1. Influenza Virus Research Center, National Institute of Infectious Diseases, Gakuen, Musashimuyayama‐shiTokyo Japan
2. Eiken Chemical Co, Ltd, Taito, Taito‐kuTokyo Japan
3. Division of MicrobiologyOsaka Institute of Public Health, Tojo‐cho, Tennoji‐kuOsaka Japan
4. Department of PediatricsShowa General Hospital, Hanakoganei, Kodaira‐shiTokyo Japan