By Daniela Hernandez and Brianna Abbott
Scientists are racing to gain a better understanding of a rare
blood-clotting disorder that affected six recipients of Johnson
& Johnson's Covid-19 vaccine and led health authorities on
Tuesday to recommend a pause in its use.
"It is quite a rare disease, and an unusual manifestation of
abnormal blood clotting in the body, but it is very serious,"
Thomas Oxley, an interventional neurologist and stroke expert at
Mount Sinai Health System in New York City, said of the clotting
The six people in the U.S. known to have been affected by the
disorder after vaccination are among almost seven million Americans
to have received the vaccine.
The reports echo similar ones involving rare clotting disorders
seen in Europe among recipients of the Covid-19 vaccine
manufactured by AstraZeneca PLC.
Doctors aren't certain what causes the clots to form. One
hypothesis, based on preliminary data from a small number of
patients in Europe, is that the AstraZeneca vaccine can cause the
body to mount an immune reaction that triggers clot formation,
according to Jean Connors, a hematologist at Brigham and Women's
Hospital. Doctors said something similar likely explains the
clotting problem seen rarely in recipients of the J&J
In two studies published in April in the New England Journal of
Medicine, German and Norwegian researchers detailed this rare
immune response, which seems to target a protein on the surface of
platelets, blood components that aid in the formation of clots.
Patients examined by the researchers developed a severe clotting
The J&J vaccine recipients who experienced the rare clotting
condition were diagnosed with cerebral venous sinus thrombosis, or
CVST, which involves the formation of clots within large blood
vessels in the brain. That was accompanied by a drop in platelet
The immune reaction can translate into unchecked clot formation,
Dr. Connors said.
The brain-vessel clots can increase pressure within the skull,
leading to hemorrhaging and stroke, doctors said.
Doctors don't have enough data yet to know whether clots
associated with the J&J shots form preferentially in blood
vessels in the brain.
The six affected recipients of the J&J vaccine, all women
between the ages of 18 and 48 years, developed symptoms six to 13
days after vaccination. Peter Marks, director of the FDA Center for
Biologics Evaluation and Research, said Tuesday during a webinar
that a man who participated in the clinical trial of the J&J
vaccine experienced similar clotting.
"This event was initially thought to be related to the study
product by the investigator and prompted a study pause," according
to an FDA briefing document on the J&J clinical trial. The
investigators' final assessment was that the complication was
unrelated to the vaccine.
Oral contraceptives can increase the risk of certain blood
clots, but Dr. Marks said most of the women who developed clots
after vaccination weren't taking the drugs. There is no clear
association between oral birth control and the clots seen in the
women, he said. The agency is continuing to evaluate various risk
"This is a very, very rare event," Dr. Marks said, adding that
the agency recommended the pause in part to educate medical
providers about spotting and treating the potential
People who develop severe headache, abdominal or leg pain or
shortness of breath within three weeks after vaccination should
immediately contact their healthcare providers, according to
federal officials. Headaches within 24 hours of getting vaccinated
are normal, doctors said. The risk of clotting among people who got
the J&J shot more than a month ago is very low, said Anne
Schuchat, the principal deputy director at the Centers for Disease
Control and Prevention.
Doctors have documented a similar disorder after treatment with
the blood thinner heparin. Dr. Connors and other blood specialists
cautioned against using that drug to treat vaccine-associated clots
because it could aggravate the condition.
The AstraZeneca vaccine and Johnson & Johnson's both use
so-called viral-vector technology, in which genetic instructions
for a key component of the novel coronavirus are placed within a
harmless virus and then injected into the body to trigger immunity
to the pathogen.
"The beauty is that you use natural systems, which are optimized
by millions of years of evolution, to deliver what you want to have
your body respond to," said Vincent Munster, chief of the virus
ecology section at the National Institute of Allergy and Infectious
Disease, which helped with development of the AstraZeneca
These vaccines offer several advantages over shots using older
technology. They seem to activate not just the antibodies that
neutralize a virus but also the memory and T-cells that keep the
immune defense alert for the long-term.
The vaccines developed by Moderna Inc. and by Pfizer Inc. with
BioNTech SE use a different technology, known as mRNA. Health
authorities haven't seen any cases of clotting with low platelet
counts associated with those vaccines, the CDC's Dr. Schuchat
In the case of the J&J vaccine, clots also might be forming
in other parts of the body but only ones in the blood vessels in
the brain have been detected so far, in part because the vaccine
has had a limited rollout in the U.S.
In Europe, there has been a wider range of abnormal clots among
the 34 million people who received the AstraZeneca shot.
"We don't know if we had given a lot more doses of the J&J
vaccine if we would have seen other [types of] clots," said Gary
Raskob, a blood-clotting expert at the University of Oklahoma.
Doctors said they would keep monitoring patients to determine
the exact mechanism behind the clotting. They said the risk of
experiencing a dangerous side effect from vaccination is very low
and is outweighed by the potential short- and long-term risks of
The pause in the J&J rollout in the U.S. comes at a
precarious time in the pandemic, with more infectious and
potentially more deadly variants taking hold around the world
creating renewed urgency to boost vaccination rates globally. Some
countries might be at greater risk than others.
"The majority of vaccines administered [in the U.S.] have been
mRNA-based -- Moderna and Pfizer," said Abraar Karan, a global
health physician at Brigham and Women's Hospital. "I don't expect
that, right now, this will have an exceptionally outsize role in
slowing down the vaccine rollout."
--Joseph Walker and Bojan Pancevski contributed to this
Write to Daniela Hernandez at Daniela.Hernandez@wsj.com and
Brianna Abbott at firstname.lastname@example.org
(END) Dow Jones Newswires
April 13, 2021 19:37 ET (23:37 GMT)
Copyright (c) 2021 Dow Jones & Company, Inc.