Younger age is the only factor to increase the risk of vaccine induced immune thrombocytopenia and thrombosis (VITT), according to a UK study that found no other predisposing medical conditions played a role in its development.
The prospective cohort study, led by Dr Sue Pavord, Consultant Haematologist at Oxford University Hospitals, looked at data from patients with 220 suspected VITT who presented to hospitals in the UK between March and June 2020, in a bid to determine clinicopathological features, risk factors, treatment, and markers of poor prognosis.
“The young age of patients was very striking, with 85% of cases occurring in the under 60s, and 56% in the under 50s,” Dr Pavord told the limbic.
“The vaccine program began its roll out in 90-year olds on January 4 and yet the condition didn’t become apparent until mid March. So I think that really shows that young age is a risk factor.”
However, the researchers did not find any other risk factors for developing the condition.
“There were no other risk factors in terms of prior medical illness, social disorders, previous history of thrombosis or being on the pill,” said Dr Pavord who suggested this would enable physicians to “reassure people with thrombotic disorders that they’re no more at risk of VITT than anybody else”.
“The thrombotic picture of VITT is really very different to normal thrombosis, it’s much more aggressive, much more life-threatening, and it’s more widespread,” she noted.
Meanwhile, the fact that the condition affects different sites in the vascular system was still vexing researchers. “There’s no obvious reason for it,” she told the limbic.
“With VITT’s sister condition, heparin induced thrombocytopenia (HIT), you would expect thrombosis to occur at a site with existing pathology. In VITT there’s no reason why it should have a predilection for one thrombotic site over another and why that should differ between people”.
The study showed that while overall mortality due to VITT was 22%, the rate of death was much higher (73%) in patients with platelet counts below 30,000 per cubic millimeter and intracranial haemorrhage.
According to the data, patients with VITT were 2.7 times more likely to die if they had cerebral venous sinus thrombosis, 1.7 times more likely to die for every 50% decrease in the baseline platelet count, 1.2 times more likely to die for every increase of 10,000 fibrinogen-equivalent units in the baseline d-dimer level, and 1.2 times more likely to die for every 50% decrease in the baseline fibrinogen level.
Criteria to diagnose VITT
The research, published in the New England Journal of Medicine, has also crucially provided further clarification on the set of characteristics which can be used by haematologists to accurately diagnose VITT, including an onset of symptoms 5-30 days post vaccination against SARS-Cov-2, the presence of thrombosis, a platelet count of <150,000 per cubic millimetre and positive anti-PF4 antibodies on the ELISA test.
“This is a really important aspect of the paper,” said Dr Pavord.
“We have defined the criteria for diagnosing cases, because publications and clinicians were often getting VITT muddled with vaccine-induced low platelets or vaccine induced thrombosis, or just non-vaccine related issues. But VITT is a really specific diagnosis with a unique pathogenesis, so it’s really important to get that diagnosis right when we’re looking at patient management and getting the true incidence of this condition.”
The researchers have shared the characteristics of VITT as determined by the study with both Public Health England (PHE) and the Medicine and Healthcare products Regulatory Agency (MHRA), so that physicians can apply the same definition criteria to each potential case.