Australian influenza report 2009 - 13-26 June 2009 (#7/09)

The Australian Influenza Report is compiled from a number of data sources, including laboratory-confirmed notifications to NNDSS, sentinel influenza-like illness reporting from general practitioners and emergency departments, workplace absenteeism, and laboratory testing. It is produced weekly from May to October. A more in-depth end of season report is also published in Communicable Diseases Intelligence.

Page last updated: 26 June 2009

Report No. 7
Week ending 26 June 2009

A print friendly version of this report is available as a PDF (227 KB)

Key Points

  • Influenza notifications are high compared to previous years reflecting the introduction and rapid transmission of H1N1 Influenza 09 throughout the Australian community as well as higher rates of detection of seasonal influenza due to high levels of testing for H1N1 Influenza 09.
  • The proportion of H1N1 Influenza 09 to seasonal influenza varies by jurisdiction with most reporting that the number of notifications of H1N1 influenza 09 is now greater than the number of seasonal influenza A notifications.
  • All Australian jurisdictions have moved to a new response phase called PROTECT in which laboratory testing is directed towards people with moderate or severe illness, more vulnerable to severe illness, or in institutional settings. This move will mean cases of H1N1 Influenza 09 are under-reported. The decrease in daily notifications does not reflect a decrease in H1N1 Influenza 09 activity.
  • To 26 June 2009, the total number of cases of H1N1 Influenza 09, was 3360 with 197 hospitalisations and 4 deaths confirmed. *
  • The Australian deaths associated with confirmed H1N1 Influenza 09 infection have been largely confined to people with pre-existing underlying medical conditions.
  • The highest reporting of confirmed cases was observed in the 10–19 year age group, likely to be related to the predominance of school based transmission in the earlier stages of the outbreak. In recent weeks, there has been a shift from school-based transmission and importation of cases from areas of high prevalence to higher levels of community transmission.
  • Clinical symptoms observed in 500 Australian confirmed cases for whom data was available include cough, fever, sore throat, runny nose and joint pain. Fever was reported in approximately seventy six per cent (380) of cases, a lower level than reported in other countries.
  • Complications and severe disease requiring hospitalisation were reported in six per cent (197) of confirmed cases, with twenty five per cent (49) of these cases being admitted to an intensive care unit.
  • Indigenous Australians are overrepresented in the hospitalisations for H1N1 influenza 09. This apparent disparity in rates between Indigenous and non-Indigenous Australians may reflect differences in underlying co-morbidities, but could also relate to health seeking behaviours. More severe illness could develop from not seeking medical help early enough, a possible issue in remote areas. There have been a number of significant clusters in Aboriginal and Torres Strait Islander communities.
  • A strain of H1N1 Influenza 09 showing resistance to osteltamivir (Tamiflu) has been identified in Denmark.** Anti-viral resistance is not uncommon in flu viruses and the World Health Organization (WHO) has stated that the Danish case is isolated and has no public health implications. The antiviral resistance finding will not prompt any changes in the WHO antiviral recommendations.
  • Seasonal Influenza notifications in Australia continue to rise, a reflection of increased detection associated with the H1N1 Influenza 09 pandemic. Syndromic and laboratory surveillance show increases in influenza-like illness and confirmed influenza, indicating that the Australian influenza season may have commenced.
  • The majority of seasonal influenza notification rates have been in the age range of 0–24 years. Notification rates are highest in the 10–14 year age group.
  • Influenza A(H3N2) is the predominant seasonal influenza subtype reported by most jurisdictions.

    *This report was produced based on data for the period ending 26 June 2009. At the time of publication however, there have been 5733 confirmed cases, 552 hospitalisations and 11 deaths confirmed.
    **At the time of publication strains showing resistance to antivirals have also been found in Japan and Hong Kong.


