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There is a small increased risk of thrombosis with thrombocytopenia syndrome (TTS), a very rare blood-clotting condition, after some Covid-19 vaccines, according to a study in The BMJ.
The analysis by an international team of researchers showed a 30% increased risk of thrombocytopenia after a first dose of Oxford-AstraZeneca compared with Pfizer-BioNTech – an absolute risk difference of 8.21 per 100,000 recipients.
An increase in risk, albeit not statistically significant, of venous thrombosis with thrombocytopenia was observed after a first vaccine dose of Janssen/Johnson & Johnson compared with Pfizer-BioNTech. But the researchers say this finding needs to be replicated in other studies before any firm conclusions can be drawn.
Risk of blood clots “should be considered” when planning further immunisation campaigns
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 a first dose of Pfizer-BioNTech.
The researchers stress that this syndrome is very rare, but say these observed risks “should be considered when planning further immunisation campaigns and future vaccine development.”
They added: “To our knowledge, this is the first multinational analysis of the comparative safety of adenovirus based compared with mRNA based Covid-19 vaccines.
“Although these events are very rare, absolute numbers of affected patients could become substantial owing to the large numbers of vaccine doses administered worldwide.”
What is thrombosis with thrombocytopenia syndrome?
TTS occurs when a person has thrombosis as well as low blood platelet counts (thrombocytopenia). It 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.
Their findings are based on routinely collected health data for over 10 million adults in France, Germany, the Netherlands, Spain, the UK, and the US who received at least one dose of a covid-19 vaccine (Oxford-AstraZeneca, Pfizer-BioNTech, Moderna or Janssen/Johnson & Johnson) from December 2020 to mid-2021.
To minimise possible error, participants were matched 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, 762,517 people receiving Janssen/Johnson & Johnson were matched to 2.8 million receiving Pfizer-BioNTech in Germany, Spain, and the US, and all 628,164 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.