aDivision of Infectious Diseases, Department of Medical Subspecialties, National Center for Child Health and Development, Tokyo, Japan

bVaccine and Infectious Disease Division, Fred Hutchinson Cancer Center, Seattle, Washington, USA

Correspondence to Chikara Ogimi, Division of Infectious Diseases, Department of Medical Subspecialties, National Center for Child Health and Development, 2-10-1 Okura, Setagaya, Tokyo 157-8535, Japan. Tel: +81 3 3416 0181; fax: +81 3 3416 2222; e-mail: [email protected]