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 Here's what we know so far:
WHAT HAS HAPPENED?
The EMA said its vaccine side effects monitoring system, as of April
4, had received 169 reports of cases of cerebral venous sinus
thrombosis (CVST), or clots in blood vessels exiting the brain, and
53 cases of splanchnic vein thrombosis (SVT), or clotting in veins
in the abdomen.
That's out of some 34 million AstraZeneca vaccine doses administered
in Britain and the European Economic Area since vaccinations started
almost three months ago.
Its safety committee carried out a review of 62 cases of CVST and 24
cases of SVT, of which 18 were fatal.
Most of the cases reported have occurred in women under 60, though
that could be misleading. Germany and Britain say many more women
received AstraZeneca's shot than men.
Most cases occurred within 2 weeks of the person receiving their
first dose.
German vaccination officials who recorded 29 cases of CVST in women
aged 20 to 59 who got the AstraZeneca vaccine said the occurrence
rate in that group was 20 times higher within 16 days of vaccination
than what would have typically been expected.
Germany's health ministry has said 1 to 1.4 cases of CVST would have
been expected during that time.
WHAT HAS BRITAIN'S REGULATOR SAID?
The Medicines and Healthcare products Regulatory Agency reviewed 79
cases of rare clotting coupled with low platelets, with 19
fatalities - 13 women and 6 men. Eleven of the deaths in people
under the age of 50 and three under 30.
All 79 cases occurred after a first dose of the vaccine.
HOW DID REGULATORS COME TO THEIR DECISION?
British officials drew on statistics from the University of
Cambridge's Winton Centre for Risk and Evidence Communication to
explain their recommendations that young people get an alternative
shot while older people still get AstraZeneca's.
According to the Centre, vaccination with the AstraZeneca shot would
reduce admissions to intensive care units by nearly seven in 100,000
people in the group aged 20-29 years old with high risk of exposure
to the virus, while 1.1 people per 100,000 people in the same group
were estimated as likely to suffer serious vaccine-related harms.
The risk of serious harm due to vaccination falls further the older
people get, and ICU admissions prevented by vaccination rise
sharply, boosting the AstraZeneca shot's benefit-to-risk ratio. The
Centre concluded only 0.4 people for every 100,000 in the 50-59 age
group would suffer vaccine-linked harm, while 95.6 ICU admissions
per 100,000 people would be prevented.
EMA investigators checked if the frequency of events was higher in
the vaccinated population than normal background rates drawn from
public health statistics or insurance records. This would be
combined with a medical analysis of each case and insight from
scientific literature.
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 In its findings, the EMA said
on March 18 that, on average, just 1.35 cases of
CVST might normally have been expected among
people under 50 within 14 days of receiving
AstraZeneca's vaccine, whereas by the same
cut-off date 12 cases had been recorded.
By comparison, four women out of 10,000 would
get a blood clot from taking oral contraception.
Key to the EMA's assessment that benefits
continue to outweigh risks are its conclusions
about likelihood of an increased risk from
clotting, compared with the benefits of
preventing COVID-19 and reducing
hospitalizations and deaths.
WHAT'S THE EU DOING NOW?
The EMA said that unusual blood clots with low blood platelets
should be listed as very rare side effects and countries should
decide on how to proceed with their national COVID-19 immunisation
campaigns.
These may vary from nation to nation, the EMA said, depending on
factors like how quickly infections are spreading and whether
vaccines are available. Its safety committee requested new studies,
and changes to ongoing ones, to provide more information about the
mechanism behind the rare clots.
AstraZeneca and European regulators have said concerns about blood
disorders did not emerge during clinical trials. ARE
THERE ANY THEORIES FOR WHAT MAY BE LEADING TO THE CLOTS?
Among possible causes for these rare cerebral sinous vein clots
being investigated are that the vaccine triggers an unusual antibody
in rare cases.
The EMA said the vaccine may trigger an immune response leading to
the unusual clotting, though it acknowledged that scientists have
yet to identify any risk factors that might predispose somebody to
clotting combined with low platelets.
German scientists at Greifswald University concluded the vaccine is
linked to the side effects, though their work still requires peer
review. Andreas Greinacher, an expert on drug-induced immune
responses, said specimens he has examined exhibited unusual
antibodies that activate blood platelets, prompting the clots.
Greinacher is still seeking clues about why some people had clots
and others did not.
(Reporting by Ludwig Burger in Frankfurt, Matthias Blamont in Paris
and John Miller in Zurich; Additional reporting by Kate Kelland and
Alistair Smout in London and Julie Steenhuysen in Chicago; Editing
by Josephine Mason and Angus MacSwan)
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