Evidence is showing that an uncommon immune response is behind dangerous, but incredibly rare blood clots, associated with some COVID-19 vaccines. But the good news is that there is evidence that doctors can use to identify and get patients to receive proper care.
A small number of the millions of people vaccinated with COVID-19 shots from AstraZeneca or Johnson & Johnson have developed severe blood clots, such as those in the sinuses that drain blood from the brain (SN: 4/7/21; 4/13 / 21). Some died.
Studies suggest that some inoculated people develop an immune response that attacks a protein called platelet factor 4 or PF4, which causes platelets to form clots. Those platelets get used to before the body can do more. Thus, these patients end up with rare clots and low platelet levels in the blood.
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Of the 23 patients who received the sting of AstraZeneca and had symptoms of clots or low platelets, 21 tested positive for antibodies to PF4, according to researchers on April 16 in the New England Journal of Medicine. Of these, 20 people developed blood clots. The finding is in addition to previous studies that found the same antibodies in additional patients who were shot AstraZeneca and had dangerous clots.
Five out of six women who had clots after receiving Johnson & Johnson’s shot in the United States also had PF4 antibodies, health officials said on April 14 during a meeting of the Advisory Committee on Immunization Practices. That advisory group of the U.S. Centers for Disease Control and Prevention is evaluating what needs to be done to suspend a temporary pause in the Johnson & Johnson jab administration that was caused by blood clot problems (SN: 4/13/ 21). One man developed cerebral sinus clots during the clinical trial of the shot and a seventh case is under investigation, the pharmaceutical company said during the meeting.
"Because we are aware of this syndrome … we know how to treat it," says Jean Connors, a clinical hematologist at Harvard Medical School and Brigham and Women's Hospital in Boston who did not participate in the studies. And unlike people who developed clots before officials identified the link, "we can diagnose it faster and treat it more appropriately if it happens, so the results are better."
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This is because vaccine-induced clots are similar to a condition called heparin-induced thrombocytopenia or HIT. Patients with HIT develop blood clots when treated with commonly used anti-coagulant heparin. Heparin binds to the PF4 protein and some people develop an immune response that attacks both molecules.
Treating vaccinated patients who have PF4 antibodies with heparin is like “adding fuel to the fire” and can cause them to develop more clots, Connors says. Four of the six American women who developed clots after receiving the Johnson & Johnson vaccine, for example, received heparin, as did the man in the clinical trial. The man recovered and a woman was discharged from hospital. Three were still hospitalized as of April 14th.
Healthcare workers can try PF4. And if a patient tests positive, there are many other anticoagulants other than heparin that doctors can use for treatment, Connors says.
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