Vaccination can be seen as a strategic decision. Accordingly, not only individual benefit aspects determine the vaccination decision, but also the common good. If the vaccination rate is high, the indirect effect of vaccination also protects unvaccinated individuals. This is called herd immunity. However, high vaccination rates also encourage free-riding: Individuals rely on the existing vaccination protection of others and benefit from the reduced likelihood of illness due to vaccinations already carried out in society without exposing themselves to the risk of the vaccination. Vaccination thus becomes a social dilemma in that it is rational not to be personally vaccinated. However, from a social point of view, vaccination is necessary to contain or even eradicate the spread of disease. In the project "Vaccine Hesitancy" this dilemma is examined in a strategic game ("iVax-Game"). The aim of the project is to identify psychological factors that contribute to vaccination readiness or free-riding. Furthermore, the iVax-Game can be used to test communication strategies in the laboratory that are promising to counteract vaccination hesitancy.