The rare blood clotting events recently associated with AstraZeneca’s COVID-19 vaccine may be caused by an autoimmune reaction to the adenovirus vector, doctors say.

Last week, the Federal Government was forced to rewrite its coronavirus vaccination plan after ATAGI recommended under-50s receive the Pfizer’s mRNA vaccine where possible, as the high-fatality rate events appeared to be occurring in younger people.

Although the mechanism remains speculative, two separate European case series published in the New England Journal of Medicine have described the syndrome and offered plausible explanations.

Both studies, one from German and Austrian researchers and another from Norway, say the events resemble a rare autoimmune reaction to the drug heparin, in which antibodies to the platelet factor 4 (PF4) protein are formed, triggering clot formation.