Managing pandemic influenza in general practice

A guide for preparation, response and recovery

3.3 How is it different from seasonal influenza?

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Seasonal influenza and pandemic influenza cause the same (or very similar) signs and symptoms. However, how often the influenza occurs, who gets ill and the severity of the disease may be quite different. Table 1 shows similarities and differences between seasonal and pandemic flu.

Seasonal influenza imposes a moderate but variable burden every year. The WHO estimates that seasonal influenza causes between 250,000 and 500,000 deaths each year.7

Unlike seasonal influenza, it is impossible to predict when and where the next pandemic will start, how quickly it will spread and what impact it will have on public health. The majority of deaths from seasonal influenza occur among people aged 65 years or older, but in a pandemic the proportion of deaths among the young increases.8

Table 1. Similarities and differences between seasonal and pandemic flu

Seasonal flu

Pandemic flu

Happens annually, usually in winter

Rarely happens (approximately every 30 years – three times in the 20th century)

Usually some immunity is built up from previous exposure

People have little or no immunity because the virus is new (hence no previous exposure)

Usually only people at high risk (eg. infants, elderly, immunocompromised, chronic illness such as chronic obstructive pulmonary disease [COPD]), not healthy adults, are at risk of serious complications

Healthy people may be at higher risk for serious complications and even death

Vaccination is available for annual flu season

Vaccination is unlikely to be available in the early stages of a pandemic

Adequate supply of antivirals is usually available

Effective antivirals may be in limited supply

Symptoms include fever, sore throat, weakness, headache, joint and muscle pain and cough

Symptoms may be more severe

Usually causes minor impact

May cause major impact on general public, including travel restrictions, closure of schools and businesses

Deaths occur each year (approximately 2800 per year)9

Potential for high death rate

Adapted from and available on the Centers for disease control and prevention website.

As was seen with the 2009 H1N1 pandemic, high death rates are not necessarily a feature of all influenza pandemics. Australia experienced 20-fold less mortality than several countries in the Americas.8 A number of factors may influence the mortality rate; these include:

  • the strength (virulence) of the virus
  • the number of people infected
  • the vulnerability of the affected populations
  • implementation of public health mitigation strategies
  • the effectiveness of preventative measures
  • the number and duration of pandemic waves
  • influenza vaccination coverage in preceding seasons
  • the use of antivirals (where indicated)
  • access to intensive care.

Although Australia as a whole experienced only a mild pandemic with H1N1, causing fewer deaths than seasonal influenza,10 not all Australians were affected equally. Indigenous Australians were found to be more vulnerable than the general Australian population. Aboriginal and Torres Strait Islander peoples had disproportionately high rates of complications with a mortality rate six times higher than non-Indigenous Australians.2 Research suggests that Australian Indigenous people may be particularly vulnerable to future infections (eg. H7N9) due to a lack of pre-existing T-cell immunity.11

  1. Department of Health and Ageing. Review of Australia’s health sector response to pandemic (H1N1) 2009: lessons identified. Canberra: Australian Government, 2011.
  2. World Health Organization. Influenza (seasonal). Geneva: WHO Media Centre, 2009.
  3. Simonsen L, Spreeuwenberg P, Lustig R, et al. Global mortality estimates for the 2009 influenza pandemic from the GLaMOR project: a modeling study. PLoS Med 2013;10:e1001558.
  4. Department of Health. Australian Health Management Plan for Pandemic Influenza. Canberra: Australian Government, 2009.
  5. Muscatello DJ, Newall AT, Dwyer DE, Macintyre CR. Mortality attributable to seasonal and pandemic influenza, Australia, 2003 to 2009, using a novel time series smoothing approach. PLoS One 2013;8:e64734.
  6. Quiñones-Parra S, Grant E, Loh L, et al. Pre-existing CD8+ T-cell immunity to the H7N9 influenza A virus varies across ethnicities. Proc Nat Acad Sci USA 2014;111:1049–54.