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    Influenza Activity in Australia

    Epidemiology of Influenza in Australia

    As Figure 1 shows, the influenza activity in 2009 started earlier than in 2008 and there was a rapid increase in the number of confirmed influenza cases (both seasonal and H1N1 Influenza 09) from week 18. The high rates of seasonal influenza seen during May and June are most likely due to the increase in testing for H1N1 Influenza 09.

    The total number of confirmed influenza cases started decreasing after 17 June 2009 as result of Australia’s transition to a new response phase called PROTECT, in which laboratory testing is directed towards people with moderate or severe illness; those more vulnerable to severe illness; and those in institutional settings.

    Figure 1: Influenza activity in Australia, by reporting week, years 2008 and 2009

    Influenza activity in Australia, by reporting week, years 2008 and 2009

    Sources: NNDSS and NetEPI



    The proportion of H1N1 influenza 09 to seasonal influenza varies across jurisdictions. Most jurisdictions have reported that the proportion of H1N1 influenza 09 has increased in recent weeks to be greater than that of seasonal influenza A. Reports from the three National Influenza Centres (NICs) showed that average proportion of influenza A positive tests that were H1N1 influenza 09 for the week ending 20 June 2009 was 64%. The following week (ending 27 June) increases in the proportion of H1N1 influenza 09 were reported, by the NICs, in WA (64% to 75%) and NT (35 % to 71%). Levels in NSW remained the same at 51%. Victoria reported a proportion of H1N1 influenza 09 of 85% for the week ending 20 June 2009. No data were available for the following week.
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    Laboratory confirmed influenza

    Note: This section EXCLUDES any notifications of H1N1 Influenza 09 that are identifiable in NNDSS. A small number of notifications reported as Influenza A are known to be H1N1 influenza 09.

    There have been 5,716 cases of laboratory confirmed influenza diagnosed and reported to the National Notifiable Diseases Surveillance System (NNDSS) to 26 June 2009 (Figure 2). There were 1,115 laboratory confirmed influenza cases in the same period last year.

    Influenza notifications are above the 5 year rolling mean for this period (Figure 2). The high notification rates are predominantly due to increased testing for influenza associated with H1N1 Influenza 09. Influenza notifications and notification rates have decreased in this reporting period (Figures 2 and 3). It is likely that this is related to the transition to the Protect phase and delays in reporting.

    Figure 2. Number of laboratory-confirmed influenza notifications, NNDSS, 1 January 2009 to 26 June 2009, by jurisdiction and week of diagnosis

    Number of laboratory-confirmed influenza notifications, NNDSS, 1 January 2009 to 26 June 2009, by jurisdiction and week of diagnosis

    SOURCE: NNDSS


    Notifications in 2009 have been predominantly from Victoria (2,097 notifications – 36.7%), Queensland (1,367 notifications – 23.9%), New South Wales (881 notifications – 15.4%) and South Australia (648 notifications – 11.3%). The Australian Capital Territory, Northern Territory, Tasmania and Western Australia accounted for the other 12.7% or 723 notifications (Table 1; Figure 3). The Northern Territory has reported the highest rate of seasonal influenza notifications per 100,000 population (117) followed by South Australia (41), Victoria (40), and the Australian Capital Territory (33). All other jurisdictions reported notification rates less that 15 per 100,000 population.

    Table 1. Number and rate of laboratory-confirmed notifications by jurisdiction, NNDSS, 1 January 2009 to 26 June 2009
    State Cases Percentage of Total Notifications Rate per 100,000 Average Rate YTD 2004-2008 Percentage of Australian  population*
    ACT
    113
    2.0%
    33.2
    22.1
    1.6%
    NSW
    881
    15.4%
    12.8
    17.8
    32.6%
    NT
    252
    4.4%
    117.2
    38.6
    1.0%
    QLD
    1367
    23.9%
    32.7
    31.0
    20.0%
    SA
    648
    11.3%
    40.9
    7.4
    7.5%
    TAS
    66
    1.2%
    13.4
    12.0
    2.3%
    VIC
    2097
    36.7%
    40.3
    10.1
    24.8%
    WA
    292
    5.1%
    13.9
    18.3
    10.1%
    AUS
    5716
    100%
    27.2
    3.6
    100.0%

