Funding: The research leading to these results was conducted as part of the activities of the EU PE&PV (Pharmacoepidemiology and Pharmacovigilance) Research Network which is a public academic partnership coordinated by the Utrecht University, the Netherlands. The project has received support from the European Medicines Agency under the Framework service contract nr EMA/2018/23/PE. The content of this abstract expresses the opinion of the authors and may not be understood or quoted as being made on behalf of or reflecting the position of the European Medicines Agency or one of its committees or working parties.

Background: In 2021, thrombosis with thrombocytopenia syndrome (TTS) was confirmed by the European Medicines Agency (EMA) as a rare side effect of the COVID-19 adenovirus vector vaccines from AstraZeneca and Janssen.

Objective: To describe the public’s knowledge of TTS and how it affected the willingness to be vaccinated with COVID-19 and other vaccines in six European countries.

Methods: From June to October of 2022, a multi-country cross-sectional online survey was conducted in Denmark, Greece, Latvia, Netherlands, Portugal, and Slovenia. The minimum target of participants to be recruited was based on the country’s population. The results were analysed descriptively.

Results: In total, 3794 respondents were included in the analysis. Across the six countries, 33.3% to 68.3% reported being familiar with signs and symptoms of TTS, although 3.1 to 61.4% of those were able to identify the symptoms correctly. The reported changes in willingness to be vaccinated against COVID-19 and with other vaccines varied per country. The largest reported change in the willingness to be vaccinated with AstraZeneca and Janssen vaccines was observed in Denmark (61.2%), while the willingness to be vaccinated with other COVID-19 vaccines changed most in Slovenia (30.4%). The smallest decrease in willingness towards future vaccination against COVID-19 was reported in the Netherlands (20.9%) contrasting with the largest decrease observed in Latvia (69.1%).

Conclusion: Knowledge about TTS seemed to have influenced the public’s opinion in Europe resulting in less willingness to be vaccinated with the AstraZeneca and Janssen COVID-19 vaccines. Willingness for vaccination against COVID-19 with other vaccines and widespread use of vaccines to prevent other diseases also differed and seemed to be determined by the approaches taken by national health authorities when reacting to and communicating about COVID-19 vaccination risks. Further investigation of optimal risk communication strategies is warranted.