London: Scientists have uncovered more knowledge about the risk of developing a very rare blood clotting condition following vaccination against COVID-19, according to a study.

Based on health data from five European countries and the US, the study shows an increased risk of the condition, called thrombosis with thrombocytopenia syndrome (TTS), after first-dose of the Oxford-Astrazeneca vaccine (same as Covishield in formulation).

The study, published in The British Medical Journal (BMJ), also shows a trend towards an increased risk after the Janssen/Johnson & Johnson vaccine, compared with the Pfizer-BioNTech vaccine.

The researchers stressed that the syndrome is very rare, but said the observed risks "should be considered when planning further immunisation campaigns and future vaccine development."

TTS occurs when a person has blood clots (thrombosis) as well as low blood platelet counts (thrombocytopenia). According to the study, it is very rare and different from general clotting conditions like deep vein thrombosis (DVT) or lung clots (pulmonary embolism).

TTS is currently being investigated as a rare side effect of adenovirus-based Covid 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, according to the study.

To address this knowledge gap, an international team of researchers set out to compare the risk of TTS or thromboembolic events associated with the use of adenovirus-based Covid vaccines with mRNA-based Covid vaccines.

Their findings were based on routinely collected health data for more than 10 million adults in France, Germany, the Netherlands, Spain, the UK, and the US who received at least one dose of a Covid vaccine (Oxford-AstraZeneca, Pfizer-BioNTech, Moderna or Janssen/Johnson & Johnson) from December 2020 to mid-2021.

Further, to minimise possible error, participants were matched by the researchers by age and sex and a range of other potentially influential factors such as pre-existing conditions and medication use were taken into account.

The researchers then compared rates of thrombosis and of thrombosis with thrombocytopenia between the adenovirus vaccines (Oxford-AstraZeneca or Janssen/Johnson & Johnson) and the mRNA vaccines (Pfizer-BioNTech or Moderna) within 28 days after vaccination.

Overall, 1.3 million first dose Oxford-AstraZeneca recipients were matched to 2.1 million Pfizer-BioNTech recipients from Germany and the UK. An additional 7,62,517 people receiving Janssen/Johnson & Johnson were matched to 2.8 million receiving Pfizer-BioNTech in Germany, Spain, and the US, and all 6,28,164 Janssen/Johnson & Johnson recipients from the US were matched to 2.2 million Moderna recipients.

A total of 862 thrombocytopenia events were found in the matched first dose Oxford-AstraZeneca recipients from Germany and the UK, and 520 events after a first dose of Pfizer-BioNTech.

When the data were pooled together, analysis showed a 30 per cent increased risk of thrombocytopenia after first dose of Oxford-AstraZeneca compared with Pfizer-BioNTech - an absolute risk difference of 8.21 per 100,000 recipients, the study found.

No differential risk of thrombocytopenia was seen after a second dose of Oxford-AstraZeneca compared with a second dose of Pfizer-BioNTech.

Similarly, no increased risk of thrombocytopenia was noted after Janssen/Johnson & Johnson compared with first dose of Pfizer-BioNTech.

This is an observational study, and the researchers acknowledge that the rarity of the condition and incomplete vaccine records may have affected the results.

Additionally, the scientists cannot rule out the possibility that some of the observed risks may have been due to other unmeasured and confounding factors.

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

"To our knowledge, this is the first multinational analysis of the comparative safety of adenovirus based compared with mRNA based Covid-19 vaccines," said the authors.

"Although these events are very rare, absolute numbers of affected patients could become substantial owing to the large numbers of vaccine doses administered worldwide," they warned.