Objectives: Hesitance and resistance to COVID-19 vaccination poses a serious challenge to achieving adequate vaccine uptake in the general population. Cross-sectional data from the early months of the pandemic indicates that approximately one-third of adults in multiple nations are hesitant or resistant to a vaccine for COVID-19. Using longitudinal data, we tracked changes in attitudes to COVID-19 vaccination during the pandemic. Study Design: A quantitative, longitudinal design. Method: Nationally representative samples of the adult general population of the Republic of Ireland (N = 1,041) and the United Kingdom (N = 2,025) were assessed for their attitudes towards COVID-19 vaccination at three points from March to August 2020. Results: Statistically significant increases in resistance to COVID-19 vaccination were observed in Irish (from 9.5% to 18.1%) and British (6.2% to 10%) adults. Conclusion: Resistance to vaccination has significantly increased in two European nations as the pandemic has progressed. Growing resistance to COVID-19 vaccination will pose a challenge to public health officials responsible for ensuring sufficient vaccine coverage.