A comparison of the effectiveness of oseltamivir for the treatment of influenza A and influenza B: A Japanese multicenter study of the 2003-2004 and 2004-2005 influenza seasons

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, >= 37.5 degrees C) and administration of the first dose of oseltamivir (0-12 h, 13-24 h, 25-36 h, and 37-48 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 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.