Factors influencing the effectiveness of oseltamivir and amantadine for the treatment of influenza: A multicenter study from Japan of the 2002-2003 influenza season

Background. To evaluate the effectiveness of oseltamivir and amantadine for the treatment of influenza with respect to various clinical factors, a prospective multicenter study of the influenza season of 2002 - 2003 was done with 2163 patients whose condition was diagnosed by an antigen-detection test kit. Methods. Oseltamivir was administered to 803 patients with influenza A (A+Os group) and 684 patients with influenza B (B+Os group). Amantadine was administered to 676 patients with influenza A (A+Am group). Results. For each group, the duration of fever (i.e., body temperature, >= 37.5 degrees C) was significantly shorter in patients who received the drug within 12 h after the onset of symptoms than in patients who received the drug 112 h after the onset. For all 3 groups, the duration of fever was shorter in patients with a highest temperature <39 degrees C than in patients with temperatures >= 39 degrees C. The duration of fever was significantly longer for the B+Os group than for the A+Os group. Multiple regression analysis found that the type of influenza, the highest body temperature, and the time between the onset of symptoms and the start of treatment are independent factor that influence the duration of fever. Conclusions. Early administration increases the benefit of anti-influenza drugs - not only the benefit of oseltamivir treatment for influenza A, but also the benefit of amantadine treatment for influenza A and oseltamivir treatment for influenza B. Oseltamivir may be less effective as a treatment for influenza B than for influenza A. A highest body temperature of >= 39 degrees C was an indicator of a longer duration of fever.