1. Department of Virology, National Institute of Hygiene and Epidemiology, Hanoi, Vietnam.
2. Department of Bacteriology I, National Institute of Infectious Diseases, Tokyo, Japan.
3. Department of Veterinary Medical Sciences, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo, Japan.
4. Vietnam Research Station, Center for Infectious Disease Research in Asia and Africa, Institute of Tropical Medicine, Nagasaki University, Nagasaki, Japan.
5. Department of Infectious Diseases Control, Hanoi Center for Disease Control, Hanoi, Vietnam.