Vaccine Preventable Diseases in Australia, 2005 to 2007

3.5 Influenza

Page last updated: 24 December 2010

Influenza virus causes annual epidemics of respiratory disease, mainly spread by droplet transmission. The disease is often indistinguishable clinically from that caused by other respiratory viruses. Asymptomatic influenza infection is well documented in serologic studies. Typical symptomatic cases have abrupt onset of fever and cough (most common), but malaise, myalgia, sore throat and headache are also prominent. Complications of influenza infection include pneumonia, otitis media and exacerbation of chronic medical conditions.1,2 In areas with temperate climate, influenza epidemics usually occur during the winter months, causing an increase in hospitalisations for pneumonia, exacerbation of chronic diseases and contributing to increased mortality, particularly among the elderly and people with high-risk underlying conditions. In areas with tropical climate, the seasonality patterns are more variable.3,4

There are three types of influenza viruses that infect humans, types A, B and C, of which types A and B are clinically important. Type A influenza viruses can be subtyped according to the antigenic differences of two surface glycoproteins, haemagglutinin (e.g. H1, H2, etc) and neuraminidase (e.g. N1, N2, etc).1 Both influenza A and influenza B viruses undergo frequent changes in their surface antigens, thus evading natural immunity acquired from previous infections. Cumulative stepwise genetic changes (point mutations) of the virus occur regularly and these mutations may result in antigenic variation, known as antigenic drift, which is responsible for seasonal outbreaks and smaller epidemics of influenza. Pandemics of influenza are caused by strains that have undergone dramatic or major genetic changes (called antigenic shift when a novel haemagglutinin is introduced), which might include reassortment (mixing) with influenza viruses that primarily infect other animal species.1

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Case definitions

Notifications

Laboratory-confirmed influenza was a nationally notifiable disease in all jurisdictions except South Australia during this reporting period. Although in South Australia influenza was not a notifiable disease, laboratory reports were collected and sent to NNDSS, and laboratory-confirmed influenza became notifiable in May 2008. Implementation of influenza notification occurred nationally in all other jurisdictions during 2001, except Tasmania (2002).

A laboratory-confirmed influenza infection is one in which an influenza virus is identified by cell culture, nucleic acid amplification testing, or influenza antigen testing, from appropriate respiratory tract specimens; OR a recent infection demonstrated by serological methods including IgG seroconversion (a significant [≥4-fold] increase in antibody level) or a single high (IgG) titre to the influenza virus.

(The criteria for the national case definitions were revised and implemented in 2008, after the period covered by this report. The revisions consisted of specifying that the influenza virus antigen detection has to be performed by a laboratory, and that the high titre qualifying for notification has to be performed by complement fixation test [CFT] or by haemagglutination inhibition [HAI]).5

Hospitalisations

The ICD-10-AM codes used to identify hospitalisations were J10 (influenza due to identified influenza virus) and J11 (influenza, virus not identified). In this report, we did not make the distinction between admissions where a virus was identified and those where it was not.

Deaths

The ICD-10 codes used to identify deaths were J10 (influenza due to identified influenza virus) and J11 (influenza, virus not identified).

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Secular trends

In the period January 2006 to December 2007, there were 13,655 notifications of laboratory-confirmed influenza, giving an average annual rate of 32.7 per 100,000 (Table 3.5.1). Notifications varied considerably between the 2 years, with 3,252 in 2006 (annual rate 15.7 per 100,000) but 10,403 in 2007 (annual rate 49.5 per 100,000). Nationally, there was a clear seasonal distribution of notifications with peaks in August in both years, as in most previous years since 2001. The median number of notifications per month was 626 (range 244–1,327) in 2005, 542 (range 204–1,058) in 2006, but 1,238 (range 407–5,094) in 2007 during the months from June to October for each of these 3 years.