    SOURCE: NNDSS, ABS (*populations 2008)


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    Figure 3. Number of laboratory-confirmed influenza notifications, NNDSS, 1 January 2009 to 26 June 2009, by jurisdiction and week of diagnosis

    Number of laboratory-confirmed influenza notifications, NNDSS, 1 January 2009 to 26 June 2009, by jurisdiction and week of diagnosis

    SOURCE: NNDSS


    National age-specific notification rates YTD show the highest rates of notifications occurred in the 10–14 year age group. Rates of influenza are higher in males than female in the 0 to 19 and 70 to 85+ age ranges (Figure 4).
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    Figure 4. Notification rates of laboratory-confirmed influenza, NNDSS, Australia, 1 January 2009 to 26 June 2009, by age group and sex

    Notification rates of laboratory-confirmed influenza, NNDSS, Australia, 1 January 2009 to 26 June 2009, by age group and sex

    SOURCE: NNDSS



    Mapping of influenza notifications by statistical division of residence indicates that there are areas within states with higher rates compared to the rest of the state or Australia (Figure 5). The numbers on the map indicate the number of cases occurring within each region.

    Figure 5. Map of notification rates of laboratory-confirmed influenza, NNDSS, Australia, 1 January 2008 to 26 June 2009, by Statistical Division of residence

    Map of notification rates of laboratory-confirmed influenza, NNDSS, Australia, 1 January 2008 to 26 June 2009, by Statistical Division of residence

    SOURCE: NNDSS


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    Laboratory Surveillance

    The Victorian Infectious Disease Reference Laboratory (VIDRL) has reported that in the fortnight ending 21 June 2009, picornavirus, influenza A and respiratory syncytial virus (RSV) were the most commonly detected respiratory viruses in Victoria (For full report see: www.vidrl.org.au).

    South Australia has reported that in the fortnight ending 21 June 2009, influenza A, rhinovirus, RSV and adenovirus were the most commonly detected respiratory viruses in South Australia.

    New South Wales reported that in the fortnight ending 5 June 2009, influenza A and RSV were the most commonly identified respiratory viruses circulating.

    The New South Wales sentinel laboratory network collects virology data from six major public laboratories and serology data from three major public laboratories.

    In the fortnight ending 5 June 2009, the average number of virology samples tested in New South Wales was 1,966 a significant increase over previous periods and previous years. The expected decrease in testing as a result of the introduction of the PROTECT phase is not yet reflected in this graph. The percentage of tests positive for influenza compared to the number of tests has increased over recent weeks to approximately 10% and is much higher than at the same time in 2007 and 2008 (Figure 6).

    Figure 6. Number of All Virology Tests for Respiratory Illness and Percentage of tests positive for influenza, NSW, 2007 to 5 June 2009

    Number of All Virology Tests for Respiratory Illness and Percentage of tests positive for influenza, NSW, 2007 to 5 June 2009

    SOURCE: NSW HEALTH ‘NSW Influenza Surveillance Report’


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    Sentinel General Practice

    Data available from the Australian Sentinel Practices Research Network (ASPREN), the Northern Territory GP surveillance system, and VIDRL up until 21 June 2009 show that ILI consultation rates have increased indicating an increasing level of ILI in the community. Rates in recent weeks are following similar trends to that seen in 2007 (Figure 7). In the last week, the presentation rate to sentinel GPs in Australia was approximately 24 cases per 1,000 patients seen.

    The first confirmed case of H1N1 Influenza 09 reported in Australia was on 9 May 2009 (week 20), corresponding with an increase of ILI consultations to GPs.

    As data from NT and VIDRL surveillance systems are being combined with ASPREN data, rates may not be directly comparable between 2007/2008 and 2009.

