evidence: In March 2021, the European Medicines Agency concluded that in an extremely small number of vaccinated individuals there is a causal link between AZD1222 administration, blood clotting and low platelet counts (thrombocytopenia), leading to 30 deaths in vaccinated individuals79. As a result, vaccine agencies in EU countries and the UK issued age-based restrictions on the use of AZD1222. Similarly, extremely rare events of thrombocytopenia were observed in the USA, with causality following Ad26.COV2-S vaccination80,81 (six deaths among more than 6.8 million vaccinated individuals). The FDA briefly paused use of Ad26.COV2-S in April 2021. Although it seems to be more strongly associated with adenovirus-based vaccines, thrombocytopenia has also been observed after mRNA vaccination82. The rates of vaccine-induced immune thrombotic thrombocytopenia (VITT) differ in different countries, with a higher rate reported in Scandinavia (1 in 10,000) than the UK83; regional differences may reflect HLA type, reporting sensitivity and pre-existing conditions84. [...] Dolgin, E. CureVac COVID vaccine let-down spotlights mRNA design challenges. Nature594, 483–483 (2021). [...] British Society for Haematology. Guidance produced from the Expert Haematology Panel (EHP) focussed on Covid-19 vaccine induced thrombosis and thrombocytopenia (VITT). https://b-s-h.org.uk/about-us/news/guidance-produced-by-the-expert-haematology-panel-ehp-focussed-on-vaccine-induced-thrombosis-and-thrombocytopenia-vitt/ (2021).