Can the AstraZeneca Vaccine Cause Blood Clots? What You Need to Know
April 08 2021 - 11:15AM
Dow Jones News
By Jason Douglas
European health regulators have advised that there may be a link
between the Covid-19 vaccine developed by AstraZeneca PLC and the
University of Oxford and very rare blood clots, and some
governments have recommended restricting its distribution among
younger people as a result. The potential side effect is rare -- so
rare it is only likely to be picked up in a real-world rollout of
the vaccine rather than in earlier clinical trials involving many
fewer people. The concern is limited to the AstraZeneca vaccine,
which isn't yet authorized for use in the U.S. Here is what you
need to know.
What decisions have European regulators made on the AstraZeneca
vaccine?
The European Medicines Agency and the U.K.'s Medicines and
Healthcare products Regulatory Agency on Wednesday issued new
guidance on the use of the AstraZeneca vaccine. Both regulators
said there might be a link between the vaccine and very rare blood
clots, some of which have been fatal. The EMA advised European
Union governments to keep using the vaccine in all age groups --
though some governments have said they would limit the use of the
shot to older people. In the U.K., a panel of scientists that
advises on vaccines said adults under 30 years old should be
offered an alternative vaccine if available. Both urged doctors to
be alert for symptoms of these clots in anyone newly vaccinated,
especially if associated with another unusual condition involving
low levels of platelets, a type of blood cell.
Why did they reach different conclusions?
Behind both decisions is an assessment of the trade-off between
the benefits of vaccination against Covid-19 and the risk of
suffering an extremely rare side effect soon after getting a shot.
The risk of severe illness and death from Covid-19 rises steeply
with age. That means for older people, the risks are more heavily
weighted in favor of vaccination. For younger people, who are much
less likely to fall gravely ill with Covid-19, the risks are more
finely balanced, especially because the latest evidence suggests
these rare clots are slightly more likely to occur in younger
people than older people.
A key thing to understand, though, is that the balance of these
risks for young people in some parts of the EU and young people in
the U.K. aren't necessarily the same, and can change over time.
That helps explain why regulators made different
recommendations.
So what kind of risks are we talking about?
Regulators said that as of April 4 there have been 169 cases of
a type of blood clot in the brain called a cerebral venous sinus
thrombosis, or CVST, in Europe since vaccinations began, and 53
cases of another rare clot. Through April 4, 34 million people had
been vaccinated in the region, giving a crude frequency of six or
seven cases per million vaccinations. Reviews of the medical
literature suggest CVST would be expected to occur in around two to
four people per million a year.
Reported cases vary considerably from country to country. The
U.K. has recorded 44 cases of CVST out of more than 20 million
vaccine doses. Germany has logged 31 cases out of 2.7 million
vaccinations and Norway five out of just 120,000 shots. Regulators
report higher incidences of clots among women than among men.
The infection fatality rate for Covid-19, meanwhile, is
estimated to be around 1%, meaning that for every million people
infected, 10,000 deaths would be expected. But for younger people,
the risk is considerably lower. The IFR for someone aged 20 to 29
is estimated to be 0.03% to 0.04%, meaning that for every million
people in that age group infected, 300 to 400 deaths would be
expected.
That isn't the end of the story. In considering whether the risk
of getting seriously ill and even dying from Covid-19 outweighs the
risk of getting a blood clot as a side effect of vaccination, it is
also worth considering the likelihood of catching the disease in
the first place.
That is where the balance of risks behind these decisions is
most pertinent. A University of Cambridge analysis shows that if
Covid-19 infection is accelerating, with new cases being detected
at a rate of 200 per 100,000 people a day, for instance, then you
might expect seven out of every 100,000 20-to-29-year-olds infected
to need intensive care over a four-month period. If daily incidence
is lower, at 0.2 cases for every 100,000 people, roughly where the
U.K. was in March, then you might expect 0.8 intensive care
admissions. The same analysis put the risk of a severe blood clot
as a consequence of vaccination in people of the same age group at
1.1 per 100,000 doses administered.
How do those risks compare to other common risks?
The chance of dying in a road accident in 50 years of driving is
one in 85, according to the British Medical Journal. Blood clots
are a side effect of contraceptive pills, and over the course of a
year can occur in between two and 12 of every 10,000 women taking
them, depending on the medication in question. Studies cited by the
U.S. Centers for Disease Control and Prevention suggest that there
is a 0.3% chance of developing a symptomatic blood clot after a
flight of longer than eight hours -- 30 per 10,000.
What else should be considered when weighing the risks of
Covid-19 and vaccination?
While for young people most Covid-19 cases are mild, scientists
are concerned about so-called long Covid, a puzzling and sometimes
debilitating set of symptoms that can persist long after infection.
Some studies have found around 10% of people infected are still
suffering symptoms 12 weeks after infection. Another consideration
is herd immunity. Preventing a virus spreading by ensuring
sufficient numbers are immune not only protects against future
outbreaks but shields those especially vulnerable to the virus.
Write to Jason Douglas at jason.douglas@wsj.com
(END) Dow Jones Newswires
April 08, 2021 11:00 ET (15:00 GMT)
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