Last checked at 
(Adds FDA investigation into vaccine design in paragraph 5,
rewrites headline)
By Julie Steenhuysen
CHICAGO, April 16 (Reuters) - Scientists at Johnson &
Johnson on Friday refuted an assertion in a major
medical journal that the design of their COVID-19 vaccine, which
is similar AstraZeneca's, may explain why both have been
linked to very rare brain blood clots in some vaccine
recipients.
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."
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)
