But the carriers are different types of adenoviruses … that’s part of the background, why is there indeed a pause now.”
“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 elicits an antibody response that activates platelets, leading to clots.”
While many experts have hesitated to say for sure whether there is a link, Schaffner said there is a growing consensus in the scientific community after none of these rare clots have been linked to the Pfizer or Moderna vaccines, which use mRNA. a different technology.
“I think we should no longer hold back on that,” Schaffner said, adding that it may be time to “accept that these are vaccine-induced but very rare events.”
But experts cautioned that even if there is a link, the current evidence suggests that the risk of developing a blood clot after getting the Johnson & Johnson vaccine is extraordinarily low – even lower than with lightning strikes.
In response to a report on one of six cases of clot published in the New England Journal of Medicine, Johnson & Johnson wrote a response on Friday, insisting that no clear link has yet been made.
At present, there is insufficient evidence to establish a causal relationship between these events and the disease [Johnson & Johnson] vaccine, “Janssen scientists said, adding that the vectors used in their vaccine and those from AstraZeneca are” substantially different “and that those differences could lead to” very different biological effects. The researchers added that “more evidence is needed” to determine the cause of this clotting, combined with low platelet counts in those who receive the COVID-19 vaccine.
Next week, the independent advisory committee of the Centers for Disease Control and Prevention will review all evidence and make a recommendation on whether or not to resume use of J&J vaccines.
Different theories
With still more questions than answers, scientists are exploring several theories as to why this type of injection – called a viral vector vaccine – can cause rare clotting problems.
Vaccines work by causing our immune system to develop antibodies to a virus, and a prevailing theory is that viral vector vaccines somehow cause an abnormal immune response, leading to blood clots.
Scientists in Germany identified a specific antibody in many people who developed blood clots after receiving the AstraZeneca vaccine – and American researchers then identified the same antibody in people who developed similar blood clots after receiving a J&J vaccine. In these individuals, the body’s immune system has formed antibodies that attach to platelets, the specialized blood cells that come together to form clots.
“It’s not the vaccine that causes it, it’s the body’s immune response to the vaccine,” said Dr. Alex Spyropoulos, a blood clot specialist and professor at the Feinstein Institutes for Medical Research.
Experts say the condition, cerebral sinus vein thrombosis, or CVST, is extremely rare.
The Food and Drug Administration and CDC said that in the six women who received a clot, the problem manifested itself six to 13 days after receiving the injection, a time frame followed by an immune response, Spyropoulos said.
It’s still not clear why a vaccine would trigger this cascade of events, but researchers said it mimics another well-documented response to heparin, a common blood thinner. For this reason, the CDC and the FDA have warned against using heparin to treat someone recently vaccinated with the Johnson & Johnson vaccine.
Existing evidence from 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 about the assumptions we make,” Spryopoulos warned, “especially given how rare these events are.”
It’s possible that similar cases will develop in men, but if the CDC advisory panel concludes that the risk is higher in women, the FDA could move towards a black box warning – a label for drugs and medical devices with potentially serious side effects – on the Johnson & Johnson vaccine for certain women.
While the abrupt discontinuation of an already approved vaccine could raise concerns and spark hesitation around the vaccine, experts said it’s a good sign that drugs are being properly screened for safety.
“I really want to stress to the public that they must continue to have faith in our concepts and the times regarding vaccines in general,” said Dr. Jason Goldman, an internal medicine physician representing the American College of Physicians on the panel of experts. , adding that members of the general public should “not let your decision to get vaccinated in general sour”.
“We are confident in the process,” Goldman added. “And we will make the right decision regarding public safety.”
Amanda Benarroch, MD, a psychiatric resident of the Mayo Clinic in Rochester, Minnesota, is a contributor to the ABC News Medical Unit. Sasha Pezenik and Sony Salzman from ABC News contributed to this report.