By Sarah Toy
Scientists are trying to understand why women appear to be more
vulnerable than men to a potentially deadly blood-clotting
condition reported among a tiny number of recipients of Johnson
& Johnson's Covid-19 vaccine.
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
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 are known to have stronger immune reactions to vaccines
"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
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,
"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
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
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
Betsy McKay contributed to this article.
Write to Sarah Toy at firstname.lastname@example.org
(END) Dow Jones Newswires
April 18, 2021 08:14 ET (12:14 GMT)
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