Between July 2005 and June 2007, there were 4,444 hospitalisations coded as influenza (an average annual rate of 10.8 per 100,000) (Table 3.5.1), with a higher number of hospitalisations (2,593) recorded in 2005/2006. There was a clear seasonal pattern nationally, with obvious increases over the winter months (Figure 3.5.1). Hospitalisations peaked in August in both 2005 and 2006. The median number of admissions per month was 92.5 (range 44–689) over the 2 years. During the months from June to October for these 2 years, the median number of admissions per month was 317.5 (range 130–689), with a peak of 689 in August 2005. Data on hospitalisation rates for the winter season in 2007, during which notification rates were higher than previous years since influenza became nationally notifiable, were not available at the time of preparing this report. The median duration of a seasonal epidemic (when the fortnightly hospitalisation count exceeded the mean count of the inter-epidemic period by ≥2 standard deviations) was 10 fortnights (20 weeks) in the years 1999–2006 (range 9–12 fortnights), based on virologically confirmed influenza hospitalisations.

Figure 3.5.1: Influenza hospitalisations and notifications,* Australia, 1993 to 2007, by month of diagnosis or admission

Figure 3.5.1:  Influenza hospitalisations and notifications, Australia, 1993 to 2007, by month of diagnosis or admission

* Notifications where the month of diagnosis was between January 2001 and December 2007; hospitalisations where the month of admission was between July 1993 and June 2007. Note that the Northern Territory, Queensland, Tasmania and Victoria did not notify influenza for the complete year in 2001.

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Table 3.5.1: Influenza notifications, hospitalisations and deaths, Australia, 2005 to 2007,* by age group

Age group
(years)
Notifications
2 years
(2006–2007)
Hospitalisations
2 years
(July 2005–June 2007)
LOS per admission
(days)
Deaths
2 years
(2005–2006)
n Rate n (§) Rate (§) Median (§) n Rate
0–4
2,904
110.0
1,241
(914)
47.8
(35.2)
2.0
(2.0)
5
0.19
5–14
1,971
35.9
383
(282)
7.0
(5.1)
2.0
(2.0)
2
0.04
15–24
1,751
30.1
494
(332)
8.7
(5.8)
1.0
(1.0)
1
0.02
25–59
4,869
24.0
1,354
(898)
6.7
(4.5)
2.0
(1.0)
7
0.03
60+
2,159
28.8
972
(534)
13.4
(7.4)
6.0
(5.0)
40
0.55
All ages
13,655||
32.7
4,444
(2,960)
10.8
(7.2)
2.0
(2.0)
55
0.13

* Notifications where the date of diagnosis was between January 2006 and December 2007; hospitalisations where the date of separation was between July 2005 and June 2007; deaths where the death was recorded between January 2005 and December 2006.

† LOS = length of stay in hospital.

‡ Average annual age-specific rate per 100,000 population.

§ Includes 1 case with unknown age.

|| Principal diagnosis (hospitalisations).

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Severe morbidity and mortality

A total of 30,823 hospital bed days with an ICD-10-AM code for influenza were recorded over the reporting period from July 2005 to June 2007. The median length of stay was much longer in people aged 60 years (6 days) compared with younger age groups (1 or 2 days) (Table 3.5.1). Influenza was the principal diagnosis for 67% of the hospitalisations where influenza was one of the separation diagnoses. There were substantially more bed days due to influenza in 2005/2006 compared with 2006/2007 (18,199 vs 12,624). Over these 2 reporting years, hospitalisations where the virus was identified (ICD-10-AM code J10) contributed to 44.5% (1,979/4,444) of all influenza hospitalisations, but 55.3% (17,043/30,823) of hospital bed days. The median length of stay was longer for hospitalisations where the virus was identified compared with those where the virus was not identified (ICD-10-AM code J11) overall (3 days vs 1 day), and for all age groups including those with longer length of stay (3.0 vs 1.5 days for those aged 0–4 years, 4 vs 1 days for those aged 25–59 years, and 9 vs 4 days for those aged ≥60 years).

