A change in the effectiveness of amantadine for the treatment of influenza over the 2003–2004, 2004–2005, and 2005–2006 influenza seasons in Japan

A retrospective study of the 2003–2004, 2004–2005, and 2005–2006 influenza seasons was done to investigate the effectiveness of amantadine and oseltamivir for treating influenza A. Commercial antigen detection kits were used for diagnosis and data were collected from 44 clinics throughout Japan, using an Internet-based system. Oseltamivir was administered to 2775 patients and amantadine to 781 patients. The durations of fever, from the time of the first drug administration and from the onset of fever, were calculated for each patient. In the 2005–2006 season, the duration of fever from the first drug administration was longer for patients who received amantadine than for those who received oseltamivir when the patients were grouped by the time from onset of fever to the start of treatment (P < 0.001 for groups administered at 0–12, 13–24, 25–36 h from the onset) and by patient age (P < 0.001 for under 16 years and P < 0.05 for 16–64 years). Mean values of duration of fever from the first drug administration were 31.3 h, 31.3 h, and 31.9 h for oseltamivir therapy, and 33.3 h, 42.7 h, and 53.3 h for amantadine therapy, in the 2003–2004, 2004–2005, and 2005–2006 seasons, respectively. Reduction in the effectiveness of amantadine over the three influenza seasons were also observed in each age group of 0–6, 7–15, and 16–64 years. The studied season was an independent factor associated with the effectiveness of amantadine by multiple regression analysis. In conclusion, the effectiveness of oseltamivir did not change, but the effectiveness of amantadine was progressively reduced over the three influenza seasons.