It turns out that an unusual immune response lies behind the dangerous but incredibly rare blood clots associated with some COVID-19 vaccines. The good news is that there is a test that doctors can use to identify and get patients the right care.
A small number of the millions vaccinated with AstraZeneca or Johnson & Johnson COVID-19 vaccines have developed severe blood clots, such as those in the sinuses that drain blood from the brain (SN: 07.04.21; 13.04.21). A few 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. These platelets get used before the body can do more. So, these patients end up with both rare clots and low blood platelet levels.
Of the 23 patients who received AstraZeneca and had symptoms of clots or low platelets, 21 were positive for antibodies to PF4, the researchers report on April 16 in New England Journal of Medicine. Of these, 20 people developed blood clots. The finding adds to previous studies that found the same antibodies in additional patients who received AstraZeneca shot and had dangerous clots.
Five of the six women who had clots after receiving Johnson & Johnson’s shots in the United States also had PF4 antibodies, health officials said on April 14 at a meeting of the Advisory Committee on Immunization Practices. This advisory group to the U.S. Centers for Disease Control and Prevention is evaluating what needs to be done to lift a temporary break in the administration of Johnson & Johnson’s jab, which has been caused by blood clots (SN: 13.04.2021). A man developed cerebral sinus clots during the clinical trial of the shooting and a seventh case is being investigated, the pharmaceutical company said during the hearing.
“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 was not involved in the study. And unlike people who developed clots before officials identified the link, “we can diagnose it faster and treat it more properly if it happens, so the results are better.”
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 the commonly used anti-coagulant drug heparin. Heparin attaches to the PF4 protein and some people develop an immune response that attacks the two molecules.
Treating vaccinated patients who have PF4 antibodies to heparin is like “adding fuel to the fire” and can cause more clots to grow, says Connors. Four of the six women in the United States 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 the hospital. Three were still hospitalized on April 14.
Healthcare workers can test PF4. And if a patient tests positive, there are many other anticoagulants besides heparin that clinicians can use for treatment, says Connors.
This story was originally published by Science News, an independent nonprofit news organization.
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