    Figure 7. Rate of ILI reported from GP ILI surveillance systems from 2007 to 2009 by week

    Rate of ILI reported from GP ILI surveillance systems from 2007 to 2009 by week

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    Emergency departments

    The Western Australia Influenza Surveillance Program collects data from eight Perth Emergency Departments (EDs). The number of ILI presentations reported in WA EDs continues to rise and is higher than numbers seen in 2007 and 2008 at the time of year (Figure 8). The number of admissions through EDs of patients with ILI is reported to have also increased slightly during the period from 20 to 22 admissions.

    In 2007, several deaths associate with influenza infection were reported in children from WA. These deaths occurred in early July 2007 (week 26) and could account for the sudden increase in ILI presentations to Perth EDs in 2007.

    Figure 8. Number of Emergency Department presentations due to ILI in Western Australia from 1 January 2007 to 21 June 2009 by week

    Number of Emergency Department presentations due to ILI in Western Australia from 1 January 2007 to 21 June 2009 by week

    SOURCE: WA ‘EDSS News’ Report


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    Paediatric hospital admissions

    The Australian Paediatric Surveillance Unit (APSU) conduct surveillance of severe complications of influenza in children aged 15 years and under. In 2009, this surveillance began on 1 June. Details of admissions are reported on a weekly basis.

    Up to 25 June 2009, 10 notifications of children hospitalised with severe complications of influenza have been reported by the APSU. Of the nine cases for which data is available, five (55.5%) cases were admitted to ICU. The average age of cases admitted to hospital is 2 years, with an age range from 1 month to 8 years.

    Absenteeism

    A national organisation provides data on the number of employees that have been on sick leave for a continuous period of more than three days. These data are not influenza or ILI specific and absenteeism may be a result of other illnesses.

    Absenteeism rates continue to rise, possibly reflecting greater rates of illness in the community. Rates in 2009 are following similar trends to those seen in 2007 and 2008 are higher and rising earlier in the year (Figure 9).

    Figure 9. Absenteeism rates, 1 January 2007 to 10 June 2009, by week

    Absenteeism rates, 1 January 2007 to 10 June 2009, by week

    SOURCE: Absenteeism data


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    H1N1 Influenza 09 in Australia

    Epidemiology of H1N1 Influenza in Australia

    As of 26 June 2009 there were 3360 confirmed cases of H1N1 Influenza 09 in Australia. The cumulative number of cases continued to rise in all jurisdictions. The reported daily rate of notifications declined in the previous 10 days as a consequence of a targeted approach to the laboratory testing of suspect cases, consistent with the PROTECT phase.

    Distribution of cases over time

    The national epidemic curve shows the distribution of confirmed cases of H1N1 Influenza 09 in Australia (Figure 10). The epidemic curve shows two peaks, however the pattern is a surveillance artefact due to a change in testing policy. The vast majority of earlier confirmed cases occurred in Victoria, where case reporting peaked in late May before declining rapidly in early June due to the change to targeted laboratory testing which was implemented on 3 June 2009 as part of the modified- SUSTAIN phase in Victoria. The reported daily notification rate of confirmed cases in most other jurisdictions increased in the first half of June 2009 and has since declined.

    On 17 June 2009, Australian jurisdictions began to transition towards a new response phase called PROTECT. In the PROTECT phase, laboratory testing is directed towards people with moderate or severe illness, or those more vulnerable to severe illness, or those in institutional settings. The counts of new cases in situation reports and other sources should be interpreted with caution and do not reflect disease incidence. The degree of reporting delay is unknown for 15% of the cases reported with no onset date.

    Figure 10. Temporal distribution of confirmed cases of H1N1 Influenza 09 in Australia, by jurisdiction

    Temporal distribution of confirmed cases of H1N1 Influenza 09 in Australia, by jurisdiction


    Source: NetEPI


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    Distribution of cases by age and sex

    The distribution of confirmed H1N1 Influenza 09 cases in Australia shows higher reporting rates in younger age groups, consistent with the age distribution of confirmed cases observed in other parts of the world.1 Of the 3344 confirmed Australian cases for whom age was known, the median age is 18. The greatest number of cases was reported in the 10–19 year age group and these accounted for thirty seven per cent (1218) of all confirmed cases (Figure 11). Children under five years of age accounted for five per cent (n=161) of cases and people over the age of 60 accounted for two per cent (n=75) of cases. Again, these data need to be interpreted with caution, as case ascertainment could bias results. In the early weeks, active surveillance in schools may have skewed reporting, while the current focus on testing more severe cases or those with underlying illness may skew the age distribution towards an older age group.

