As the Johnson & Johnson COVID-19 vaccine is still suspended in the US after reports of a rare but severe blood clotting disorder in a small number of the approximately 7 million people who got the chance, health experts are now focusing on what may be behind the unusual adverse events.
Doctors, scientists, and public health experts are turning to Europe for clues, where a similar vaccine made by AstraZeneca – not yet authorized in the US – has also been linked to a number of rare blood clots.
“The AstraZeneca and Johnson & Johnson vaccines are manufactured in a similar way,” said Dr. William Schaffner, a physician at Infectious Diseases at Vanderbilt University, said with expertise in preventive medicine and health policy. “But the carriers are different types of adenoviruses … it’s part of the background information. Why is there a silence now?”
“It’s hard to say if this is the same problem,” said Dr. Richard Kuhn, Ph.D., a virology expert at Purdue University, said, “but the vaccine appears to elicit an antibody response that activates platelets, leading to blood clots.”
While many experts are hesitant to make sure there is a link, Schaffner said there is a growing consensus in the scientific community after none of these rare blood clots were linked to the Pfizer or Moderna vaccines containing mRNA, a other technology, do not use.
“I think we no longer have to watch over it,” Schaffner said, adding that it may be time to “accept the fact that it is vaccine-induced but very rare incidents.”
But experts have warned that, even if 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 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 down it on Friday, insisting that a clear connection had not yet been established.
“At present, there is insufficient evidence to establish a causal link between these events and the [Johnson & Johnson] vaccine, “Janssen scientists said, adding that the vectors used in their vaccine and AstraZeneca’s” differ significantly “and that the differences could lead to” very different biological effects. The researchers added that “more evidence is needed” to determine the cause of this clot, combined with a low platelet count in those receiving the COVID-19 vaccine.
Next week, the independent advisory committee of the Centers for Disease Control and Prevention will review all evidence and give a recommendation on whether to resume the use of J&J vaccines.
Different theories
Still with more questions than answers, scientists are investigating various theories as to why this type of shot – called a viral vector vaccine – can rarely cause clotting problems.
Vaccinations work by stimulating our immune systems to develop antibodies against a virus, and a common theory is that viral vector vaccines somehow cause an abnormal immune response, leading to blood clots.
Scientists in Germany have identified a specific antibody in many people who develop blood clots after receiving the AstraZeneca vaccine, and U.S. researchers have identified the same antibody in individuals who have had similar blood clots after receiving a J&J vaccine. . In these subjects, the body’s immune system produced antibodies that attach to platelets, the specialized blood cells that combine to form blood clots.
“It’s not the vaccine that causes it – it’s the body’s immune response to the vaccine,” said Dr. Alex Spyropoulos, a specialist in blood clots and professor at the Feinstein Institutes for Medical Research, said.
Experts believe that the condition, cerebral venous sinus thrombosis, or CVST, is extremely rare.
The Food and Drug Administration and CDC said the problem six to 13 days after receiving the shot in the six women who experienced a blood clot, a time frame that follows with an immune response, Spyropoulos said.
It is still not clear why a vaccine would cause 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 FDA have warned against the use of heparin for the treatment of anyone recently vaccinated with the Johnson & Johnson vaccine.
Existing evidence from the United States and Europe suggests that women may be at greater risk – perhaps in part because women are increasingly likely to develop CVST, based on previous data.
“We have to be careful with the assumptions we make,” Spryopoulos warned, “especially given how infrequent these events were.”
It is possible that similar cases will develop in men, but if the CDC advisory panel concludes that the risk in women is higher, the FDA could move to a black box warning – a label for drugs and medical devices with potentially serious side effects effects – on the Johnson & Johnson vaccine for certain women.
Although the sudden discontinuation of an already authorized vaccine may cause concern and the hesitation surrounding the vaccine, experts said it is a good sign that drugs are being properly monitored for safety.
“I really want to emphasize to the public that they need to remain confident in our concepts and the times regarding vaccines in general,” said Dr. Jason Goldman, a physician in internal medicine who represents the American College of Physicians in the expert panel, said. that members of the general public ‘should not allow it to make you decide to be vaccinated in general.’
“We do have confidence in the process,” Goldman added. “And we will make the right decision regarding public safety.”
Amanda Benarroch, MD, a resident in psychiatry at Mayo Clinic in Rochester, Minnesota, makes a contribution to the ABC News Medical Unit. ABC News, Sasha Pezenik and Sony Salzman contributed to this report.