From January 2005 to December 2006, influenza was recorded as the underlying cause of death in 55 cases in the AIHW National Mortality Database. Of these, 40 (73%) were aged 60 years (rate 0.55 per 100,000), and 5 (9%) were aged 0–4 years (rate 0.19 per 100,000) (Table 3.5.1). Three of the 5 deaths (60%) in children aged <5 years were coded as death due to influenza with virus identified (ICD-10 code J10), whereas only 8 of the 50 deaths (16%) occurring at other ages were coded as death due to influenza with virus identified. Although there is no requirement for clinical information to be entered on NNDSS for influenza cases (and whether death was the outcome was not recorded in 69% of notifications in this reporting period), 22 deaths were recorded on NNDSS among influenza notifications in 2006–2007 (5 deaths in 2006, 17 deaths in 2007); these included 1 death in 2006 and 5 deaths during 2007 in children aged <5 years, 3 of which were in Western Australia.

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Age and sex distribution

Similar to previous years, during the period January 2006 to December 2007, the notification rate of laboratory-confirmed influenza was substantially higher among children aged <5 years (110.0 per 100,000) than in other age groups (Table 3.5.1, Figure 3.5.2 and Figure 3.5.3). Among those aged <5 years, the highest rate of notifications was in those <1 year of age, and the rate declined progressively with every year of increase in age. There were year-to-year variations in notification rates from 2002 to 2007, but the trends were similar for all age groups. The substantial increase in notifications during 2007 was not restricted to any particular age group but occurred across the age spectrum (Figure 3.5.3). Although the absolute increase in notification rates was most marked in the 0–4 years age group in 2007, the rate ratio (3.3), compared with the 2006 rates, was similar to other age groups <50 years, decreasing to 2.7 for those aged 50–64 years and 2.4 for those aged ≥65 years.

Figure 3.5.2: Influenza notification rates, Australia, 2006 to 2007 and hospitalisation rates 2005/2006 to 2006/2007,* by age group

Figure 3.5.2:  Influenza notification rates, Australia, 2006 to 2007 and hospitalisation rates 2005/2006 to 2006/2007, by age group

* Notifications where the date of diagnosis was between January 2006 and December 2007; hospitalisations where the date of separation was between July 2005 and June 2007.

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Figure 3.5.3: Annual influenza notification rates, Australia, 2002 to 2007,* by age group and year of diagnosis

Figure 3.5.3:  Annual influenza notification rates, Australia, 2002 to 2007, by age group and year of diagnosis

* Notifications where the date of diagnosis was between January 2002 and December 2007.

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From July 2005 to June 2007, the hospitalisation rate was also highest in children aged <5 years (47.8 per 100,000), and was substantially higher than that of other age groups (Table 3.5.1, Figure 3.5.2). Hospitalisation rates were progressively higher with increasing age among people aged ≥50 years, ranging from 5.7 per 100,000 for those aged 50–54 years to 26.4 per 100,000 for those aged ≥85 years (Figure 3.5.2). Among children aged <5 years, the hospitalisation rate was highest among infants (annual average of 115.9 per 100,000 population aged <1 year, ranging from 95.9 per 100,000 in 2006/2007 to 136.7 per 100,000 in 2005/2006), and progressively lower with increasing age (Figure 3.5.2). The proportion of hospitalisations for which the virus was identified (ICD-10-AM code J10) progressively decreased with increasing age group from 0–4 years (84%) to 25–29 years (13%). The proportion of hospitalisations for which the virus was identified was 23% and 34%, respectively, for those aged 30–59 years and ≥60 years.

Notification rates were higher among males than females at both ends of the age spectrum (those aged <15 years and ≥70 years), but the overall male to female ratio was close to unity (1:1.03). A similar pattern in the male to female ratio was also observed for hospitalisation rates (higher among males compared with females in those aged <15 years and ≥60 years), with the overall male to female ratio also close to unity (1:1.02).

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Geographical distribution

There was a wide variation by jurisdiction in the average notification rate in the 2-year review period of 2006–2007, ranging from 11.7 per 100,000 in South Australia (where influenza only became officially notifiable in 2008) to 18.5 per 100,000 in New South Wales to 76.6 per 100,000 in Queensland (see also Appendix 6.2). Crude average hospitalisation rates were similarly varied, with the highest rate reported in Western Australia (16.2 per 100,000 population, n=660) and the lowest in the Australian Capital Territory (4.4 per 100,000, n=29) for the 2-year review period of July 2005–June 2007 (see also Appendix 6.3). The variation among jurisdictions was greater in notification rates than in hospitalisation rates (rate ratio 6.5 for notification rates and 3.7 for hospitalisation rates between the jurisdictions with the highest and the lowest rates).

