Annual Report of the National Influenza Surveillance Scheme, 2009.

The 2009 influenza season was considered a significant season triggered by the April 2009 emergence of a novel influenza A virus prompting a World Health Organization (WHO) declaration of a public health emergency of international concern. The overall number of notifications in the Australian 2009 influenza season was the highest since national reporting to the National Notifiable Diseases Surveillance System (NNDSS) began in 2001, and substantially higher than in prior years. Over 59,000 notifications were reported to the NNDSS, almost ten times the five year mean and representing a crude notification rate of 272.1 per 100,000. Australia's first case of confirmed influenza A(H1N1) pdm09 was identified in early May 2009. By the end of 2009, there were 37,755 laboratory confirmed cases, including 5,085 hospitalisations and 188 deaths notified. Traditionally the age distribution of influenza notifications has rates highest in very young children and the elderly, however in 2009 with the predominance of the pandemic virus, notifications were highest in older children and younger adults. Although influenza can cause very severe and fatal illness, particularly in the elderly, the impact of influenza A(H1N1) pdm09 in younger healthy adults, Aboriginal and Torres Strait Islander peoples, pregnant women and people with existing medical co-morbidities was proportionally greater than normal seasonal outbreaks, even though the absolute number of such cases remained low.1 The establishment of a number of surveillance systems during the pandemic enabled an enhanced assessment of the epidemiological, clinical and virological characteristics to inform public health responses.