    Figure 11. Laboratory confirmed cases of H1N1 Influenza 09 in Australia to 26 June 2009, by age group and sex

    Laboratory confirmed cases of H1N1 Influenza 09 in Australia to 26 June 2009, by age group and sex

    Source: NetEPI



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    Geographic distribution of cases

    The greatest proportion of all cases reported in Australia to June 26 2009 have occurred in Victoria (45.8%), followed by New South Wales (19.5%) and Queensland (15.6%) (Table 1). Crude rates per capita vary across the jurisdictions, with the greatest rates per 100,000 populations occurring in the Northern Territory (54.9), Australian Capital Territory (30.3) and Victoria (29.4) (Table 2). The relatively high rate in the Northern Territory has been influenced by two clusters (one in a remote community and one in a visiting group of students) and the small population of the territory. It should also be noted that the crude rates do not take into account differences in population structure.

    Table 2. Notifications and crude rates of laboratory-confirmed H1N1 Influenza 09, by jurisdiction to 26 June 2009
    Jurisdiction Total H1N1 Influenza 09 notifications Percentage of national H1N1 Influenza 09 notifications Crude notification rate per 100,000 population
    ACT
    103
    3.1%
    30.3
    NSW
    650
    19.5%
    9.4
    NT
    118
    3.5%
    54.9
    QLD
    522
    15.6%
    12.5
    SA
    157
    4.7%
    9.9
    TAS
    72
    2.2%
    14.6
    VIC
    1528
    45.8%
    29.4
    WA
    187
    5.6%
    8.9
    AUS
    3337*
    100%
    15.9


    * No jurisdiction identifier was available for 23 confirmed cases
    Source: NetEPI


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    Aboriginal and Torres Strait Islander people/communities

    As of the 25 June 2009, two per cent (n=74) of confirmed cases are identified as Indigenous in the NetEpi database. In approximately 20% of the cases (600/3030) the Indigenous field in NetEpi has not been completed.

    The median age of Indigenous confirmed cases is 17 years of age (range 1-54 years of age). Fifty three per cent (n=31) are aged less than 20 years of age. Fifty nine per cent (n=35) are female and forty one per cent (n=24) are male.

    Fifty-four per cent (n=40) of Indigenous confirmed cases are in the Northern Territory, followed by twenty eight per cent (n=21) in Queensland, seven per cent (n=5) in New South Wales, four per cent (n=3) in Western Australia, three per cent (n=2) in Victoria and Tasmania and one case is located in the Australian Capital Territory. In the Northern Territory, the number of cases within the Aboriginal population is higher than would be expected given the proportion (30%) of Aboriginal people in the Territory.

    Fifteen Indigenous cases are reported as having being hospitalised. The majority of these cases (n=11) have been in the Northern Territory, with more than twenty seven per cent of Indigenous confirmed cases requiring hospitalisation. This compares with four per cent (n=2) of non-indigenous confirmed cases requiring hospitalisation in the Northern Territory.
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    Clinical characteristics—general

    The clinical spectrum of H1N1 Influenza 09 is still being defined in Australia and other affected countries. To date, available information suggests that the vast majority of confirmed cases of H1N1 Influenza 09 have been characterised by self-limiting, uncomplicated respiratory illness and symptoms similar to those of seasonal influenza. Complications and severe disease requiring hospitalisation have been reported in a minority of cases and 238 deaths* worldwide have been attributed to H1N1 Influenza 09.

