Background: Recently, many Japanese physicians have claimed that oseltamivir is less effective in children with influenza B virus infection. This study assesses the effectiveness of oseltamivir against influenza A (H3N2) and influenza B in children on the basis of the duration of febrile illness.
Methods: We used oseltamivir to treat 127 children with influenza A (H3N2; mean age, 6.97 years [range, 1-15 years]) and 362 children with influenza B (mean age, 5.16 years [range, 1-15 years]) in outpatient clinics. The duration of fever after the start of oseltamivir therapy was compared in the influenza A group and the influenza B group.
Results: The mean duration of fever after the start of oseltamivir therapy was significantly greater in the influenza B group than in the influenza A (H3N2) group (2.18 days vs. 1.31 days, respectively; P<.001). The difference was marked in young children (1-5 years old; 2.37 days for the influenza B group vs. 1.42 days for the influenza A group) but was not significant among older children (11-15 years old). The 50% inhibitory concentration of oseltamivir against influenza B virus was 75.4+/-41.7 nmol/L and was substantially higher than that for type A (H3N2) virus (0.3+/-0.1 nmol/L). Only 3 (1.6%) of 192 influenza B viruses were resistant to oseltamivir.
Conclusions: Oseltamivir is much less effective against influenza B virus infection in young children, probably because of the low sensitivity of influenza B viruses to oseltamivir. The effectiveness of oseltamivir against influenza B is influenced by age and host immunity. A few oseltamivir-resistant influenza B strains were isolated before the start of oseltamivir therapy.