For all jurisdictions, notification rates were higher in 2007 compared with 2006, and the hospitalisation rates higher in 2005/2006 than 2006/2007.

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Influenza virus types and subtypes

The great majority of the influenza notifications reported to the NNDSS recorded the influenza type (13,095/13,665, 95.8%). Of those where the influenza type was known, most (85.9%) were type A (n=11,245), 13.5% (n=1,764) were type B, and both A and B were documented in 0.7% of notifications (n=86). Subtype or strain information of influenza type A was recorded in 1,218 notifications (8.9% of all notifications, 10.8% of type A influenza). Of these notifications where subtype was known, 30.6% were of subtype A/H1 and 69.4% were of subtype A/H3. Queensland and Victoria reported most of the notifications where subtyping was documented. More comprehensive Australian data from the WHO Collaborating Centre for Reference and Research on Influenza on the influenza subtypes for the respective years of this review period have been published in the annual reports of the National Influenza Surveillance Scheme (see Comment section below).6,7

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Comment

The seasonal patterns seen in both the notifications of laboratory-confirmed influenza (since 2001) and the hospitalisations with influenza virus identified (ICD-10-AM code J10) were similar, with trends of annual variation corresponding to each other. Notification and hospitalisation rates are substantially higher in children aged <5 years, and in those aged <1 year in particular, compared with all other age groups, even people aged ≥85 years. This is likely to reflect the patterns of health care use, and, in particular, diagnostic testing for respiratory viruses, which is much more common in the young age group.

It should be noted that, although notification data are a specific, but not sensitive, indicator of disease incidence, and hospitalisation data are an indicator of more severe cases, neither of these data sources is adequate for assessment of the true population burden due to influenza. The likelihood of diagnosing influenza by laboratory tests in a patient with respiratory or other symptoms compatible with influenza varies with the clinical threshold for obtaining a diagnostic specimen, the quality of the specimen, and other context factors. In addition, the role of influenza in exacerbating chronic disease in the elderly is minimally reflected in these surveillance data, especially in notifications data and data on the underlying cause of death. In a retrospective population-based study conducted in Sydney, excess hospitalisation rates attributable to influenza in children aged <18 years were estimated to be 2–11 times higher than the hospitalisation rate calculated from hospitalisations where influenza was coded as the principal discharge diagnosis, depending on the chosen estimation method and the age group.8 A more recent prospective cohort study conducted in a paediatric hospital in Sydney found that only 43% of the principal discharge diagnoses of hospitalisations of children aged <5 years with laboratory-confirmed influenza were coded as influenza; other diagnoses included lower respiratory tract infection, pneumonia and febrile convulsion.9 Deaths and hospitalisations coded as due to influenza are widely acknowledged to substantially underestimate the true number of deaths and hospitalisations attributable to influenza.10–13

There was a marked increase in notifications of laboratory-confirmed influenza in 2007. This is likely to be a result of significant antigenic drift of the circulating strains of the influenza virus away from those that predominated in the previous few years and vaccine strains, against which a greater level of immunity would be expected in the population. While the majority of the types of influenza virus in the notified cases are reported, available information on subtypes is limited. Instead, additional information on antigenic drift is available from an alternate source, the WHO Collaborating Centre for Reference and Research on Influenza in Melbourne, Victoria.

