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 Vaccine-induced immune thrombocytopenia and thrombosis (VITT) is a
combination of blot clots and low platelet levels which has been
labelled as a rare side effect of the viral vector COVID vaccines
made by AstraZeneca and Johnson & Johnson.
A higher occurrence of the side-effect in younger people has led to
many countries to put age restrictions on AstraZeneca's shot.
Around 85% of those who suffered rare blood clots after vaccination
with AstraZeneca's COVID-19 shot in Britain were under 60 even
though more of the shots were given to the elderly, the study found,
in one of the fullest characterisations of the syndrome so far.
It found that in those aged under 50, incidence was around 1 in
50,000, in line with previous estimates, and experts said the study
reinforced prior understanding of risk-benefit calculation of
vaccination.
Sue Pavord, a consultant haematologist at Oxford University
Hospitals who led the research, said the incident usually affected
young people who were otherwise healthy, and was especially
dangerous if it resulted in bleeding in the brain.
But she added that an initial spike of cases of the side-effect had
subsided as the impact of Britain's decision to offer under-40s
alternative shots in May filtered through.
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 "We haven't seen new cases for the last four
weeks or so and this has been a tremendous
relief," she told reporters.
The condition had an overall mortality rate of
23%, but that rose to 73% in cases with clots in
the brain known as cerebral venous sinus
thrombosis (CVST), though treatments like blood
plasma exchange increased the survival rate for
severe cases to 90%.
The researchers said they hoped the study would
inform vaccination strategy but emphasised the
importance of getting vaccinated, especially
given much higher rates of other types of clots
in severely ill COVID-19 patients.
The paper was published in the New England
Journal of Medicine.
Out of 294 possible cases analysed, 220 were
found to be definite or possible cases of VITT,
all of which followed the AstraZeneca rather
than the Pfizer vaccine.
Multiple clots were found in around one-third of
cases, and almost all those hospitalised with
the condition experienced it between five to 30
days after a first dose of AstraZeneca vaccine.
J&J's single-shot vaccine is not being rolled
out in Britain.
(Reporting by Alistair Smout; editing by David
Evans)
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