BACKGROUND: To compare the effectiveness of oseltamivir for treatment of 
influenza A and influenza B, we conducted a prospective, multicenter study of 
the 2003-2004 and 2004-2005 influenza seasons. The study included 3351 patients 
in whom influenza had been diagnosed by use of an antigen detection test kit.
METHODS: Oseltamivir was administered to 1818 patients with influenza A and 1485 
patients with influenza B. No anti-influenza drugs were administered to 21 
patients with influenza A or to 27 patients with influenza B. Patients receiving 
oseltamivir therapy were divided into 4 groups according to the time between the 
onset of fever (temperature, > or = 37.5 degrees C) and administration of the 
first dose of oseltamivir (0-12 h, 13-24 h, 25-36 h, and 37-48 h). The patients 
were also divided into 4 subgroups on the basis of age (0-6 years, 7-15 years, 
16-64 years, and >64 years). Virus isolation was performed after completion of 
oseltamivir therapy for 44 patients with influenza A and 31 patients with 
influenza B.
RESULTS: The duration of fever was significantly shorter for patients with 
influenza A and B who were treated with oseltamivir than for patients who were 
not treated with an anti-influenza drug (P<.001 for both). The time until the 
patient became afebrile after the initial administration of oseltamivir and the 
duration of fever were significantly longer for patients with influenza B than 
for patients with influenza A for the 0-12 h, 13-24 h, 25-36 h, and 37-48 h 
groups (P<.001) and for all age groups (P<.001). After 4-6 days of oseltamivir 
therapy, the influenza B virus reisolation rate (51.6%) was significantly higher 
than the influenza A virus reisolation rate (15.9%) (P<.001).
CONCLUSION: Oseltamivir is less effective for influenza B than for influenza A 
with regard to duration of fever and virus persistence, irrespective of patient 
age or the timing of administration of the first dose.