Objective: This study aimed to describe the change in knowledge, attitudes, concerns, perceptions, preventive practices, and vaccination intention at two key time points of the COVID-19 pandemic and to assess whether these changes varied by level of pandemic fatigue.Methods: Data included in this study came from the third and the ninth round of the COSMO-Spain cross-sectional study. A general linear model was used to investigate the interaction terms between rounds and levels of pandemic fatigue.Results: Changes between rounds were observed in knowledge, attitudes, concerns, perceptions, behaviours, and vaccination intention. Significant interactions between rounds indicated that those with low levels of pandemic fatigue had a greater increase in knowledge, lower decrease in concerns, greater decrease in agreement with the decisions made, and lower increase in vaccination intention compared with those with high pandemic fatigue.Conclusion: As a pandemic evolves, it becomes necessary to consider the level of pandemic fatigue of the population and how this affects knowledge, concerns, and agreement with the measures adopted, as they influence the population’s adherence to public health recommendations aimed at controlling infections and protecting the most vulnerable.