Our study has several limitations. Most importantly, not every patient eligible for sampling was included, frequently because the patient refused or clinic staff failed to identify patients with illnesses that met the case criteria. These data were not recorded, so the effect of this bias cannot be estimated. As previously discussed, ILI is not a frequently used diagnosis outside this surveillance program, and most cases with this clinical syndrome were diagnosed as common cold. Many of the ILI cases documented were miscategorized in the clinic as pneumonia but were subsequently found not to meet the case defi nition, explaining the presence of persons hospitalized with ILI. Overall, these factors may bias toward sampling of case-patients who had more severe symptoms. Also, screening took place in only 1 of the 2 hospital outpatient clinics. However, because both are general clinics, the impact of this screening is likely to be refl ected in the absolute number of cases detected rather than in the proportion of ILI and pneumonia cases caused by infl uenza viruses. Regarding laboratory data, the likelihood of confi rmation of infl uenza infection is associated with the clinical case defi nitions in use: the strict ILI case defi nition used in our surveillance has a sensitivity of 98.4%-100% but a specifi city of only 7.1%-12.9% (25) . In another study, the probability of having a positive infl uenza virus PCR was directly related to magnitude of fever (26) . Therefore, given the bias toward severe cases, we may have considerably underestimated the impact of infl uenza in Maela.