Analysis by this Centre of 657 viable viral isolates or clinical specimens from Australian laboratories in 2006 identified 402 (61.2%) to be A(H3N2) strains, 24 (3.6%) to be A(H1N1) strains, and 231 (35.2%) to be influenza B strains. Those 2006 A(H3) strains were mostly antigenically similar to the 2006 vaccine strain A/New York/55/2004. Among the small number of A(H1) strain isolates, there was a drift away from the 2006 vaccine strain A/New Caledonia/20/99. The great majority (93.5%) of the 231 influenza B viruses analysed were antigenically related to the 2006 vaccine strain B/Malaysia/2506/2004 (B/Victoria/2/87-lineage viruses).6

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However, in 2007, many more influenza virus isolates or specimens were received by the WHO Influenza Centre from Australian laboratories, and the proportional distribution of the influenza subtypes differed substantially from that of 2006. Of the 1,406 viable isolates, 826 (58.7%) were identified as A(H3N2) strains, 483 (34.4%) as A(H1N1) strains, and 97 (6.9%) as influenza B strains. The 2007 Australian A(H3N2) viruses were antigenically similar to either the 2007 vaccine strain A/Wisconsin/67/2005 (which was different from the dominant A/H3 strain in 2006) or the newly emergent variant A/Brisbane/10/2007. There was also significant antigenic drift in A(H1N1) viruses away from the 2007 vaccine strain A/New Caledonia/20/99. Among the relatively small number of influenza B viruses isolated in 2007, only 21% were antigenically related to the 2007 vaccine strain B/Malaysia/2506/2004 (B/Victoria/2/87-lineage viruses), while the remaining 79% were closely related to emergent B/Florida/7/2004-like viruses (B/Yamagata-lineage).7

Moreover, although A(H3N2) viruses constituted the majority (58.7%) of the Australian isolates analysed in 2007, co-circulating A(H1N1) viruses constituted a substantial proportion (34.4%). This was the highest annual proportion of A(H1N1) viruses found in Australia since 2001, when A(H1N1) was the predominant strain. The extensive co-circulation of two influenza A strains might have contributed to the higher incidence of influenza in 2007.7

The increase in the number of reported deaths among notified cases in 2007, including 5 deaths in children aged <5 years, is also consistent with an overall true increase in disease incidence. Although the reporting of death as an outcome of laboratory-confirmed influenza in the notification data is incomplete, it is likely that a notified case that died of influenza would be captured, especially in young children. The Australian Government Department of Health and Ageing, collating information from multiple surveillance sources, identified a total of 7 deaths caused by influenza in Australian children aged <5 years in 2007.7 Surveillance based on influenza-like illness syndrome and absenteeism revealed a higher case incidence compared with previous years.7

The increase in notifications in 2007 is also likely to be partly related to increased diagnostic testing, possibly prompted by influenza deaths in young children being widely reported in the media.14 However, the increase in notifications was not restricted to young children but occurred in all age groups. At the time of preparing this report, the corresponding hospitalisation and death data for the winter season in 2007 were not yet available.

Apparent differences in notification and hospitalisation rates between jurisdictions should be assessed with caution as they may primarily reflect differences in likelihood and availability of virological testing or in coding practices. Hospitalisation data referred to in this report are based on discharge coding and it is possible that some of those hospitalisations coded with the less specific set of codes for influenza without virological confirmation (i.e. ICD-10-AM J11 codes) may be due to other respiratory pathogens such as respiratory syncytial virus (RSV), parainfluenza virus, picornavirus, adenovirus or coronavirus.15–17

Annual seasonal influenza vaccination is currently recommended for all Australians aged ≥65 years, all Aboriginal and Torres Strait Islander people aged ≥15 years, and all individuals aged ≥6 months who are predisposed to severe influenza or its complications, for example, those with chronic medical conditions such as chronic pulmonary or cardiovascular disease, pregnant women, residents of nursing homes and other long-term care facilities, and homeless people and those providing care to them.18 From 1999, publicly funded seasonal influenza vaccine has been available for Australians aged ≥65 years, and for Aboriginal and Torres Strait Islander people who are aged ≥50 years or who have chronic medical conditions that predispose them to severe influenza. From January 2010, eligibility for free vaccine, has been extended to include all Aboriginal and Torres Strait Islander people aged 15–49 years, all individuals aged ≥6 months who have a medical condition that predisposes them to severe influenza, and pregnant women.19 In Western Australia influenza vaccine has been available free for all children aged 6 months to 5 years since 2008.20 The latest published estimations of influenza vaccination uptake, from the Adult Vaccination Survey conducted by the AIHW, were 79.1% in Australians aged ≥65 years for 2004,21 and 77.5% for 2006, which were not significantly different statistically.6

