In Depth: Possible explanations for the rare blood clots after Johnson & Johnson vaccination

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Tue, 2021-04-20 11:24 — mike kraft

The race to untangle the secrets of rare, severe blood clots after Johnson & Johnson vaccination Making sense of rare, possibly related adverse events after vaccination can be notoriously tricky, but in this case, U.S. health officials have a blueprint. Carolyn Y. Johnson Washington Post

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Making sense of rare, possibly related adverse events after vaccination can be notoriously tricky, but in this case, U.S. health officials have a blueprint. European scientists’ detective work on similar cases in March, paired with decades of painstaking research into an obscure immune reaction to the anticoagulant drug heparin, have given them a probable — but not certain — mechanism, just weeks after the cases began to be detected.

Scientific questions remain about what component of the vaccines might be triggering the reaction, and who is at risk. But the syndrome is so similar to the rare heparin-related reactions that scientists have given the vaccine-triggered reaction a similar name, established a probable link and identified a widely available diagnostic test. It’s clear that heparin shouldn’t be given because it may worsen the clots, but other treatments exist on the shelves of virtually any hospital, although they cannot undo the damage caused by severe clots.

In a letter Friday to the New England Journal of Medicine, scientists from Johnson & Johnson said that “evidence is insufficient to establish a causal relationship” between their shot and the clots, and called for more evidence to clarify the symptoms observed in people who had been vaccinated. But many experts, including U.S. government health officials, said the immune explanation is the leading theory.

On Monday, Rochelle Walensky, director of the Centers for Disease Control and Prevention, said health officials were examining “a handful” of additional cases to judge if they were the same rare reaction. Understanding the frequency of the events will help inform decision-making about how the vaccine should be used.

“I’m encouraged that it hasn’t been an overwhelming number of cases,” Walenksy said.

The rapid progress in understanding the clots highlights the dividends yielded by scientific research on seemingly narrow topics — and stands in stark contrast to the plight of other vaccines, in which scientists can struggle for years to understand why an adverse event occurs.

“It’s actually extraordinary we know so much about this already,” said Theodore Warkentin, a professor in the department of pathology and molecular medicine at McMaster University in Canada, who is one of the world’s experts on the heparin-triggered syndrome.

“Imagine if heparin had never been invented, but otherwise the world was the same. People would be getting these low platelets and clotting, and we wouldn’t even know where to start. … There was a road map.”

For years, a relatively small circle of physicians and scientists have single-mindedly focused on unraveling the science behind a rare reaction to heparin that can cause both clotting and low platelet counts. ...

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