Background: Seroepidemiological studies have revealed that influenza C virus is widely distributed globally. However, because the isolation of this virus is difficult, there have been few reports on its clinical features.
Methods: Between December 1990 and November 2004, 84,946 respiratory-tract specimens were obtained from patients < or = 15 years old. On the basis of the results of isolation of virus, we examined the clinical data on children infected with influenza C virus.
Results: Of 170 children infected with influenza C virus, 157 (92.4%) were < 6 years old. Fever (frequency, 90.0%), cough (frequency, 74.1%), and rhinorrhea (frequency, 61.8%) were the most frequent symptoms. The mean duration of fever was 2.88 days (standard deviation, 1.66 days). Of the 170 children, 29 were hospitalized, and 21 (72.4%) of these 29 had lower-respiratory-tract illness such as pneumonia, bronchitis, and bronchiolitis. The rate of hospital admission was significantly higher in children < 2 years old than in children 2-5 years old (30.4% vs. 11.9%; P = .0043).
Conclusions: Influenza C virus is a significant cause of upper-respiratory-tract illness in children < 6 years old, and the risk of complications with lower-respiratory-tract illness is particularly high in children < 2 years old.