    Information on symptoms was available for 500 Australian confirmed cases. Among these cases, symptoms were cough (83%); a measured temperature of greater than 38 degrees or good history of fever (76%); sore throat (54%); runny nose (54%); joint pain (42%); fatigue (39%;) and headache (35%).

    A total of 382 deaths have been reported globally since this report was compiled. For the most recent figures access the latest situation Report at http://www.healthemergency.gov.au/internet/healthemergency/publishing.nsf/Content/updates


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    Severe cases and hospitalisations

    As of 26 June 2009, six per cent (n=197) of confirmed cases of H1N1 Influenza 09 in Australia have been hospitalised. The number of cases per day requiring hospitalisation has been increasing since mid June (Figure 12) and has more than doubled since the previous week (Week 26). This rise is to be expected with the increasing detection of H1N1 Influenza 09 in the community and may also reflect improved reporting of hospitalisation data by states and territories.

    Twenty five per cent (n=49) of hospitalised cases have been admitted to an intensive care unit (ICU), with the proportion of cases requiring admittance to ICU on any given day ranging from twenty two per cent to thirty nine per cent (Figure 13). It is likely that most cases admitted to an ICU would be ventilated.

    Figure 12. Hospitalisations of Influenza H1N1 09, 15 June 2009 to 26 June 2009, Australia

    Hospitalisations of Influenza H1N1 09, 15 June 2009 to 26 June 2009, Australia

    Source: NetEPI



    Figure 13. Proportion of hospitalised Influenza H1N1 09 confirmed cases admitted to ICU compared to cases hospitalised*, 15 June 2009 to 26 June 2009, Australia

    Proportion of hospitalised Influenza H1N1 09 confirmed cases admitted to ICU compared to cases hospitalised*, 15 June 2009 to 26 June 2009, Australia

    * From the beginning of the H1N1 Influenza 09 outbreak, 197 confirmed cases have been hospitalised with 18 cases admitted to ICU. These numbers are very small and the proportion admitted to ICU should be interpreted with caution.
    Source: NetEPI


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    Four people in Australia with confirmed H1N1 Influenza 09 infection had died as of the 26 June 2009. The first death, on 19 June 2009, was a 26 year old Indigenous man from WA who had several serious underlying medical conditions. The second death, on 20 June 2009, was a 35 year old man from Victoria who is believed to have had significant underlying medical conditions including type 2 diabetes and obesity. The third, on 24 June 2009, was a 50 year old woman from Victoria who had life-threatening cancer. The fourth, on 25 June, was a 71 year old Melbourne woman.

    A total of eleven* deaths have been reported since this report was compiled. For the most recent figures please access the latest Situation Report at http://www.healthemergency.gov.au/internet/healthemergency/publishing.nsf/Content/updates


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    International seasonal influenza surveillance

    Southern Hemisphere

    During weeks 21-22, seasonal influenza activity continued to increase in some countries in the southern hemisphere. South Africa reported regional outbreaks of seasonal influenza A (H3) with low levels of influenza B circulating. New Zealand reported local levels of seasonal influenza A (H1 and H3) activity and Peru reported local levels of seasonal influenza A (H1) activity. Brazil also reported increased levels of influenza A (H3).

    In New Zealand, since 2009 seasonal influenza reporting commenced on 27 April 2009, influenza A (H1N1) has been the predominant seasonal influenza strain of all subtyped isolates. Of the 18 seasonal influenza A (H1N1) isolates that have been tested, all isolates (100%) were resistant to oseltamivir.2

    Northern Hemisphere

    Europe, the United States of America (USA) and Canada are continuing to report influenza activity related to H1N1 Influenza 09. In the northern hemisphere, seasonal influenza activity has come to an end in most countries.3 The northern hemisphere seasonal influenza reporting period usually ends at the end of April or May.

