With Johnson & Johnson COVID-19 still on break in the US after reports of a rare but severe blood clotting disorder in a small number of the approximately 7 million people who received the vaccine, health experts are now focusing on on what might be behind those unusual side events.
Doctors, scientists and public health experts are heading to Europe for clues, where a similar vaccine produced by AstraZeneca – not yet licensed in the US – has also been linked to a number of rare blood clots.
“The AstraZeneca and Johnson & Johnson vaccines are made similarly,” said Dr. William Schaffner, an infectious disease physician at Vanderbilt University with expertise in preventive medicine and health policy. “But carriers are different types of adenoviruses … that’s part of the basic information, why there really is a break 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 the vaccine triggers an antibody response that activates platelets, leading to clots.”
While many experts have hesitated to say for sure if 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 Pfizer or Moderna vaccines, which use mRNA. different technology.
“I don’t think we should be more shy about it,” Schaffner said, adding that it might be time to “accept that these are vaccine-induced events, but very rare.”
But experts have warned that while there is a link, current evidence suggests that the risk of developing a blood clot after receiving Johnson & Johnson vaccine is extremely low – lower, in fact, than being struck by lightning.
Responding to a report on one of six cases of clots published in the New England Journal of Medicine, Johnson & Johnson received a response on Friday, insisting that a clear link had not yet been established.
“At present, there is insufficient evidence to establish a causal relationship between these events and [Johnson & Johnson] The vaccine, “Janssen scientists said, adding that the vectors used in their vaccine and AstraZeneca are” substantially different “and that these differences could lead to” quite different biological effects. “The researchers added that” more evidence is needed “to further clarify the cause of this clotting, 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 make a recommendation on the resumption of J&J vaccine use.
Different theories
Even with more questions than answers, scientists are exploring various theories as to why this type of vaccine – called a viral vector vaccine – could cause rare clotting problems.
Vaccines work by causing our immune system to develop antibodies against a virus, and a predominant theory is that vector viral vaccines somehow trigger an abnormal immune response, leading to blood clots.
Scientists in Germany have identified a specific antibody in many people who developed clots after receiving the AstraZeneca vaccine – and US researchers later identified the same antibody in people who developed similar clots after receiving a J&J vaccine. In these subjects, the body’s immune system has formed antibodies that attach to platelets, specialized blood cells that bind 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 Institute for Medical Research.
Experts say that the condition, cerebral venous sinus thrombosis or CVST, is extremely rare.
The Food and Drug Administration and the CDC said that in the six women who experienced a clot, the problem occurred six to 13 days after receiving the vaccine, a time frame that follows an immune response, Spyropoulos said.
It is not yet clear 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 the use of heparin to treat anyone recently vaccinated with the Johnson & Johnson vaccine.
Existing evidence in the United States and Europe suggests that women may be at higher risk – probably 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 have been.”
Similar cases may develop in men, but if the CDC advisory group concludes that the risk is higher in women, the FDA could turn to a black box warning – a label for drugs and medical devices with potentially serious side effects – Johnson & Johnson Vaccine for certain women.
Although abruptly discontinuing an already authorized vaccine may raise concerns and stimulate hesitation around the vaccine, experts say it is a good sign that the drugs are being properly screened for safety.
“I really want to emphasize to the public that they need to remain confident in our concepts and in times of vaccines in general,” said Dr. Jason Goldman, an internal medicine physician representing the American College of Physicians in the panel. “Your decision to vaccinate in general should not leave this sour.”
“We have confidence in the process,” Goldman added. “And we will make the right decision on public safety.”
Amanda Benarroch, MD, a psychiatric resident at the Mayo Clinic in Rochester, Minnesota, is a contributor to the ABC News Medical Unit. Sasha Pezenik and Sony Salzman of ABC News contributed to this report.