A nationwide analysis of over 2.5 million Scottish adults who received a first dose of the Oxford–AstraZeneca or Pfizer–BioNTech vaccine revealed a small increased risk of an autoimmune bleeding disorder known as ‘immune thrombocytopenic purpura’ (ITP) associated with the Oxford–AstraZeneca vaccine. The research, published in Nature Medicine, also suggests that there may be evidence of an increased risk of other bleeding and vascular events associated with the Oxford–AstraZeneca vaccine. These very small risks are important but rare, and are comparable with those of other vaccines, including vaccines against hepatitis B, measles, mumps and rubella, and influenza.
Clinical trials have demonstrated that the Oxford–AstraZeneca (ChAdOx1) and Pfizer–BioNTech (BNT162b2) vaccines against SARS-CoV-2 have been generally well tolerated, although there have been a small number of reports of serious side effects. The UK’s Medicine and Healthcare products and Regulatory Agency received 209 reports of thrombocytopenic cases and thromboembolic cases (bleeding disorders and blood clot disorders, respectively) after 22 million first doses and 6.8 million second doses of the ChAdOx1 vaccine.
To investigate the possibility of an association between vaccination against SARS-CoV-2 and the development of blood disorders, Aziz Sheikh and colleagues examined cases of vaccine-related bleeding and vascular events among 2.53 million adults in Scotland (57% of the adult population 18 years of age and older) who received their first doses of vaccines against SARS-CoV-2 — the most common being ChAdOx1 or BNT162b2 — between December 2020 and April 2021. The authors found that for these people, the ChAdOx1 vaccine was associated with a slightly increased risk of ITP — a disorder that can cause minor bruising in some patients and excessive bleeding and long-term illness in others — up to 27 days after vaccination. This was at an estimated frequency of 1.13 cases per 100,000 first-dose vaccinations. The analysis also revealed very small increased risks of other arterial blood clot and bleeding events associated with ChAdOx1 up to 27 days after vaccination. However, there were insufficient data to conclude that there was an association between ChAdOx1 and cerebral venous sinus thrombosis (a rare condition in which a blood clot forms in the brain). They found no evidence of an increased risk of the adverse events studied associated with the BNT162b2 vaccine.
The authors emphasize that these findings need to be understood within the context of the very clear benefits of the ChAdOx1 vaccine. The risk of developing a serious adverse event related to the vaccine is far lower than the risk of serious illness or death caused by SARS-CoV-2, particularly for elderly and other vulnerable populations. Further research involving younger people (as relatively few people 40 years of age or under were included in this study due to the targeting of the vaccination program by age and underlying illnesses) and to assess responses to second doses of the vaccines is needed.
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