There have been progressively widening indications for influenza immunisation in children in the USA, where the Advisory Committee on Immunization Practices (ACIP) in 2003 recommended routine annual influenza vaccination of healthy American children aged 6–23 months22,23 based on the high burden of illness.12,24,25 In 2006, the ACIP extended its recommendation to include children up to the age of 5 years,26 and, more recently, to include all children to the age of 18 years commencing 2008/2009.27 Australian data in this report, as well as those published in previous studies, suggest a similarly significant burden of illness of more severe influenza in young children.8,28 The Australian Technical Advisory Group on Immunisation has been deliberating on influenza vaccination for children aged from 6 months up to 5 years.29 In addition to disease burden, other aspects, such as vaccine efficacy and safety for recipients of this age group, the feasibility of implementation, and community acceptability of a universal influenza vaccination program for healthy children, need to be considered.30,31

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References

1. Nicholson KG, Wood JM, Zambon M. Influenza. Lancet 2003;362(9397):1733–1745.

2. Bridges CB, Katz JM, Levandowski RA, Cox NJ. Inactivated influenza vaccines. In: Plotkin SA, Orenstein WA, Offit PA, eds. Vaccines. 5th edn. Philadelphia, PA: Saunders Elsevier, 2008.

3. Monto AS. Epidemiology of influenza. Vaccine 2008;26(Suppl 4):D45–D48.

4. Roche P, Spencer J, Hampson A. Annual report of the National Influenza Surveillance Scheme, 2001. Commun Dis Intell 2002;26(2):204–213.

5. Communicable Diseases Network Australia. Surveillance case definitions for the Australian National Notifiable Diseases Surveillance System (revised 2008). Available from: http://www.health.gov.au/internet/main/publishing.nsf/Content/cdna-casedefinitions.htm Accessed on 24 August 2009.

6. O’Brien K, Barr IG. Annual report of the National Influenza Surveillance Scheme, 2006. Commun Dis Intell 2007;31(2):167–179.

7. Owen R, Barr IG, Pengilley A, Liu C, Paterson B, Kaczmarek M. Annual report of the National Influenza Surveillance Scheme, 2007. Commun Dis Intell 2008;32(2):208–226.

8. Beard F, McIntyre P, Gidding H, Watson M. Influenza related hospitalisations in Sydney, New South Wales, Australia. Arch Dis Child 2006;91(1):20–25.

9. Iskander M, Kesson A, Dwyer D, Rost L, Pym M, Wang H, et al. The burden of influenza in children under 5 years admitted to The Children’s Hospital at Westmead in the winter of 2006. J Paediatr Child Health 2009;45(12):698–703.

10. Dushoff J, Plotkin JB, Viboud C, Earn DJ, Simonsen L. Mortality due to influenza in the United States–an annualized regression approach using multiple-cause mortality data. Am J Epidemiol 2006;163(2):181–187.

11. Nichol KL, Nordin J, Mullooly J, Lask R, Fillbrandt K, Iwane M. Influenza vaccination and reduction in hospitalizations for cardiac disease and stroke among the elderly. N Engl J Med 2003;348(14):1322–1332.

12. Izurieta HS, Thompson WW, Kramarz P, Shay DK, Davis RL, DeStefano F, et al. Influenza and the rates of hospitalization for respiratory disease among infants and young children. N Engl J Med 2000;342(4):232–239.

13. Armstrong BG, Mangtani P, Fletcher A, Kovats S, McMichael A, Pattenden S, et al. Effect of influenza vaccination on excess deaths occurring during periods of high circulation of influenza: cohort study in elderly people. BMJ 2004;329(467):660.

14. Review of notifiable diseases, 2007. Vaccine preventable diseases. Disease Watch – the Western Australian Communicable Diseases Bulletin 2008;12(2):9. Available from: http://www.public.health.wa.gov.au/cproot/1397/2/DW%20%2012%20no%202%20Mar%202008vfinal_web.pdf Accessed on 11 May 2009.

