Zanamivir Treatment Is Equally Effective for Both Influenza A and Influenza B

Tothe Editor—We previously reported that oseltamivir was less effective against influenza B than it was against influenza A in a study of the 2002–2003 influenza season; these findings were similar to those in a report in 2007 by Sugaya et al. [1,2–3]. However, the effectiveness of another neuraminidase inhibitor, zanamivir, has not been compared between influenza A and influenza B. Therefore, we performed a preliminary study of the effectiveness of zanamivir for the treatment of 67 patients with influenza A and 100 patients with influenza B (with influenza being diagnosed using commercial antigen detection kits) [3, 4] during the 2001–2002, 2002–2003, 2003–2004, 2004–2005, and 2005–2006 seasons (table 1). The percentage of patients who were afebrile at 24 h or 48 h after the first inhalation of zanamivir was analyzed as a parameter of the effectiveness of zanamivir treatment. There was no significant difference between patients with influenza A and patients with influenza B with respect to the percentage of patients who were afebrile at 24 h (49.3% vs. 36%) or at 48 h (79.1% vs. 80%). The percentage of afebrile patients at 24 h and 48 h after the first inhalation of zanamivir. In our previous study, the mean duration of fever (±SD) in patients with influenza A and patients with influenza B was 31.2 ± 23.7 h and 47.1 ± 30.8 h, respectively, after the first dose of oseltamivir and 47.9 ± 26.0 h and 65.4 ± 32.8 h, respectively, after the onset of fever [3]. In addition, the mean duration of fever (±SD) after onset of fever was 82.4 ± 36.0 h and 78.3 ± 41.9 h in patients with influenza A and patients with influenza B, respectively, who were not treated with antiinfluenza drugs [3]. Studies of in vitro antiviral activity of oseltamivir or zanamivir against laboratory strains of influenza virus that used culture and enzymatic assays have suggested that influenza B virus is less susceptible than influenza A virus to oseltamivir and zanamivir [5]. However, the reported difference of the mean inhibitory concentration of 50% between influenza A and B viruses was less for zanamivir (2.09 nM vs. 4.15 nM) than it was for oseltamivir (0.73 nM vs. 11.53 nM). These findings may explain our results in a clinical context, showing that oseltamivir is less effective against influenza B than it is against influenza A and that zanamivir is equally effective against both. We are now studying the effectiveness of zanamivir against influenza A and influenza B among a large number of patients identified during the 2006–2007 season. In conclusion, zanamivir is more effective than oseltamivir for the treatment of influenza B. Potential conflicts of interest. All authors: no conflicts.