Contents

10 Influenza

10.3 Epidemiology

10.3.1 New Zealand epidemiology

The impact of influenza in New Zealand over the past 20 seasons, since the sentinel surveillance systems commenced, has been substantial in terms of GP consultations, hospitalisations and deaths. The highest burden of disease is in the very young, the elderly, pregnant women, those with co-morbid conditions, people from low income groups, and Māori and Pacific ethnic groups. Future considerations to prevent influenza-related hospitalisations, particularly among young children, could include a childhood immunisation programme.

Influenza surveillance

New Zealand experiences the typical temperate climate epidemiology for influenza, and although influenza activity can occur throughout the year, the peak incidence is usually around the winter months (Figure 10.1). Ongoing surveillance of influenza is carried out by the four regional virus diagnostic laboratories, and by the Institute of Environmental Science and Research (ESR) virology laboratory. The regional virus diagnostic laboratories report respiratory virus diagnoses to ESR.

Sentinel general practice surveillance, as part of the WHO Global Program for Influenza Surveillance, operates nationally during the ‘influenza season’ from May through September each year. Sentinel general practices, distributed at approximately 1:50,000 population, record the daily number of consultations that fit a case definition for an influenza-like illness (ILI) and collect respiratory samples for virus. Weekly consultation and virus detection data is forwarded by regional coordinators to ESR. The surveillance data and virology laboratory data are compiled weekly onto the ESR website (www.esr.cri.nz). During periods of Pandemic Alert, the surveillance system is enhanced by the inclusion of additional sentinel general practices and other sites.

In addition to the influenza surveillance systems described above, the SHIVERS study (Southern Hemisphere Influenza, Vaccine Effectiveness, Research and Surveillance) is collecting New Zealand data to help better understand the burden of disease and how to prevent its spread. Two new surveillance systems have been established in Auckland – one hospital-based and one general practice-based. Data from SHIVERS is available on the ESR website, including reports on mild to severe acute respiratory disease.

Weekly reports

The national weekly consultation rate is used to describe the overall level of ILI activity, using a set of threshold values: a weekly rate of 50–249 consultations per 100,000 patients is considered indicative of normal (baseline) seasonal influenza activity; 250–399 indicates higher than expected activity; while 400 and over indicates an epidemic level of disease. Figure 10.1 shows the national weekly ILI consultation rates from 2008 to 2013. In 2013 the weekly consultation rates were below the baseline level of activity.3

Figure 10.1: National weekly consultation rates for influenza-like illness, 2008–2013

Figure 10.1: National weekly consultation rates for influenza-like illness, 2008–2013

Source: Institute of Environmental Science and Research

Hospitalisations and deaths

In 2013 there were 782 hospitalisations for influenza, fewer than in 2012 (1076) (Figure 10.2). There were 205 deaths during the period 2000 to 2011: nine were children aged under 5 years, 126 were adults aged 65 years and older and one was a pregnant woman.

Figure 10.2: Hospitalisations for influenza, 2000–2013, and mortality, 2000–2011

Figure 10.2: Hospitalisations for influenza, 2000–2013, and mortality, 2000–2011

Source: Institute of Environmental Science and Research (hospitalisations) and the Ministry of Health (mortality)

SHIVERS hospitalisation data4 from the Auckland and Counties Manukau DHBs showed high hospitalisation rates in the very young and elderly populations (Figure 10.3), as well as for Pacific peoples, Māori and those from low-income groups.

Figure 10.3: Age-specific influenza hospitalisation rates among residents from Auckland and Counties Manukau DHBs (SHIVERS data), 29 April–29 December 2013

Figure 10.3: Age-specific influenza hospitalisation rates among residents from Auckland and Counties Manukau DHBs (SHIVERS data), 29 April–29 December 2013

Source: Institute of Environmental Science and Research

Hospital data for pneumonia and influenza includes both those cases coded as influenza and cases diagnosed with pneumonia that are secondary to, or a complication of, influenza. The primary diagnosis coded is pneumonia, but this underestimates the burden of disease associated with influenza.

Previous modelling data suggests that for every death attributable to influenza, a further 7.7 deaths are associated with complications of influenza.5 Overseas modelling has found a similar patterns of under-diagnosis, with a factor of 3.7 for the Netherlands6 and 10 for the UK.7

Circulating influenza strains

In 2013 a total of 2066 viruses were typed and subtyped. Of these, influenza B was the predominant strain (45 percent of all typed and subtyped viruses), then influenza A(H3N2) (41 percent) and the 2009 pandemic strain, A(H1N1)pdm09 (15 percent); see Figure 10.4.

Figure 10.4: Influenza viruses, by type, 2000–2013

Figure 10.4: Influenza viruses, by type, 2000–2013

Source: Institute of Environmental Science and Research

10.3.2 Influenza immunisation uptakeTop

The number of influenza vaccine doses distributed in 2013 was higher than in previous years, including 2010 (the year after the pandemic) (see Figure 10.5). Funded vaccine uptake for individuals aged 65 years and older was 70 percent, a small increase from 2012 (65 percent).

Figure 10.5: Influenza vaccine uptake, 1990–2013

Figure 10.5: : Influenza vaccine uptake, 1990–2013

Funded vaccine for individuals aged 65 years and older was introduced in 1997. Funded vaccine for individuals aged under 65 years with certain medical conditions was introduced in 1999.

Source: Institute of Environmental Science and Research

Since 2010 the Ministry of Health has requested that all DHBs provide influenza immunisation coverage data for their staff at the end of each influenza season. National influenza immunisation coverage for DHB staff is still very low, but it has steadily increased from 45 percent in 2010 to 58 percent in 2013.

10.3.3 Pandemic influenza A(H1N1)pdm09Top

A new influenza pandemic caused by a novel strain (ie, not covered in previous vaccines) of influenza A(H1N1) commenced in 2009. The pandemic strain became the predominant cause of influenza in New Zealand, with 3211 confirmed cases reported between 1 April and 31 December 2009, including 48 deaths. It was estimated that 30 percent of New Zealanders (1.3 million) had immunity to A(H1N1)pdm09 by March 2010, and an estimated 18 percent (800,000) were infected with the virus during the first wave, including one child in every three.8

Risk factors for severe H1N1 disease included obesity, pregnancy,9 diabetes mellitus and Pacific or Māori ethnicity.8

10.3.4 Avian influenza associated with human casesTop

Human infections and outbreaks following interspecies transmission of avian influenza viruses have been reported since 1997. Most cases have been associated with direct or indirect contact with infected birds.

Avian influenza is a rare disease in humans but requires close monitoring. In New Zealand, illness due to highly pathogenic avian influenza virus (HPAI) is a notifiable disease. Further information can be found on the Ministry of Health website (www.health.govt.nz).

Avian influenza A virus subtypes H5N1, H7N9 and H9N2 have caused human infections, with the H5N1 subtype established in domestic poultry throughout Southeast Asia, and in wild birds or domestic poultry in Europe and Africa. The H5N1 virus is highly pathogenic, with a mortality rate of over 50 percent. There has been no evidence of ongoing person-to-person transmission. Human infections with H7N9 virus were first reported in China in March 2013, with a mortality rate of approximately one-third of cases. At the time of writing, there was no evidence of ongoing person-to-person transmission.

Monitoring, surveillance and response for new pandemic strains are in place. See section 10.8.3.