Background: People’s perceived risk to be infected and to have severe illness has been thought as a motivational source of adherence to behavioral measures during the COVID-19 crisis. Methods: We used online self-reported data, spanning 20 months of the COVID-19 crisis in [blinded] (n = 241,275; 34% vaccinated; July 2020 - March 2022). Results: The findings demonstrate, especially among vaccinated persons, that people’s perceived severity was more prominent than perceived probability for infection, up until Omicron emerged. At both the between-persons and between-day levels, perceived severity was the most strongly related to autonomous motivation, a pattern that was less pronounced for unvaccinated people towards the end of the crisis. Conclusions: These findings show that variation in risk perceptions largely accounts for the variation in both between-individuals’ and day-to-day variation in motivation to adhere to the measures, thereby showing a sensitivity to the characteristics of the variants of the virus and the role of one’s vaccination status.