CONTEXT: Routine immunization of groups at high risk for influenza has been progressively implemented as a matter of Brazilian public health policy. Although the benefits of the vaccination for healthy young adults are still controversial, it has been offered yearly to hundreds of thousands of Brazilian workers, generally as part of wellness initiatives in the workplace. OBJECTIVE: To study the characteristics of subjects that accepted or refused to be vaccinated against influenza and to report on respiratory symptoms in both groups, one year after the campaign date. DESIGN: A prospective observational study. SETTING: Workers at a subsidiary of an international bank in São Paulo, Brazil. PARTICIPANTS: 124 persons that did not accept and 145 that voluntarily accepted the vaccine completed 12 months of follow-up. MAIN MEASUREMENTS: Data concerning gender, age, tobacco use, and any history of chronic respiratory illness such as asthma, bronchitis, rhinitis, and repetitive upper-respiratory infections, were recorded at the time of vaccination. After that, workers were asked monthly by questionnaire or telephone about respiratory symptoms, days of work lost and medical consultations. RESULTS: The results showed statistically significant differences regarding age (P = 0.004) with the vaccinated group (V) being younger than the non-vaccinated (NV) one, and with reference to previous repetitive upper-respiratory infections being higher among the V group (P < 0.0001). During the follow-up, the V group reported more occurrences of upper respiratory symptoms (P < 0.0001), due to both non-influenza (P < 0.0001) and influenza-like illness (P = 0.045). Differences were also found between V and NV groups concerning days off work and number of medical consultations due to upper-respiratory symptoms and non-influenza illness. Gender and history of repetitive upper-respiratory infections were the best predictors of influenza-like illness-related events. CONCLUSIONS: The making of previous reference to repetitive upper-respiratory infections was a major difference between those who accepted or rejected the vaccine. The vaccination itself was not sufficient to reduce the number of occurrences of respiratory symptoms and related absenteeism to levels similar to those found among non-vaccinated people.