title: The authors reply.
authors.affiliation: WHO Collaborating Centre for Reference and Research on Influenza,Melbourne,Australia.
authors.forename: S G
authors.initials: SG
authors.lastname: Sullivan
authors.affiliation: WHO Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong,Hong Kong Special Administrative Region,China.
authors.forename: B J
authors.initials: BJ
authors.lastname: Cowling