Covid-19 vaccines and rare blood clots: Are women at greater risk?

U.S. health officials called for a pause in the use of the J&J vaccine on Tuesday after six women—out of roughly seven million people who received the vaccine in the U.S.—developed blood clots in the brain and other parts of the body within two weeks of vaccination.

The move came after similar clotting problems were reported in at least 140 people in Europe, the majority of them women, who received the Covid-19 vaccine made by AstraZeneca.

At least 34 million in Europe have received the AstraZeneca vaccine, which hasn’t been approved for use in the U.S.

J&J said Wednesday that it would delay the rollout of its vaccine in Europe and pause vaccinations in its vaccine trials. A federal committee said Wednesday that more information was needed to determine whether use of the vaccine should resume, be discontinued or limited to certain groups of the population. In a letter published Friday in the New England Journal of Medicine, three J&J scientists said there wasn’t enough evidence to say that the company’s vaccine caused the six clotting cases.

AstraZeneca said last week that it is working to update its vaccine labels to list blood clots as a rare side effect. The company also said it is working to understand what could explain the events. Some countries have restricted or suspended use of its vaccine.

Scientists said the tiny number of cases of the clotting condition—and the fact that more women than men may have gotten the AstraZeneca vaccine—make it hard to draw firm conclusions about whether women are at higher risk. Over time, they said, men may prove to be as vulnerable to the rare condition as are women.

Women are known to have stronger immune reactions to vaccines than men.

“It’s a blessing and a curse," Dr. Sabra Klein, a professor of molecular microbiology and immunology at Johns Hopkins Bloomberg School of Public Health, said of women’s stronger immune response. It could help explain why Covid-19 tends to be less deadly in women than in men. But it could also underlie women’s greater likelihood of experiencing severe side effects after vaccination, as well as their greater risk of autoimmune diseases, in which the immune system attacks the body, she said.

In two studies, published in April in the New England Journal of Medicine, researchers found that the clotting condition seen in some recipients of the AstraZeneca vaccine closely resembled a rare condition called heparin-induced thrombocytopenia, or HIT. The condition, which in rare instances affects people given the blood-thinning drug heparin, involves both abnormal clotting and a drop in levels of clot-forming blood components known as platelets.

A case report of a 48-year-old woman who received the J&J vaccine described a similar phenomenon: low platelet levels and blood clots in both her brain and digestive system.

HIT is triggered by an errant antibody reaction that activates platelets, causing them to clump together to form clots. In the case of the clotting problems following vaccination, the patients didn’t receive heparin, just a vaccine. Scientists now are calling the condition vaccine-induced immune thrombotic thrombocytopenia, or VITT.

“When we talk about women being more at risk, my suspicion is that it is…an immunologic reaction and there’s some molecule [in the vaccine] that is mimicking heparin," said Dr. Jean Connors, a hematologist at Brigham and Women’s Hospital in Boston.

Both the J&J and AstraZeneca vaccines use what’s known as viral vector technology, in which genetic instructions for a key component of the novel coronavirus are placed within a harmless virus and injected into the body to trigger an immune response to the actual pathogen.

Theodore Warkentin, an expert on HIT at McMaster University in Ontario, Canada, said his research has shown that women who receive heparin after cardiac or orthopedic surgery are more likely than men to experience the condition.

“It could be that this vaccine effect, which does mimic this adverse drug reaction in many ways—there could be a female predisposition," said Dr. Warkentin, who is also studying vaccine-linked blood clots.

Some doctors have speculated that women’s apparently heightened risk for the clotting disorder following vaccination with the J&J or AstraZeneca vaccines could be explained by pregnancy or the use of birth-control pills. Both pregnancy and the hormones found in birth-control pills can boost the body’s clot-making ability.

Of the six women who developed clots after getting the J&J vaccine, however, only one was using the hormones, and none had any known blood-clotting disorders. And the clots that affected the women following vaccination differ from clots seen in the general population, which typically aren’t accompanied by a drop in platelet levels.

In a late-stage J&J trial, only one individual had this type of blood-clotting reaction after getting the vaccine, the company said, out of more than 43,000 people who were vaccinated. The clotting disorder wasn’t reported among any of the tens of thousands of patients who received the vaccine in the AstraZeneca trials.

This story has been published from a wire agency feed without modifications to the text