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Mortality from influenza

Mortality from influenza: Comparing deaths from seasonal and pandemic influenza

The number of deaths reported from the 2009 influenza A(H1N1) pandemic are sometimes compared to the number of deaths from seasonal influenza. One perspective is that fewer people have died from the 2009 influenza pandemic than from ordinary seasonal influenza. This has led to the 2009 influenza pandemic sometimes been characterised as a mild pandemic, or even as a ‘false pandemic’.

When comparing the deaths from seasonal influenza with the reported deaths from the pandemic influenza, the following has to be taken into account: 

For seasonal influenza exact number of deaths are not accurately measured in any EU country. Instead, statistical methods are used to estimate mortality through looking at the surges of deaths (excess mortality) that take place in association with influenza epidemics. 

There have been around 3,000 confirmed deaths from the pandemic influenza. This is based on confirmed cases of influenza (usually through laboratory tests). This is a very different way of measuring mortality from the estimate method.

It is thus grossly misleading to compare the estimates from non-pandemic years with the numbers of confirmed deaths from the pandemic.

Accurate assessments of pandemic deaths will likely be possible only one to two years from now, and then only in certain countries with sophisticated surveillance systems.

It is also misleading to say that the 2009 influenza pandemic is “just like seasonal influenza”, even if the numbers of deaths end up being similar. The pattern of deaths from the 2009 pandemic influenza has differed significantly from the previous seasonal influenza in several ways: 

  1. In the cases reported to ECDC with severe respiratory infection from pandemic influenza who died, nearly 80% are in people under 65 years, while for previous seasonal flu the proportion is about 90% in people over 65. 
  2. 30% of the pandemic deaths are in entirely healthy people.
  3. Surveillance of all-cause mortality in a number of EU countries has shown an increase in deaths among children during the pandemic as found by the EUROMOMO system and published in Eurosurveillance. (See Eurosurveillance., Volume 15, issue 5, 4 February).
  4. A number of deaths have been from Acute Respiratory Distress Syndrome – this is very rare in seasonal influenza.
  5. But it remains true that the majority of deaths have been in people with chronic underlying conditions, following the pattern of the old seasonal influenza.

Importantly not all people who die as a result of influenza have the infection recognized and in some cases the tests are done too late to detect the infection. In other instances there are no tests available. As a consequence it is important to note that the deaths reported as being due to influenza are always an underestimate, this is especially true in the case with seasonal influenza affecting older people but is probably less the case for this pandemic.

Resources

1.WHO Pandemic (H1N1) 2009 Briefing note of 22 December 2009: Comparing deaths from pandemic and seasonal influenza

2.Eurosurveillance, Volume 15, Issue 1, 07 January 2010, Editorial: ’A new decade, a new seasonal influenza: the Council of the European Union Recommendation on seasonal influenza vaccination’. A Nicoll 

3.Eurosurveillance, Volume 15, Issue 5, 04 February 2010: ‘Higher all-cause mortality in children during autumn 2009 compared with the three previous years: pooled results from eight European countries’, A Mazick

4. ECDC Forward look risk assessment for the 2009 pandemic influenza A(H1N1) and future influenza seasons of 08 March 2010

Links
How does 2009 H1N1 flu compare to seasonal flu in terms of its severity and infection rates?

- Table comparing Seasonal Influenza 2000/1 to 2008/9 and 2009 Pandemic Influenza

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