Exposure to a first dose of the AstraZeneca COVID-19 vaccine (ChAdOx1) but not the Pfizer vaccine (BNT162b2) appears to be associated with a small increased risk of idiopathic thrombocytopenic purpura (ITP).

An analysis of the first four months of the Scottish COVID-19 vaccination program, comprising 2.53 million people using data from linked databases, found ITP occurred in 1.13 cases per 100,000 vaccinations with ChAdOx1. The adjusted rate ratio (aRR) in the period 0–27 days after vaccination with ChAdOx1 was 5.77.

“For the 22 patients with post-vaccination ITP and for whom platelet counts were available after vaccination, all but two had counts below 100,000 per µl.”

About half (48%) of patients with post-ChAdOx1 ITP events had prior prescriptions that could induce ITP, compared to 35% of those who were unvaccinated at the time of their ITP event.

The study, published in Nature Medicine, said for other thrombocytopenia events, excluding ITP, the aRR in the period 0–27 days after vaccination with ChAdOx1 vaccination was 1.42. No increased risk of thrombocytopenia events was found for the BNT162b2 vaccine.

There was also no association between vaccination with either vaccine and VTE events overall, or the subgroups of DVT or PE.

The study said there was six post-vaccination cerebral venous sinus thrombosis (CVST) events – an insufficient number to adequately power an analysis. Post-vaccination CVST events were recorded for both vaccines and two individuals died.

There was a moderate increased risk of arterial thromboembolic events after ChAdOx1 vaccination (aRR 1.22) but not after BNT162b2 vaccination (aRR 0.92).

“For CVST (and other rare conditions), there were insufficient numbers to draw any reliable conclusions other than, if there is any association, it is likely to represent an extremely rare outcome.”

Hemorrhagic events were also increased after the ChAdOx1 vaccination (aRR 1.48) but not after BNT162b2 vaccination (aRR 0.92).

“We found increased risk for post-vaccination ITP and arterial thromboembolic and hemorrhagic events combined associated with increasing age (especially over 60), male sex, having certain comorbidities (such as heart failure, coronary heart disease, peripheral vascular disease, severe mental illness, sickle cell disease, prior stroke, type 1 and 2 diabetes and chronic kidney disease (stage 5)), very high blood pressure and smoking,” the study said.

Benefits outweigh the risks 

The investigators, led by epidemiologist Professor Colin Simpson, Associate Dean—Research at the Victoria University Wellington, said the very small risks of adverse events needs to be balanced against the very clear benefits of the ChAdOx1 vaccine.

However the increased risks associated with ChAdOx1 might warrant alternative vaccines for individuals at low COVID-19 risk when supply allows.

Professor Simpson told the limbic that VITT and venous thromboembolic/CVST case reports post-vaccine have received considerable media attention.

“Across 2.5 million vaccine doses of which 1.7 million were Oxford/AstraZeneca, we saw very few CVST events and these were seen mainly in the unvaccinated. There were so few CVST events, in fact, we couldn’t perform a reliable statistical analysis – i.e. we didn’t have sufficient power to do an analyses for this outcome.”

“Furthermore we didn’t see a signal for the venous thromboembolic outcome. In terms of the possible safety signals for arterial thromboembolic and haemorrhagic events and the consistent finding across the different analyses with idiopathic thrombocytopenic purpura, this is the first national published epidemiological study to report this.”

“In the wider context this isn’t an entirely surprising finding, as these ITP signals have been found with other common vaccines e.g. flu vaccine.”

He added that ITP was very rare post-vaccine, likely to be non-severe, and manageable in the vast majority of individuals.

“We can see from our work, that even if the AstraZeneca vaccine does have an increased risk of ITP, the benefits greatly outweigh the risks and it is important that we continue from a public health perspective to promote the vaccine.”