1. Institute of Pathology, St. Luke’s Medical Center, Quezon City 1112, Philippines
2. College of Public Health, University of the Philippines Manila, Manila 1000, Philippines
3. Accelerated Disease Control Branch, Global Immunization Division, Centers for Disease Control and Prevention, Atlanta, GA 30333, USA
4. Vaccine-Preventable Diseases and Immunization Unit, Division of Programmes for Disease Control, Western Pacific Regional Office, World Health Organization, Manila 1000, Philippines