During a crisis, society calls for individuals to take prosocial actions that promote crisis management. Indeed, individuals show higher willingness to help after a disaster. However, the COVID-19 pandemic presents significant differences, as it is a long-term crisis that affects all individuals and poses a direct health threat. Therefore, we propose that the pandemic negatively affects willingness to help, specifically blood donation intentions. It requires a high level of willingness to donate blood beyond the crisis outbreak, as more blood will be needed when postponed operations resume. Comparing blood donation intentions from a pre-pandemic study to results from a six-wave (bi-weekly) panel study conducted in Germany during the first pandemic phase (April to June 2020), we find lower blood donation intentions, especially in the medium and long term. While active donors show increased awareness of ability and eligibility to donate at the beginning of the pandemic compared to pre-pandemic, they feel significantly less able to donate as the pandemic progresses. Perceived ability to donate of inactive donors significantly decreases in the pandemic phase compared to the pre-pandemic phase. Crucially, both active and inactive donors feel less responsible and less morally obliged to donate, resulting in an overall negative pandemic effect on blood donation intent. The COVID-19 pandemic compromises blood donations endangering the life-saving blood supply. These alarming results offer evidence-based grounds for practical implications for driving donations in the event of a pandemic.