Last checked at 
By Julie Steenhuysen and Kate Kelland
CHICAGO/LONDON, April 13 (Reuters) - With two COVID-19
vaccines now under scrutiny for possible links to very rare
cases of blood clots in the brain, U.S. government scientists
are focusing on whether the specific technology behind the shots
may be contributing to the risk.
In Europe, health regulators said last week there was a
possible link between the AstraZeneca Plc vaccine and
169 cases of a rare brain blood clot known as cerebral venous
sinus thrombosis (CVST), accompanied by a low blood platelet
count, out of 34 million shots administered in the European
Economic Area.
The U.S. Food and Drug Administration on Tuesday recommended
temporarily halting use of the Johnson & Johnson vaccine
after reports of six cases of CVST in women under age 50 among
some 7 million people who received the shot in the United
States.
Both vaccines are based on a new technology using
adenoviruses, which cause the common cold, that have been
modified to essentially render them harmless. The viruses are
employed as vectors to ferry instructions for human cells to
make proteins found on the surface of the coronavirus, priming
the immune system to make antibodies that fight off the actual
virus.
Scientists are working to find the potential mechanism that
would explain the blood clots. A leading hypothesis appears to
be that the vaccines are triggering a rare immune response that
could be related to these viral vectors, FDA officials said at a
briefing on Tuesday.
The U.S. agency will analyze data from clinical trials of
several vaccines using these viral vectors, including J&J's
Ebola vaccine, to look for clues.
None of the previous vaccines using viral vectors have been
administered at close to the scale of the AstraZeneca and J&J
COVID-19 shots, which may explain why a potential link to blood
clots only materialized during these massive vaccination
programs.
The technology has also been used in coronavirus vaccines
developed in China and Russia.
Peter Marks, director of the FDA’s Center for Biologics
Evaluation and Research, was reluctant to declare the blood clot
issues a "class effect" shared by all adenovirus vector
vaccines, but he sees marked similarity in the cases.
"It's plainly obvious to us already that what we're seeing
with the Janssen (J&J) vaccine looks very similar to what was
being seen with the AstraZeneca vaccine," Marks said. "We can't
make some broad statement yet, but obviously, they are from the
same general class of viral vectors."
'AT THE BEGINNING'
In Europe, scientists are exploring a number of hypotheses,
including looking more broadly at the way the SARS-CoV-2 virus
itself affects blood coagulation.
One team in the Netherlands plans to conduct lab studies
that expose specific types of cells and tissues to the vaccines
and monitor how they react. They will also explore whether any
risks could be limited further by reducing the vaccine dose.
"There are many hypotheses, and some of them may play a
role," said Eric van Gorp, a virologist at Erasmus Medical
Centre in Rotterdam. "We are at the beginning, and - as it goes
in research - it may be that we can find the clue at once, or it
may be that it goes step by step."
Other scientists were struck by the parallel between the J&J
and AstraZeneca shots.
Danny Altmann, a professor of immunology at Imperial College
London, said the similar blood clotting incidents associated
with both "is clearly noteworthy for defining mechanism." There
has been no sign of such problems with the vaccines made by
BioNTech SE with Pfizer Inc or Moderna Inc
using a different technology.
"It would be interesting to know more about Sputnik V - also
a similar adenovirus vaccine," Altmann said. The Russian vaccine
developed by the Gamaleya Institute in Moscow uses two different
human cold viruses - including the Ad26 virus in the J&J shot.
The issue might also affect the adenovirus vector vaccine
from China's CanSino Biological, experts said.
Examining whether there is some common link to adenoviruses
is "a reasonable speculation, and it's a line of research and
investigation. But that doesn't mean it's proven," said John
Moore, a professor of microbiology and immunology at Weill
Cornell Medical College in New York.
Moore, who took part in an informal White House briefing
with other scientists on Tuesday, said the FDA and U.S. Centers
for Disease Control and Prevention are working closely with
health officials in Europe to determine whether the syndromes
linked to the AstraZeneca and J&J vaccines are the same.
An important clue may lie in the fact that the reported
events typically appear around 13 days after the shot, which is
the period in which antibodies might be expected to appear.
"This is speculation, but the timing of something happening
after about 13 days on average is suggestive of an immune
response to a component to the vaccine," Moore said.
Investigations of this sort could take years. But like the
vaccines themselves that were produced in record time, Moore
believes there will be so much effort put into the research that
it will more likely be resolved within weeks.
"It's so clearly important," he said.
(Reporting by Julie Steenhuysen and Kate Kelland; Additional
reporting by Michael Erman Maplewood, N.J.; Editing by Michele
Gershberg and Bill Berkrot)
