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 The United States earlier this week paused distribution of the J&J
vaccine to investigate six cases of a rare brain blood clot known as
cerebral venous sinus thrombosis (CVST), accompanied by a low blood
platelet count, in U.S. women under age 50, out of about 7 million
people who got the shot.
The blood clots in patients who received the J&J vaccine bear close
resemblance to 169 cases in Europe reported with the AstraZeneca
vaccine, out of 34 million doses administered there.
Both vaccines are based on a new technology that uses a modified
version of adenoviruses, which cause the common cold, as vectors to
ferry instructions to human cells.
The U.S. Food and Drug Administration is scrutinizing this design
behind both vaccines to see if it is contributing to the risk.
In a letter on Friday in the New England Journal of Medicine, J&J
scientists refuted a case report published earlier this week by Kate
Lynn-Muir and colleagues at the University of Nebraska, who asserted
that the rare blood clots "could be related to adenoviral vector
vaccines."
In an interview with Reuters on Thursday, Dr. Anthony Fauci, the top
U.S. infectious disease expert and an adviser to the White House,
said the fact that they are both adenovirus vector vaccines is a
"pretty obvious clue" that the cases could be linked to the vector.
"Whether that is the reason, I can't say for sure, but it certainly
is something that raises suspicion," Fauci said.
In the correspondence on Friday, Macaya Douoguih, a scientist with
J&J's Janssen vaccines division, and colleagues pointed out that the
vectors used in its vaccine and the AstraZeneca shot are
"substantially different" and that those differences could lead to
"quite different biological effects."
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 Specifically, they noted that
the J&J vaccine uses a human adenovirus while
the AstraZeneca vaccine uses a chimpanzee
adenovirus. The vectors are also from different
virologic families or species, and use different
cell receptors to enter cells.
The J&J shot also includes mutations to
stabilize the so-called spike protein portion of
the coronavirus that the vaccine uses to produce
an immune response, while the AstraZeneca
vaccine does not.
"The vectors are very different," said Dr. Dan
Barouch of the Center for Virology and Vaccine
Research at Harvard’s Beth Israel Deaconness
Medical Center in Boston, who helped design the
J&J vaccine.
"The implications of issues with one vector for
the other one are not clear at this point," he
said in an interview earlier this week.
The J&J scientists said in the letter there was
not enough evidence to say their vaccine caused
the blood clots and they continue to work with
health authorities to assess the data.
A panel of advisers to the U.S. Centers for
Disease Control and Prevention are expected to
meet on April 23 to determine whether the pause
on use of the J&J vaccine can be lifted.
 (Reporting by Julie Steenhuysen; Editing by Bill
Berkrot)
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