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LONDON, Aug 11 (Reuters) - No new cases of a rare and severe
blood clots following vaccination with AstraZeneca's
COVID shot have been reported in Britain in recent weeks after a
decision to restrict its use in under-40s, British scientists
said on Wednesday.
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.
"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)
