Delivering a Healthy WA
Disease WAtch

2012 Influenza season at a glance

The 2012 influenza season commenced earlier and was more intense than anticipated but appears to have peaked in mid-July (week 27) and is now declining.

Notification and laboratory surveillance data

Between 1 January and 26 August 2012, the Communicable Disease Control Directorate (CDCD) received 4315 notifications of laboratory-confirmed influenza, more than double the number received for the full year in both 2011 (1887 notifications) and 2010 (1633 notifications) (Figure 1).The early start to the season and sharp rise in notifications was reminiscent of the 2009 pandemic year. Notably, the Department of Health has also been notified of more outbreaks of influenza in residential aged-care facilities in 2012 than in any previous year.

Figure 1 is a graph showing influenza notifications in Western Australia by week, 2010 to 2012.

Figure 1: Influenza notifications by date of onset, 2010 to 26 August 2012.

This year's large increase in notifications has been associated with a marked decline in the number of detections of the 2009 A/H1N1 pandemic strain – which had continued to be the dominant strain in both 2010 and 2011 – and a concomitant increase in detections of both influenza A/H3N2 and B viruses (Figure 2). Up to 19 August 2012, PathWest has subtyped 2059 influenza isolates, comprising 63.4% A/H3N2, 35.4% B, 0.8% A/H1N1 and 0.4% mixed influenza virus infections.

Figure 2 is a graph showing influenza subtypes 2010 to 26 August 2012.

Figure 2: Influenza notification subtypes, 2010 to 26 August 2012.

Surveillance in the northern hemisphere over the 2011–12 winter demonstrated a declining match between circulating A/H3 and B influenza virus strains and the strains included in both the 2011–12 northern hemisphere and the 2012 southern hemisphere influenza vaccine formulations (namely, an A/California/7/2009 (H1N1)-like virus; an A/Perth/16/2009 (H3N2)-like virus; and a B/Brisbane/60/2008-like virus). Not surprisingly, the same drift strains (an A/Victoria/361/2011 (H3N2)-like virus; and a B/Wisconsin/1/2010-like virus) have predominated in Australia this winter and it is likely that relatively reduced levels of immunity against these strains have contributed to the intensity of the 2012 influenza season.

The 2012 season has also differed from the 2 previous seasons in terms of a pronounced geographical division of influenza subtype activity. Influenza subtype A/H3N2 has been the predominant cause of influenza in the Perth metropolitan area and southern part of WA, whereas influenza B was the predominant circulating strain in the Kimberley and Midwest regions during the inter-seasonal period from January through to May, causing 72% and 73% of influenza infections in these areas, respectively. Interestingly, influenza B virus was responsible for only 25% of infections in the Pilbara region during the same period.

Sentinel GP and ED influenza-like illness surveillance

Hospital emergency departments in Perth experienced high demand in early winter associated with the sharp increase in influenza activity. More than 1000 patients presented with influenza-like illness (ILI) symptoms at the peak in week 27, and presentations remained high in the following 2 weeks (Figure 3). However, the number of ILI presentations has since fallen to levels similar to those experienced at the same time in 2011 and 2010.

Figure 3 is a graph showing influenza-like illness presentations to Perth hospital emergency departments 2010 to 26 August 2012.

Figure 3: Influenza-like illness presentations to Perth hospital emergency departments, 2010 to 26 August 2012.

Similarly, reflecting the burden of respiratory disease in the community, data collected at GP practices participating in the Sentinel Practitioners Network of Western Australia also demonstrated a peak in ILI consultations (29.6 per 1000 consults) in week 27 (Figure 4). ILI consultations have since declined and are now lower than the peak levels reached in 2011 and 2010. Between 1 January and 26 August 2012, sentinel practitioners submitted a total of 1263 ILI samples to the PathWest laboratory. Of these, 43.8% (553) tested positive for influenza virus.

Figure 4 is a graph showing influenza-like illness presentations to WA sentinel GP practices, 2010 to 26 August 2012.

Figure 4: Influenza-like illness presentations to WA sentinel GP practices, 2010 to 26 August 2012.

Conclusion

The combined evidence from influenza notifications to the Department of Health, laboratory data, and sentinel GP and hospital emergency department indicators of community influenza-like illness, suggests that influenza activity is gradually declining in Western Australia, after an unusually early and abrupt period of high activity, peaking in mid-July. Other respiratory viruses, particularly respiratory syncytial virus, as well as the tail-end of the 2011–12 pertussis epidemic, have also contributed to the burden of respiratory illness over winter. The early influenza season followed higher than normal levels of inter-seasonal influenza activity during the summer and autumn months, especially in the north of the state.

The virtual disappearance of the A/H1N1 2009 pandemic influenza virus, and the relative rise in A/H3N2 and B viruses worldwide over the past 12 months – including in Australia – would appear to reflect the after effects of the rapid ecological ascendancy of the A/H1N1 2009 virus associated with pandemic spread in 2009, and its subsequent decline due to a high levels of herd immunity gained through natural infection and/or vaccination since 2009, and the fact that there has been little antigenic drift in the virus.

The intensity of the 2012 influenza season in Western Australia and other parts of Australia was not expected, given relatively mild activity during the preceding northern hemisphere winter.

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