1. Department of Maternal and Child Health, Faculty of Medicine; The University of Tokyo; Bunkyo-ku Tokyo 113-0033 Japan
2. Department of Pediatrics; Toho University School of Medicine; Ota-ku Tokyo 143-8541 Japan
3. Ehime Prefectural Institute of Health; Matsuyama Ehime 790-0003 Japan
4. Public Health Laboratory; John Radcliffe Hospital; Oxford OX3 9DU United Kingdom