Influenza activity update - 2013 winter season
Western Australia has experienced a relatively mild and late influenza season in 2013. To date (23 September) there have been 1613 notifications of laboratory-confirmed influenza.
At the same time in 2012 there had been 4998 notifications. As shown in Figure 1, notifications in 2013 have tracked similarly to the moderate seasons experienced in 2010 and 2011, with peak activity considerably lower and later than in 2012 and the 2009 pandemic season. Based on trends in notifications and other indicators, such as percentage of positive laboratory tests and influenza-like illness syndromic surveillance undertaken in sentinel general practices and hospital emergency departments, it appears that activity for 2013 is currently around the peak.
Figure 1 - Influenza notifications in Western Australia by week, 2009 to 22 September 2013
2012 was an early and intense season, associated with the re-establishment of influenza A/H3N2 virus as the dominant subtype (estimated 62% of detections), a significant increase in influenza B circulation (37%) and the virtual disappearance of the 2009 A/H1N1 pandemic virus (1%). To date in 2013, A/H3N2 has continued to be the major influenza virus circulating in WA (60%), along with a diminished level of influenza B virus (13%) and a moderate resurgence in 2009 A/H1N1 circulation (27%), as demonstrated in Figure 2. Interestingly, in other states, A/H1N1 has been the most frequently reported influenza A subtype this year, and influenza B virus has also been more common than in WA, particularly in Victoria.
Figure 2 - Trends in influenza virus detections at PathWest laboratories, by type and subtype for the 12 months to 22 September 2013
Syndromic surveillance for influenza-like illness in sentinel general practices has also indicated that 2013 has been a relatively mild and late-peaking influenza season in WA, as shown in Figure 3. Hospital emergency department surveillance has demonstrated a similar pattern. Non-influenza viruses including respiratory syncytial virus, human metapneumovirus and parainfluenza viruses have contributed significantly to the overall burden of respiratory illness during winter, but are now in decline.
Figure 3 - Rate of influenza-like illness presentations to Sentinel Practitioners Network of WA by year, 2009 to 22 September 2013.
Consistent with the lower burden of influenza in the community, as indicated by notification and syndromic surveillance data, there have been numerically fewer adverse outcomes recorded in 2013 compared to 2012. Although the proportion of notified cases hospitalised this year (25.2%) is similar to that recorded in 2012 (27%), in absolute terms there have been around 1000 fewer admissions of influenza cases to public hospitals. Similarly, while 27 and 33 deaths were identified in persons notified with influenza in 2011 and 2012, respectively, only 13 cases have been identified to date in 2013. Ten influenza outbreaks have been reported in residential aged-care facilities to this point in 2013, but there appears to have been relatively little spread within these facilities, probably reflecting generally high vaccine coverage in residents and effective control strategies, including infection control measures and use of oseltamivir for treatment of cases and targeted prophylaxis in other residents.
Virus surveillance has indicated continuing high sensitivity of circulating influenza virus strains to oseltamivir and a reasonably good match to the 2012 southern hemisphere trivalent influenza vaccine formulation.Readers interested in further progress of the 2013 influenza season can access up-to-date surveillance data in the weekly Virus WAtch publication.