During the celebrations of the religious ceremony known as World Youth Day in Sydney, Australia, in July 2008 (WYD2008), which was winter-time in Australia, there was a significant influenza outbreak which affected a number of people. This study assessed epidemiologic data and respiratory samples collected from attendees to that mass gathering who sought treatment for influenza-like illness at emergency clinics in Sydney. Isolated influenza viruses were compared with seasonal influenza viruses from the 2008 influenza season. From 100 infected attendees, numerous strains were identified: oseltamivir-resistant influenza A(H1N1) viruses, oseltamivir-sensitive influenza A(H1N1) viruses, influenza A(H3N2) viruses and strains from both influenza B lineages (B/Florida/4/2006-like and B/Malaysia/2506/2004-like). The authors showed that new viruses were introduced and the pre-WYD2008 seasonal viruses were amplified by the event. The authors argue that mass gatherings can have substantial, complex and unpredictable effects on community influenza activity on grounds of the mix of virus from different origins typical of these events.
via . ECDC