Epidemiology of influenza B in Australia: 2001‐2014 influenza seasons

BACKGROUND Influenza B is characterised by two antigenic lineages: B/Victoria and B/Yamagata. These lineages circulate together with influenza A during influenza seasons, with varying incidence from year to year and by geographic region. OBJECTIVE To determine the epidemiology of influenza B relative to influenza A in Australia. METHODS Laboratory-confirmed influenza notifications between 2001 and 2014 in Australia were obtained from the Australian National Notifiable Diseases Surveillance System. RESULTS A total of 278 485 laboratory-confirmed influenza cases were notified during the study period, comprising influenza A (82.2%), B (17.1%) and 'other and untyped' (0.7%). The proportion of notifications that were influenza B was highest in five- to nine-year-olds (27.5%) and lowest in persons aged 85 years and over (11.5%). Of all B notifications with lineage determined, 77.1% were B/Victoria and 22.9% were B/Yamagata infections. Mismatches between the dominant B lineage in a season and the trivalent vaccine B lineage occurred in over one-third of seasons during the study years. In general, influenza B notifications peaked later than influenza A notifications. CONCLUSION The proportion of circulating influenza B in Australia during 2001-2014 was slightly lower than the global average and was dominated by B/Victoria. Compared with influenza A, influenza B infection was more common among older children and young adults and less common in the very elderly. Influenza B lineage mismatch with the trivalent vaccine occurred about one-third of the time.