New insights into extremely rare COVID-19 vaccine-related blood clots

Updated: Aug 25

Southampton has contributed to new research into extremely rare blood clots appearing after COVID-19 vaccination. The findings point the way to better treatments for those affected.

Most people who have the COVID-19 vaccine experience no serious side effects. However, 1.4 people per 100,000 experience vaccine-induced immune thrombotic thrombocytopenia (VITT).

VITT can lead to abnormal blood clot formation, blocking blood vessels in the brain. This cerebral venous sinus thrombosis (CVST) results in stroke - bleeding and swelling in the brain.

It also drives a drop in blood platelets levels. Platelets are key to stopping bleeding and less of them means brain bleeds can be more severe.

Findings support treatment choices

Southampton consultant and physician,

Dr Richard Marigold collaborated on this landmark study. Led by University College Hospital London, the results were published in The Lancet.

The study team compared outcomes of 70 patients with VITT-associated CVST with 25 patients with non-VITT CVST. They found VITT-associated CVST resulted in higher rates of death, or dependency on others after discharge. Yet they also showed treatment choice could change this.

Death or dependency rates were lower for VITT-CVST patients who had received either immunoglobulin or non-heparin anti-clotting drugs. Immunoglobulin is a mix of antibodies that modify the body’s immune response – a key part of stroke-induced injury.

Dr Marigold said: “The data supports this rare condition being treated with non-heparin blood thinning medication and intravenous immunoglobulin, reducing mortality and increasing the chances of patients returning home to independent living “However, we would urge caution in interpreting this observational data as it is not a randomised controlled trial, and more studies are needed to establish how effective the medications are. Further research is required to explain the underlying mechanisms causing this reaction to prevent it happening and improve future vaccine design. In addition, clinicians need to be aware of the early markers of this condition, as without recognition and prompt treatment, the outcome can be poor.”

Not a reason to avoid COVID-19 vaccination

Although VITT-associated CVST has severe impacts, it is extremely rare.

Professor Saul Faust, Director of the NIHR Southampton Clinical Research Facility and lead for COVID-19 vaccine studies at UHS, comments:

“Studies such as this, alongside the national MHRA and public health surveillance systems for medicines and vaccines side effects, are key to ensuring those with rare side effects are identified and the mechanisms and potential treatments understood. However, it is vital to view these results in context, as VITT is incredibly rare. It is also worth bearing in mind that, in age groups offered the AstraZeneca vaccine, COVID-19 infection itself can cause blood clotting problems.

“Vaccination is critical to protecting us all, the NHS and to stemming the emergence of new – potentially more lethal – variants. The vaccines in use passed through rigorous trials and continue to be monitored for safety and side-effects. What this study shows is that colleagues across the NHS are acting fast to understand and tackle such issues when they do arise.”

Dr Richard Perry, lead author and Consultant Neurologist at the National Hospital for Neurology and Neurosurgery at University College Hospital London said:

“With an illness of such severity, often in young patients who were previously fit and well, doctors have been desperate for evidence regarding treatments that might prevent some of the death and disability that arises from this condition.

“While an observational study is not the ideal platform to provide evidence for which medications work, it may be a long time before we have evidence from randomised clinical trials, the gold standard for testing new treatments. For the moment we are dependent on observational studies like CAIAC for our evidence.”