Flu vaccination for healthcare workers is the best way to prevent nosocomial influenza. A vaccination promotion campaign was launched at the Rouen University Hospital during the 2014–15 flu season, featuring posters, postcards, and a slogan contest. The objective of this study was to evaluate the effect of this campaign on vaccination coverage among healthcare professionals at the Rouen University Hospital. A self-administered questionnaire survey collected vaccination status before and after the promotional campaign (2013/14 and 2014/15 seasons) and the reasons for vaccination or refusal to be vaccinated. Vaccination rates were compared using McNemar’s chi-square test. A study of determinants of vaccination was conducted using logistic regression among staff with inconsistent vaccination status from one season to the next. The participation rate was 17.7%. Vaccination coverage was 44.9% (95% CI 1.9–47.8) and 39.4% (95% CI 36.7–42.1) for the 2013/14 and 2014/15 seasons (p = 0.004).The observed decrease was significant among nurses (p = 0.03) and nursing assistants (p < 0.03). Only 5.3% of healthcare professionals were vaccinated following the campaign. Perceived lack of vaccine efficacy was the main reason for non-vaccination (41.4%). Participation in the slogan contest (OR = 3.5, 95% CI 1.32–9.1) and “discussing vaccination” (OR = 4.4, 95% CI 1.6–12.4) were major determinants of vaccination uptake. The use of participatory approaches should be prioritized in future campaigns.