1. Department of Virology; Osaka City University Medical School; Osaka Osaka 545-8585 Japan
2. Department of Pediatrics; Osaka City University Medical School; Osaka Osaka 545-8585 Japan
3. Laboratory of Biophysics; Osaka City University Medical School; Osaka Osaka 545-8585 Japan
4. Environment and Public Health Bureau; City of Osaka; Osaka Osaka 530-8201 Japan