Executive summary

This extended report aims to provide a broad overview of the epidemiology and virology of the 2009 pandemic in the European Union and European Economic Area (EU/EEA) countries (27 EU Member States (MS) and Norway and Iceland). Relevant background information on influenza epidemics and pandemics, notably their variability and unpredictability, is provided. The main trends and information are derived from the analysis and interpretation of the epidemiological and virological data and other analyses provided to the European Centre for Disease Prevention and Control's (ECDC) European Surveillance system (TESSy) through the European Influenza Surveillance Network (EISN).

These data and analyses show that, following its emergence in North America, the pandemic virus started to be transmitted in Europe around week 16/2009. This virus met the previously determined criteria for a pandemic in Europe as it did elsewhere. Surveillance suitable for the pandemic was rapidly developed and agreed upon by ECDC and the EU/EEA MS, with input from the World Health Organization (WHO) and countries already affected from outside Europe. This built on pre-existing systems, but included new elements to monitor the situation among those severely affected by the pandemic virus. In addition, epidemic intelligence and targeted science-watch methods were employed to determine, as early as possible, important parameters needed for informed risk assessments, adjusting projections and informing counter-measures.

The European Influenza Surveillance Network reported an initial spring/summer wave of transmission that appeared in most countries, but was only striking in a few countries, especially the United Kingdom. The rate of transmission briefly subsided as the summer progressed, but then accelerated again in the early autumn just after the re-opening of schools. This time it affected all countries, as an autumn/winter wave was seen to progress from west to east across the continent. The World Health Organization officially declared the pandemic over in week 32 of 2010.

In most countries, the autumn/winter wave of infection was sharp in shape, lasting approximately 14 weeks and was accompanied by a similar wave of hospitalisations and deaths. However, there was heterogeneity in the severity of disease as it varied from place to place, even within countries. In all, 2900 official deaths were reported by EU/EEA countries in the first 12 months during which the MS made extra efforts to collect these data. However, is recognised this will be only a proportion of the true burden of deaths due to the pandemic. An excess of allcause deaths in school-aged children was detected. Though this was an influenza virus never seen previously, prior exposure to a presumably antigenically similar influenza virus circulating before the mid-1950s ensured that many older people in Europe had some prior immunity. This fact, not unique to the 2009 pandemic, explains two of its notable differences from interpandemic, or seasonal, influenza: the overall lower mortality and the higher than expected relative burden of illness and fatality rates in young people. Though many older people appeared to be protected, those that were not showed the highest case fatality rates of any age group.

The pandemic virus displaced the previously dominant interpandemic influenza A viruses in Europe; though influenza B viruses still appeared at a low level late in the season. Only a low number of pandemic viruses were found to be resistant to oseltamivir and of these, very few seemed to be capable of being transmitted from one human to another. Though the pandemic viruses are not identical, there is little evidence of significant drift or the emergence of dominant new variants to date. One variant-A(H1N1)-D222G-has been suggested to be associated with more severe disease, though causation has not been established.

Although anecdotal evidence suggests that there were more mild and asymptomatic cases in comparison to the interpandemic influenza, there were enough cases of acute respiratory distress syndrome (ARDS)-a condition very rarely seen with interpandemic influenza-to stress intensive-care services in many places. Young children experienced the highest rates of disease, and country reports reveal that the highest rates of infection were in school-aged children. These high rates of illness passed particular burdens onto primary services, hospital paediatric services and especially intensive-care units in some localities.

Some limited data from serological surveys are now becoming available and support the surveillance data indicating higher rates of transmission than suspected from the clinical signs. However, these are not yet sufficient to make reliable predictions concerning what will happen next winter (2010/2011), and for this purpose the experience of the Southern Hemisphere temperate countries in the European summer period of 2010 has been most revealing.

At an early stage, the pandemic was much less severe than what had been feared. This was highlighted in the early ECDC Risk Assessments*, WHO reports and briefings given by ECDC to national and European authorities. With low rates of absenteeism, there was also little impact on services outside of the health sector. This and other features meant that this was arguably the most benign pandemic for which Europe could have hoped.