Researchers are trying to get to the bottom of why rare blood clots have been found in individuals who received some types of coronavirus vaccine.
A small number of people who have received the Johnson & Johnson or AstraZeneca vaccine have suffered rare blood clots. The condition has not been reported with the Moderna or Pfizer/BioNTech vaccine, which was made using a different type of technology.
Due to clotting concerns, the Food and Drug Administration and the federal Centers for Disease Control and Prevention have called for a temporary halt in administering the Johnson & Johnson vaccine. The AstraZeneca vaccine, which has been widely used in Europe, has not received federal approval to be used in the U.S.
Doctors are calling the condition vaccine-induced immune thrombotic thrombocytopenia, according to CNN, which implies a link to the vaccines, without saying there is a direct cause.
Some experts think enough of a link has already been shown, according to ABC.
“The AstraZeneca and Johnson & Johnson vaccines are made in a similar way,” said Dr. William Schaffner, an infectious disease physician at Vanderbilt University.
“I think we shouldn’t be coy about that any longer,” Schaffner said, saying it is time to “accept the fact that these are vaccine-induced but very rare events.”
Dr. Richard Kuhn, Ph.D., a virology expert at Purdue University, was more cautious,
“It’s hard to say if it’s the same problem,” he said, “but it does seem the vaccine triggers an antibody response that activates platelets, leading to clots.”
A study led by Dr. Marie Scully, a hematologist at University College London Hospitals, studied 22 patients who received the AstraZeneca vaccine, and found that the individuals who suffered a clotting reaction produced unusual antibodies most often seen before as a reaction to the drug heparin.
“The risk of thrombocytopenia and the risk of venous thromboembolism after vaccination against SARS-CoV-2 do not appear to be higher than the background risks in the general population, a finding consistent with the rare and sporadic nature of this syndrome,” the new study said.
“The events reported in this study appear to be rare, and until further analysis is performed, it is difficult to predict who may be affected. The symptoms developed more than 5 days after the first vaccine dose.”
“In all cases reported to date, this syndrome of thrombocytopenia [low platelet count] and venous thrombosis [blood clot] appears to be triggered by receipt of the first dose of the [AstraZeneca] ChAdOx1 nCoV-19 vaccine,” the study said.
“It’s not the vaccine that’s causing it — it’s the body’s immune response to the vaccine,” said Dr. Alex Spyropoulos, a blood clot specialist and professor at the Feinstein Institutes for Medical Research, according to ABC News.
In a commentary on the new study, Dr. Douglas Cines of the University of Pennsylvania and Dr. James Bussel of Weill Cornell Medicine noted that some people are at greater risk than others.
“Most of the patients included in these reports were women younger than 50 years of age, some of whom were receiving estrogen-replacement therapy or oral contraceptives. A remarkably high percentage of the patients had thromboses at unusual sites,” they wrote.
The CDC is still looking for more facts ahead of a Friday meeting about the clotting concerns.
“We need to know what the size of the problem is,” said Dr. Kevin Ault, professor and division director with the University of Kansas Medical Center.
“So we’re going to shake the trees in the databases that the CDC has and we also need to know what the denominator is — is it just young women or the whole population that’s been vaccinated?”
“There are still a fair number of people in the United States who have been vaccinated in the last two weeks,” Ault said.
“We’ve seen these reactions within two weeks, so it doesn’t sound like a very long time, but we’ll have a fair amount of data in just those nine or ten days.”
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