Background: The processes involved in how individuals attribute symptoms to illness are important for understanding the basis of symptom complaints. In a study of patients undergoing travel vaccination, we investigated the association of trait negative affect (NA) and perceived sensitivity to medicines to reported symptoms and symptoms attributed by the patient to treatment.Methods: A sample of 121 patients attending a traveller's medical centre for vaccination prior to overseas travel completed a questionnaire prior to vaccination measuring trait NA, their perceived sensitivity to medicines, self‐rated health, as well as recent symptoms and doctor visits. Patients were assessed 20 min after the vaccination and again at seven days for their current symptoms and symptoms attributed to the vaccination.Results: We found both the number of symptom complaints and symptoms attributed to the vaccination immediately after treatment to be significantly associated with patients' perceived sensitivity to medicines and recent symptom complaints. At seven days, symptom complaints were also associated with perceived sensitivity to medicine as well as trait NA, while symptoms attributed by patients to the vaccination at seven days were significantly related to trait NA. At the seven‐day assessment, high trait NA was predictive of associating a far wider range of general symptoms to the vaccination.Conclusion: The results suggest that trait NA does not cause an increase in the reporting of physical symptoms in an illness or intervention that causes highly specific or immediate symptoms. However, trait NA plays an important role in the process of misattributing common symptoms to intervention or illness‐related causes and this process may help explain the association between trait NA and symptom repo