15. Druce J, Tran T, Kelly H, Kaye M, Chibo D, Kostecki R, et al. Laboratory diagnosis and surveillance of human respiratory viruses by PCR in Victoria, Australia, 2002–2003. J Med Virol 2005;75(1):122–129.

16. Turner J, Tran T, Birch C, Kelly H. Higher than normal seasonal influenza activity in Victoria, 2003. Commun Dis Intell 2004;28(2):175–180.

17. Miller ER, Fielding JE, Grant KA, Barr IG, Papadakis G, Kelly HA. Higher than expected seasonal influenza activity in Victoria, 2007. Commun Dis Intell 2008;32(1):63–70.

18. National Health and Medical Research Council. The Australian Immunisation Handbook. 9th edn. Canberra: Australian Government Department of Health and Ageing, 2008.

19. Australian Government. Immunise Australia Program. Fact sheet for immunisation providers: influenza vaccination 2010. Available from: http://www.immunise.health.gov.au/internet/immunise/publishing.nsf/Content/IMM123-cnt/$File/imm123-fs-2010.pdf Accessed on 16 March 2010.

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20. Communicable Disease Control Directorate, Western Australia Department of Health. Paediatric influenza immunisation: the 2008 metropolitan trial. Disease Watch – the Western Australian Communicable Diseases Bulletin 2008;12(1):1–4. Available from: http://www.public.health.wa.gov.au/cproot/1247/2/DW%20%2012%20no%201%20Jan%202008v3web.pdf Accessed on 23 June 2010.

21. Australian Institute of Health and Welfare. 2004 Adult vaccination survey: summary results. AIHW Cat. No. PHE 56. Canberra: Australian Institute of Health and Welfare, Australian Government Department of Health and Ageing, 2005. Available from: http://www.aihw.gov.au/publications/phe/avssr04/avssr04.pdf Accessed on 11 May 2009.

22. Centers for Disease Control and Prevention. Recommended childhood and adolescent immunization schedule—United States, January–June 2004. MMWR Morb Mortal Wkly Rep 2004;53(1):Q1–Q4.

23. American Academy of Pediatrics Committee on Infectious Diseases. Recommendations for influenza immunization of children. Pediatrics 2004;113(5):1441–1447.

24. Centers for Disease Control and Prevention. Update: influenza activity – United States, 2003–04 season. MMWR Morb Mortal Wkly Rep 2004;53(13):284–287.

25. Neuzil KM, Mellen BG, Wright PF, Mitchel EF, Jr., Griffin MR. The effect of influenza on hospitalizations, outpatient visits, and courses of antibiotics in children. N Engl J Med 2000;342(4):225–231.

26. Centers for Disease Control and Prevention. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP) [erratum appears in MMWR Morb Mortal Wkly Rep 2006;55(29):800]. MMWR Recomm Rep 2006;55(RR–10):1–42.

27. Fiore AE, Shay DK, Broder K, Iskander JK, Uyeki TM, Mootrey G, et al. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP), 2008. MMWR Recomm Rep 2008;57(RR–7):1–60.

28. Milne BG, Williams S, May ML, Kesson AM, Gillis J, Burgess MA. Influenza A associated morbidity and mortality in a Paediatric Intensive Care Unit. Commun Dis Intell 2004;28(4):504–509.

29. Australian Government Department of Health and Ageing. ATAGI Bulletin, 38th meeting, 16–17 October 2008. Available from: http://www.immunise.health.gov.au/internet/immunise/publishing.nsf/Content/atagi-meet38bulletin Accessed on 11 May 2009.

30. Isaacs D. Should all Australian children be vaccinated against influenza? Questions of cost-effectiveness, vaccine efficacy and feasibility are yet to be answered. Med J Aust 2005;182(11):553–554.

31. Edwards KM, Griffin MR. Influenza vaccination of children: can it be accomplished? J Infect Dis 2006;194(8):1027–1029.

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