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AstraZeneca vaccine linked to bleeding disorder, Pfizer not – Expert Reaction

New Zealand-led research shows the Oxford-AstraZeneca Covid-19 vaccine is associated with a small increased risk of a rare bleeding disorder.

However, the study – which looked at more than 2.5 million people vaccinated in Scotland – finds no link between clotting or bleeding and the Pfizer vaccine, which is being used in New Zealand.

The SMC asked experts to comment on the findings. 

Professor Peter McIntyre, medical advisor, Immunisation Advisory Centre, comments:

“This is a nice study using the same data source and with the same authors as an earlier paper comparing vaccine effectiveness for AstraZeneca and Pfizer vaccines in Scotland. Immune thrombocytopenic purpura (ITP) is not so rare as central venous sinus thrombosis, but is also much less likely to cause fatal outcomes. Although I have to confess to being a paediatrician, and so not familiar with what ITP looks like in older adults, especially with co-morbidities and pre-existing platelet or clotting disorders, which would be substantially more common in the background than in children.

“The authors acknowledge that they did not have access to hospital records – and one would expect that any severe incidence cases of ITP requiring therapy would have been in hospital. They also were not able to have cases reviewed by an expert panel and so relied on coded diagnoses and inferred therapy for ITP, which in general practice would have been follow-on rather than primary therapy. They also did a self-controlled case series analysis, which is a nice methodological approach which was pioneered for ITP post-MMR and DTP vaccination in children. It allows comparison of the risk of an event in post-vaccine periods compared with pre-vaccine in the same individual, which as the authors say can control for residual confounding which I suspect is more of an issue in this dataset, dominated by older adults who would have many other causes for thrombocytopaenia.

“The lack of any association by either case control or Self Controlled Case Series method for Pfizer recipients is also reassuring with respect to study findings because the same biases should have been operating. Interesting that in a data set of the size they had available, they did not identify any of the thrombocytopaenia cases which have been reported in Europe and the UK with much larger populations, while these latter countries did not identify excess cases of ITP.

“The takeaways from this study for a New Zealand audience are:

  • The Pfizer vaccine is coming up roses again.
  • Context is important: the risk of various severe outcomes, including death, increased in the elderly at-risk population to the extent that the risk/benefit calculation is still strongly in favour of using AstraZeneca.
  • Scotland has an almost identical population size to New Zealand, and of course New Zealand has many similarities in terms of data set availability to Scotland, except for the lack of good primary care data (hospital data except for use of prescription data) and timeliness of data availability. Therefore, the capacity for rapid analysis is not as good but as there is no COVID-19 in New Zealand, if such a study were repeated here, that would not be a confounding factor.”

No conflict of interest declared.