Two recent studies the severity and mortality features of the 2009 pandemic in the UK and MexicoArchived

Scientific Advance 2011-10-27

Two recent studies the severity and mortality features of the 2009 pandemic in the UK and Mexico The United Kingdom Changes in severity of 2009 pandemic A/H1N1 influenza in England: a Bayesian evidence synthesis Presanis AM, Pebody RG, Paterson BJ, et al.  BMJ 2011; 343:d5408

The UK was unusual in Europe in having two substantial pandemic 2009 waves in the Late Spring / Summer and Autumn / Winter waves in 2009.(1,2)  Uniquely though this allowed comparison of the severity characteristic of the two phases.  The main objective of this study was to assess the impact and severity of the pandemic in England during these two waves of activity that occurred there by the end of February 2010. This was done by estimating the probabilities of cases leading to severe events and the proportion of the population infected, performing a ‘Bayesian’ synthesis of all available relevant surveillance data from June 2009 to February 2010. These data included: pre-existing influenza surveillance systems (including both estimated numbers of symptomatic cases from GP consultations and cross sectional population serological surveys); systems set up in response to the pandemic (including follow-up of laboratory confirmed cases up to end of June 2009 obtained from what is known in the UK as the First Few 100s and the Fluzone surveillance databases); retrospective and prospective follow-up of confirmed hospitalised cases; and reported deaths associated with the 2009 influenza A(H1N1) pandemic.  The main outcome measures investigated by the researchers were age specific and wave specific probabilities of infection and symptomatic infection resulting in hospitalisation, intensive care admission, and death, as well as infection attack rates (both symptomatic and total). The authors also estimated the probabilities of intensive care admissions and deaths given hospitalisation over time to allow for an evaluation of potential changes in severity across waves.

Results are presented from the two waves associated with stratified probabilities (Table) The authors also estimated that around 35% of infections were symptomatic. The estimated probabilities of infections resulting in severe events during the Late Spring / Summer wave were calculated.

An estimated 30% of hospitalisations were detected in surveillance systems in the Spring / Summer wave, compared with 20% in Autumn / Winter wave. In addition, across the two waves a mid-estimate infection attack rate of 11.2% of the population of England was calculated, rising to 29.5% in the 5-14 year old group. After performing sensitivity analysis the infection attack rate was thought to have varied between 5.9% and 28.4%. A probability of an infection leading to death during the second wave was calculated, this ranging from a high probability of 0.017% to a low probability of 0.0027%. In a synthesis the paper concluded that there was a mild 2009 pandemic (characterised by case and infection severity ratios increasing between waves) and low ascertainment rates, highlighting the importance of systems enabling early robust estimation of severity able to inform optimal public health responses.

Mexico Mortality Burden of the A/H1N1 Pandemic in Mexico: A Comparison of Deaths and Years of Life Lost to Seasonal Influenza  Charu V, Chowell G, Palacio Mejia LS, et al. Clin Infect Dis. (2011) 53 (10): 985-993. First published online: October 5, 2011

This study combining the strengths of Mexican authorities and an established United States group specialising in influenza research.  Excess mortality rates and years of life lost (YLL) for pandemic and seasonal influenza in Mexico were estimated and laboratory-confirmed death reports were evaluated.  The methodology employed to estimate excess mortality and YLL was based on monthly age-specific and cause-specific death rates from January 2000 through to April 2010, as well as on population-based surveillance of the activity of the influenza A(H1N1)2009 pandemic virus. The age-stratified laboratory-confirmed A(H1N1) death reports were obtained from an active surveillance system covering 40% of the Mexican population.

The investigation estimation that the 2009 pandemic was associated with an excess of all-cause deaths of 11.1 per 100,000 population and 445,000 YLL during the waves of the 2009 pandemic  in Mexico from April to December 2009.  In addition it was estimated that pandemic mortality burden was 0.6-2.6 times that of a typical influenza season and lower than that of the severe 2003-2004 seasonal influenza epidemic in Mexico.  Moreover, individuals included in the age groups 5-19 and 20-59 years were disproportionately affected when compared to what was observed for seasonal influenza. The results described here suggest that Mexico experienced a high burden of mortality from the 2009 influenza A(H1N1) pandemic.

ECDC Comment (27 October 2011)

There are major methodological problems associated with estimating the burden of disease and mortality due to the 2009 pandemic and comparing it with what is experienced with seasonal influenza .(3)  ECDC published a Scientific Advance on this topic in April 2011, referring then to the pandemic burden in a specific EU country, the Netherlands (4).  Aside from the technical difficulties of deciding what is meant by ‘severity’ in the simple terms of case fatality rates has varied considerably by  time and place. For example the 1918 pandemic was sensitive to social conditions with poorer countries experiencing higher death rates(5) as does seasonal influenza.(6). Over time the 1918 pandemic both became more severe, and then declined in severity.(7)  In Europe at least the last (1968) pandemic was more severe in its second winter.(8)  It will be important to monitor the severity and impact of the 2009 pandemic as over time it joins the established influenza to become the ‘new’ seasonal influenza.(9)   

References

1. Health Protection Agency and others Epidemiological report of pandemic (H1N1) 2009 in the UK October 2010 
2. Amato-Gauci A, Zucs P, Snacken R, Ciancio B, Lopez V, Broberg E, Penttinen P, Nicoll A, on behalf of the European Influenza Surveillance Network (EISN). Surveillance trends of the 2009 influenza A(H1N1) pandemic in Europe. Euro Surveill. 2011;16(26):pii=19903.
3. ECDC, Mortality from influenza: Comparing deaths from seasonal and pandemic influenza 
4. ECDC The burden of pandemic influenza A(H1N1)2009 in the Netherlands, 18 Apr 2011.
5. Murray CJL Lopez AD, Chin B, Feehan D, Hill KH Estimation of potential global pandemic influenza mortality on the basis of vital registry data from the 1918-20 pandemic: a quantitative analysis, Lancet (2006); 368: 2211-2218 
6. Anon Influenza Outbreak Madagascar, July - August 2002 MMWR November 15, 2002 / 51(45);1016-1018
7. Kilbourne ED. Influenza pandemics of the 20th century. Emerging infectious diseases. 2006 Jan;12(1):9-14.
8. Rizzo C, Bella A, Viboud C, Simonsen L, Miller MA, Rota MC, et al. Trends for influenza-related deaths during pandemic and epidemic seasons, Italy, 1969-2001. Emerging infectious diseases. 2007 May;13(5):694-9.
9. Nicoll A, Sprenger M. The end of the pandemic – what will be the pattern of influenza in the 2010-11 European winter and beyond Euro Surveill. 2010;15(32):pii 19637.