New research, published in the journal Efficacy and Mechanism Evaluation from the National Institute of Health and Care Research (NIHR), confirms there is a much higher risk of arterial and venous thrombosis (blood clots) after having a COVID-19 infection, compared to the risk from vaccination against the virus.
The study, ‘Understanding mechanisms of thrombosis and thrombocytopenia with adenoviral SARS-CoV-2 vaccines: a comprehensive synopsis’, represents the most detailed review to date of a rare, COVID-19 associated blood clot condition, called vaccine-induced immune thrombotic thrombocytopenia (VITT). It also confirms that while the condition is serious, it remains extremely rare.
The publication marks the latest findings from the Thrombotic Thrombocytopenia Syndrome (TTS) Consortium— a research collaboration supported by the NIHR and the UK Government’s Vaccine Taskforce. The TTS Consortium has been working to understand the risks and biological mechanisms behind the already-established association of the rare blood-clotting syndrome VITT with some COVID-19 vaccines.
The consortium has employed a comprehensive approach across five work packages. The first work package of the study, enabled by the British Heart Foundation Data Science Centre at Health Data Research UK, focused on large-scale cohort studies covering nearly the entire English and Welsh population within the NHS England NHS England Secure Data Environment, a secure data analysis platform for approved researchers.
Analysis revealed that the risk of both arterial and venous blood clots is markedly higher following COVID-19 infection – especially within the first week. The risk of venous clotting was over 30 times higher immediately after infection, for example.
By contrast, the risk following vaccination was far lower – and only linked to the first dose of the AstraZeneca vaccine. The study demonstrated that second doses of the AstraZeneca vaccine, and any doses of the mRNA vaccines, did not carry the same risk of rare cerebral blood clots and low platelet counts.
Encouragingly, the research found that existing treatments like intravenous immunoglobulin (IVIg) and certain antiplatelet drugs may help block the harmful platelet activation and inflammation which results in VITT events.
Professor Sir Munir Pirmohamed, corresponding author for the study, and also co-lead of HDR UK’s Medicines in Acute and Chronic Care Driver programme and HDR UK North, said:
“This Consortium’s research has provided vital new insights into understanding and hopefully preventing these rare but serious reactions. A key finding is the need for much better data infrastructure in the UK – which would allow real-time assessment of how well the vaccines are working, and whether any harm is occurring.”
The researchers recommend setting up a national reference laboratory and registry to track and diagnose TTS cases more efficiently. The study utilised national and regional level linked health data – but the researchers met with some limitations, as currently available national data feeds do not include hospital patients, and lack detailed coding which can help differentiate between different conditions or diagnoses.
The TTS Consortium’s work represents a landmark effort to understand how rare clotting syndromes develop, and how patients can be better protected. Another key part of this effort was a separate workstream focused on vaccine safety using national and regional linked health data, which was led by study co-author Professor Cathie Sudlow. The workstream, Evaluating SARS-CoV-2 vaccine safety using national and regional linked health data, was supported by HDR UK’s Programme Director Lara Edwards, and involved scientific leadership from across the HDR UK Regional Network.
Read the full paper