    Oseltamivir Resistance Global update

    A WHO summary of the prevalence of anti-viral drug use in currently circulating seasonal influenza viruses during the northern hemisphere influenza season 2008-2009, reports that ninety six per cent of seasonal influenza A (H1N1) isolates tested from 36 countries worldwide were resistant to oseltamivir compared to the 2007-2008 northern hemisphere influenza season which was fifteen per cent.4,5
    Limited data are available on anti-viral resistance for H1N1 Influenza 09. The United States of America Centers for Disease Control (CDC) have reported characterisation of 13 isolates, with none of the isolates testing resistance to oseltamivir.6 An isolate of H1N1 Influenza 09 has been identified in Denmark showing resistance to osteltamivir (Tamiflu). Anti-viral resistance is not uncommon in flu viruses and the World Health Organization (WHO) has stated that the Danish case is isolated and has no public health implications.
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    Data considerations

    NetEpi

    All jurisdictions are reporting H1N1 influenza 09 cases using NetEpi, a web-based outbreak case reporting system. Analyses of Australian cases are based on clinical onset date, if this information is available. Where an onset date is not available, notification date has been used. Victorian cases use a calculated onset date which is the earliest available date calculated from specimen date, onset date, notification date or detection date. This assumption was made for all calculations and data on which the figures are based.

    National Notifiable Diseases Surveillance System (NNDSS)

    NNDSS comprises of notifications from jurisdictions of laboratory-confirmed influenza cases. Laboratory-confirmed influenza is notifiable in all jurisdictions in Australia.

    Laboratory Surveillance data

    Laboratory testing data are extracted from the ‘NSW Influenza Report,’ ‘The 2009 Victorian Influenza Vaccine Effectiveness Audit Report’ (VIDRL) and the ‘South Australian Seasonal Influenza Report’. These reports are provided weekly.

    Sentinel General Practice Surveillance

    The Australian Sentinel Practices Research Network (ASPREN) has Sentinel GPs who report influenza-like-illness (ILI) presentation rates in NSW, SA, ACT, VIC, QLD, TAS and WA. As jurisdictions joined ASPREN at different times and the number of GPs reporting has changed over time, the representativeness of ASPREN data in 2009 may be different to that of previous years. ASPREN data are sent to the Surveillance Branch on a weekly basis. Northern Territory GP surveillance data are sent to the Surveillance Branch on a weekly basis. VIDRL influenza surveillance data are sent to the Surveillance Branch on a weekly basis.

    Sentinel Emergency Department (ED) data

    WA - ED surveillance data are extracted from the ‘EDSS News’ Report. This report is provided weekly.

    Paediatric hospital admissions data

    Reports of ICU admissions are provided to the Surveillance Branch on a weekly basis by the Australian Paediatric Surveillance Unit.
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    Attachments

    Characteristics of H1N1 Influenza 09—global perspectives

    Epidemiology

    To date, the vast majority of cases in all countries have occurred among adolescents and young adults. Males and females are similarly affected in all countries. Data compiled by the World Health Organisation (WHO) from Chile, countries of the European Union and the European Free Trade Association, Japan, Panama and Mexico indicate that approximately twenty five per cent of cases were aged 0–9 years, thirty six per cent were 10–19 years of age, seventeen per cent were 20–29 years of age, nine per cent were 30–39 years of age, seven per cent were 40–49 years of age and five per cent were more than 50 years of age.1

    A study on serum cross reactive antibody response to the novel influenza A (H1N1) virus after vaccination with seasonal influenza vaccine revealed that, at baseline, cross-reactive antibody was present in six to nine per cent of those aged 18-64 years and in thirty three per cent of those aged greater than 60 years, demonstrating that the younger age groups were more uniformly susceptible to infection with influenza A (H1N1).77 The rate of H1N1 Influenza 09 notifications per 100,000 population in Australia (13.8) is lower than that of Chile (25.5) and Canada (19.2), and higher than that of Japan (0.7), Argentina (3.0), the UK (4.7), the USA (7.0), Mexico (7.2) and New Zealand (8.9)(Table 3).[8

