Editorial: Pandemic (H1N1) 2009 Influenza in Europe

  Published: 17.02.11 Updated: 17.02.2011 13:06:40

K. Kutsar, Editor-in-Chief, EpiNorth Journal

Citation: Kutsar K. Influenza A(H1N1) Pandemic in Europe. EpiNorth 2010;11:73-4.

WHO and European Commission were notified of the novel influenza A(H1N1) outbreak on 24 April 2009 after the new virus had been identified in Mexico and the USA. The pandemic virus spread rapidly across North America, then to Europe and the rest of the world. A public health emergency of international concern according to International Health Regulations (IHR) 2005 and a novel influenza pandemic was declared by WHO on 11 June 2009. In most European countries an initial spring/summer wave of transmission appeared. The rate of virus transmission briefly subsided as the summer progressed and then accelerated again in the early autumn just after the reopening of schools. This time all European countries were affected by an autumn/winter wave, sharp in shape and lasting approximately 14 weeks, progressing from west to east across the continent.

The new influenza virus was antigenically similar to the influenza virus circulating before the mid-1950s, ensuring that many older people had some prior immunity. This explains two of the pandemic differences from seasonal influenza; the lower overall mortality and the higher than expected relative burden of disease and fatality rates among young people. The highest rates of influenza were in school-age children and 25-30% of deaths were in previously entirely healthy young adults and outside the traditional risk groups. Regarding the other differences, a) the timing of the pandemic was different, starting „out of season” for Europe, b) the new influenza virus almost entirely displaced the preceding circulating influenza A viruses, seasonal A(H1N1) and A(H3N2), and coexisted with some influenza B viruses, c) schools and households were main settings for virus transmission, d) the prevalence of pandemic virus infection was considerably over 15% (estimated at 5-10% for seasonal influenza), e) higher proportion of asymptomatic and mild infections than in seasonal influenza, and f ) pregnant women and obese people were at special risk. In November 2009 ECDC produced projections to prepare for a reasonable worst case scenario of the pandemic with clinical attack rates (the proportion of exposed population with illness) up to 20%, mortality rates up to 3 per 100,000 population, and hospitalisation rates up to 100 per 100,000 population.

Specific pandemic influenza vaccines induced a good immune response with a good safety profile. Oseltamivir provided good prophylaxis as well as treatment effect. A small degree of antiviral resistance appeared. The pandemic virus variant A(H1N1)-D222G was suggested to be associated with more severe disease, but the virus did not mutate to a more lethal form.

WHO announced the end of the pandemic on 10 August 2010. Based on experience with past influenza pandemics, it is expected the A(H1N1)pdm influenza virus will take on the behaviour of a seasonal influenza virus and continue to circulate for some years to come.