Potential impact of mandatory helmet when biking.

INTRODUCTION: Head injuries are a common and severe outcome of bicycle-related incidents, but they are preventable with a helmet. In Denmark, helmet use is not mandatory, and only half of cyclists wear helmets. Estimating the prevention potential of mandatory helmet use may inform evidence-based injury prevention policy. We aimed to estimate the number of preventable head injuries and severe head injuries under mandatory helmet use in 2020 and 2024. METHODS: We conducted a retrospective study using emergency department data from Odense and Svendborg University Hospital, where helmet use is systematically recorded. Cyclists treated for head injuries (minor or major) following bicycle-related incidents were included. The potential impact of mandatory helmet use was estimated-, assuming a post-legislation helmet use rate of 92%, based on evidence from New Zealand. A switch fraction was applied to account for compliance, and meta-analytic-based relative risk reductions of 48% for minor injuries and 60% for severe head injuries were used to estimate the number of potentially preventable head injuries. RESULTS: In total, 2,393 cyclists were treated for minor or major head injuries during the five-year period (1,366 (57%) unhelmeted). Overall, 57% were male, and the mean age was 38 years (SD 21 years). Severe head injuries were sustained by 511 cyclists, 275 of whom (53%) were unhelmeted. Extrapolation suggests that approximately 5,733 minor and 1,806 severe head injuries could have been prevented. CONCLUSION: The findings support helmet use as an effective prevention strategy to reduce head injuries among bicyclists. FUNDING: None. TRIAL REGISTRATION: Not relevant.