    Table 3. Notification rates for H1N1 Influenza 09 in a selection of affected countries
    Country Rate per 100,000 population
    Japan
    0.7
    Argentina
    3.0
    UK
    4.7
    USA
    7.0
    Mexico
    7.2
    New Zealand
    8.9
    Australia
    13.9
    Canada
    19.2
    Chile
    25.5


    Transmission dynamics

    The basic reproduction number (R0) is the average number of cases generated by a case over the course of their infectious period in a wholly susceptible population. It is useful for estimating the transmission potential of an infectious agent. Three different epidemiological analyses of data from Mexico gave basic reproduction number estimates in the range of 1.4 to 1.6, whereas a genetic analysis gave a central estimate of 1.2.9 This suggests that transmissibility of H1N1 Influenza 09 is substantially higher than that of seasonal influenza, and comparable with lower estimates of R0 obtained from previous influenza pandemics.9 A study in Japan estimated the R0 to be as high as 2.3, although the authors suggested that this relatively high value may have been influenced by high contact rates among adolescents.10

    The secondary attack rates of seasonal influenza within households range from five per cent to fifteen per cent. Current estimates of the secondary household attack rates of H1N1 Influenza 09 range from twenty two per cent to thirty three per cent.1
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    Clinical characteristics

    Globally, most cases have been mild, although severe disease has been reported both among known groups at higher risk of adverse outcomes following influenza and in previously healthy young adults, leading WHO to classify the severity of this pandemic to date as ‘moderate’.1

    Several reports suggest that a large proportion of hospitalised cases have had at least one underlying health condition. Almost one-half of the patients hospitalized in the United States, and 21 of 45 (46%) fatal cases in Mexico for whom data are available had underlying conditions, including pregnancy, asthma, other lung diseases, diabetes, morbid obesity, autoimmune disorders and associated immunosuppressive therapies, neurological disorders and cardiovascular disease.11 Of 567 hospitalized patients in New York City, eighty per cent had at least one known risk factor for severe illness or complications due to influenza. Asthma was the most common risk factor, present inforty one per cent of all laboratory-confirmed hospitalized cases. Other important risk factors included pregnancy (28% of 142 women of childbearing age hospitalized with confirmed disease), children less than 2 years of age (12%) and patients with diabetes (11%), immunodeficiency (9%) and cardiovascular disease (9%).12
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    References

    1. World Health Organisation: Weekly Epidemiological Record No. 25, 2009. Available from: http://www.who.int/wer/2009/wer8425.pdf; accessed 23 June 2009.
    2. New Zealand Public Health Surveillance Weekly Influenza Update, weeks 22 & 23.
    Available from: http://www.surv.esr.cri.nz/virology/influenza_weekly_update.php Accessed 19 June 2009.
    3. WHO, seasonal influenza activity in the world weeks 20 & 21, 12 June 2009. Available from: http://www.who.int/csr/disease/influenza/update/en/index.html
    4. WHO Influenza A virus resistance to oseltamivir and other antiviral medicines, 4 June 2009. Available from: http://www.who.int/csr/disease/influenza/2008-9nhemisummaryreport/en/index.html Accessed 19 June 2009.
    5. WHO Influenza A (H1N1) virus resistance to oseltamivir – Last quarter 2007 to 2 June 2008.
    Available from: http://www.who.int/csr/disease/influenza/ResistanceTable200806013.pdf Accessed 19 June 2009.
    6. WHO Influenza A virus resistance to oseltamivir and other antiviral medicines, 4 June 2009. Available from:
    http://www.who.int/csr/disease/influenza/2008-9nhemisummaryreport/en/index.html Accessed 19 June 2009.
    7. Serum cross-reactive antibody response to a novel influenza A (H1N1) virus after vaccination with seasonal influenza vaccine. MMWR Morb Mortal Wkly Rep 2009 May 22; 58(19): 521-4. Available at: http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5819a1.htm
    8. World Health Organisation: Influenza A(H1N1) update 53, 24 June 2009. Available at: WHO | Influenza A(H1N1) - update 53
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