The risk of developing a very rare blood-clotting condition known as thrombosis with thrombocytopenia syndrome (TTS) is thereafter vaccination against the COVID-19 virus, new research has revealed.

According to the researchers, TTS occurs when a person has blood clots (thrombosis) as well as low blood platelet counts (thrombocytopenia).

It is very rare and different from general clotting conditions like deep vein thrombosis (DVT) or lung clots (pulmonary embolism). This syndrome is currently being investigated as a rare side effect of adenovirus-based COVID-19 vaccines, which use a weakened virus to trigger an immune response against coronavirus, but no clear evidence exists on the comparative safety of different types of vaccines.

For the study, published in the British medical journal (BMJ), the researchers stressed that this syndrome is very rare, but these observed risks should be considered when planning further immunisation campaigns and future vaccine development.

Based on health data from five European countries and the US, it shows a small increased risk of TTS after a first dose of the Oxford-AstraZeneca vaccine, and a trend towards an increased risk after the Janssen/Johnson & Johnson vaccine, compared with the Pfizer-BioNTech vaccine.

However, this was a well-designed study that allowed comparison of available vaccines with each other, rather than with no vaccination, and the results were consistent after additional analyses, suggesting that they withstand scrutiny, the study mentioned.