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4. NSW Influenza Surveillance Report 19 to 25 September 2009 (Issued 2 October 2009)

NSW Influenza Surveillance Report 19 to 25 September 2009 (Issued 2 October 2009)

Summary

Influenza activity in NSW for the week ending 25 September, 2009 was at low levels. A small number of specimens were positive for influenza A [both H1N1 influenza 09 (human swine influenza) and seasonal influenza H1 and H3]. Respiratory syncytial virus (RSV) was the most common respiratory virus detected for this report week.

NSW influenza surveillance

The aim of influenza surveillance is to monitor general trends in influenza rather than the total number of people who are infected each year. The surveillance program has 4 main parts:

1.Influenza-like illness (ILI) presentations to 49 Emergency Departments¹
In the week to 25 September, ILI presentations to emergency departments (EDs) were similar to that of the previous week and were at low levels (rate 1.7 per 1000 presentations) (Figure 1). Presentations were mainly for mild illnesses, although 7% of patients presenting with ILI were admitted. Rates of presentations for all age groups were similar.
2.Laboratory diagnoses of respiratory infections²
Virology
In the week to 25 September, of the 593 samples tested by participating laboratories 8 (1.3%) were positive for influenza A. Of these 5 (63%) tested positive to H1N1 influenza 09 and 2 (25%) tested positive for seasonal influenza; one was H1 and the other was H3. The number of samples positive for influenza diminished and activity was localised. The number of influenza cases reported is only a small proportion of all influenza circulating in the community. RSV was the most commonly identified respiratory virus circulating for this week (Figure 2 & Table 1).
Serology
Please see Table 1.

3. Deaths due to influenza or pneumonia³
Death registration data show that as of 11 September 2009, there were 96 pneumonia or influenza deaths per 1000 deaths in NSW, which was below the seasonal threshold of 154 per 1000.
4. Outbreaks⁴
Nil reported outbreaks this week.

National activity

Seasonal influenza A strains circulating this year are the same strains contained in the 2009 seasonal vaccine. Influenza B strains match more closely with those in the 2009-10 Northern Hemisphere vaccine (B/Brisbane/60/2008-like virus) (Department of Health & Ageing).

WHO has recommended that the vaccines for use in the 2010 southern hemisphere influenza season contain the following antigens:

• an A/California/7/2009 (H1N1)-like virus;
• an A/Perth/16/2009 (H3N2)-like virus; and
• a B/Brisbane/60/2008-like virus (WHO).

International activity

Pandemic (H1N1) influenza virus continues to be the predominant influenza virus circulating. In the northern hemisphere pandemic H1N1accounted for 57% of influenza viruses and in the southern hemisphere it accounted for 94% of the total . On average, pandemic (H1N1) influenza accounted for 58% of all influenza detected worldwide for the week ending 7 September. Other influenza viruses detected included: influenza A H3 (22%), seasonal A H1 (4%), A not subtyped (16%), and B (2%) (Health Protection Agency, UK).

1. The 49 Emergency Departments included in this report are Albury, Auburn, Bankstown/Lidcombe, Belmont, Blacktown, Blue Mountains District, Bowral, Calvery Mater Newcastle, Camden, Campbelltown, Canterbury, Cessnock, Children's Hospital at Westmead, Coffs Harbour, Concord, Fairfield, Goulburn, Gosford, Griffith, Hornsby, John Hunter, Lismore, Lithgow, Liverpool, Maitland, Manly, Manning Base, Mona Vale, Mount Druitt, Nepean, Port Macquarie, Prince of Wales, Royal North Shore, Royal Prince Alfred, Ryde, Shellharbour, Shoalhaven, Singleton, St George, St Vincent's, Sutherland, Sydney Hospital, Sydney Children's, Tamworth, The Tweed, Wagga Wagga, Westmead, Wollongong and Wyong Hospitals.

Rates of ILI less than 2.0 /1000 presentations are considered "low", 2.0 to 3.9 /1000 presentations "moderate", 4.0 to 5.9 /1000 presentations "high" and greater than 6.0 /1000 presentations "very high".

2.Influenza laboratory diagnoses using virology are reported by South Eastern Area Laboratory Services (SEALS), Institute of Clinical Pathology and Medical Research (ICPMR), South West Area Pathology Services (SWAPS), Pacific Laboratory Medicine Services (PaLMS), Royal Prince Alfred Hospital (RPAH), Hunter Area Pathology Services (HAPS) and Children's Hospital at Westmead (CHW). Laboratory diagnoses by serology are reported by ICPMR, Royal Prince Alfred Hospital (RPAH) and SEALS but some results may reflect previous infections or immunisation are not reported weekly.

Point of care testing is a test that allows rapid qualitative detection of influenza A and B as well as RSV. It is currently used at HAPS, ICPMR and PaLMS.

3. Deaths data are sourced from the New South Wales Registry of Births, Deaths and Marriages and include deaths with "pneumonia" or "influenza" reported as a direct or contributing cause of death. Recent deaths referred to a Coroner are excluded. The interval between death and death data availability is usually at least 7 days, and so these data are one week behind reports from emergency departments and laboratories. In addition, previous weekly rates may change due to longer delays in reporting some deaths.

It should be noted that influenza infection may not be known at time of death certification and only very few certificates mention "influenza". Pneumonia has many other causes, so monitoring the pneumonia death rate is a crude indicator of influenza deaths.

Influenza, when it is circulating, is known to cause excess mortality over and above expected seasonal death rates. The predicted seasonal baseline is obtained by fitting a 'robust regression' model to estimate the baseline. Robust regression modelling limits the influence of outliers due to past epidemics. This is important because the aim of this surveillance is to identify outliers (or excess mortality) due to influenza epidemics. This is based on Serfling's method (Serfling RE. Methods for current statistical analysis of excess pneumonia-influenza deaths. Public Health Reports 1963; 78(6): 494-506.) The epidemic control limit is 1.2 standard errors of prediction from the regression model . This limit has been found to provide a reasonable balance between false alarms (control limit exceeded when influenza not circulating) and true alarms (control limit exceeded when influenza circulating). If the observed mortality rate remains above the upper confidence limit for 2 or more weeks, then this could indicate that the excess mortality may be due to circulating influenza.

4. Outbreaks of influenza voluntarily reported to Public Health Units from institutional settings such as residential care facilities

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Report prepared by Communicable Diseases Branch, NSW Health, in collaboration with: NSW Emergency Department Data Collection (HOIST), and NSW Real-time Emergency Department Surveillance System (Centre for Epidemiology and Research, NSW Department of Health), SEALS, ICPMR, SWAPS, PaLMS, HAPS, CHW, RPAH. Laboratories report weekly via a web base system designed by Andrew McNamara and Tim Churches, Centre for Epidemiology and Research, NSW Health. Enquires to Robin Gilmour, ph. 02 9424 5875, e-mail: robin.gilmour@doh.health.nsw.gov.au