Overview of EuroFlu influenza surveillance data

This section provides an overview of the data reported to WHO/Europe through EuroFlu during weeks 16–39/2009. It includes epidemiological and virological monitoring data from sentinel and non-sentinel sources, as well as virological data from enhanced surveillance of pandemic cases. During this period, the EuroFlu platform was modified to include pandemic surveillance indicators.

During the period covered, 44 of the 53 countries in the European Region reported data to EuroFlu. In contrast to previous seasons, during which mainly virological surveillance was performed and an interseason bulletin was published, countries continued routine epidemiological surveillance. In addition to routine monitoring, many countries enhanced their surveillance, to detect the first pandemic cases and to describe the epidemiological, clinical and virological features of the disease. Newly affected countries typically tried to test all identified cases but, as domestic transmission increased, they switched to monitoring performed by seasonal influenza sentinel surveillance networks.

Situation overview

During the summer, high-intensity influenza activity was reported for England (the United Kingdom), Ireland and Malta, indicating a level higher than usually seen for seasonal influenza. Twelve countries reported medium intensity. The following countries reported widespread activity for at least one week in the period of weeks 16–39/2009: Belgium (week 39), Israel (weeks 27–39), Ireland (weeks 38–39), the Netherlands (week 39), Sweden (weeks 30–31) and the United Kingdom (England, weeks 28–33; Wales, weeks 30–33). Many countries only reported sporadic or localized cases over the summer. The EuroFlu platform offers season tables giving an overview of the intensity and geographic spread for all countries.

A substantial increase of multiple weeks above baseline levels of influenza was observed for Ireland, Israel, Norway, Sweden and the United Kingdom (England, Northern Ireland and Wales). These levels of activity were similar to normal seasonal and historical influenza activity, except in the United Kingdom, where England had a somewhat higher rate of ILI per 100 000 population than in the last five seasons. The age groups most frequently presenting with ILI to GP s during periods of influenza circulation were school-aged children (5–14 years) in Ireland and the United Kingdom (Northern Ireland), adults (15–64 years) in Sweden and children (aged 0–14) in Israel. The increase in ILI was substantiated by an increase in virological detections in all countries except Norway, where few influenza virus detections were reported. The increased levels of ILI in Norway may represent increased public concern about influenza, rather than a substantial rise in incidence. The percentage of sentinel samples testing positive for influenza in the Region as a whole decreased from 14.4% to 3.6% from week 16 to week 24. From week 25 to week 39, this percentage increased again to about 10% (range: 7.0–14.4%).

In the WHO European Region, virus detections from both sentinel and non-sentinel sources peaked in weeks 30–31.  Peaks for individual countries may differ, however, and are influenced by surveillance practices (enhanced versus monitoring), as discussed above.

For a number of countries (e.g. Belgium, France) an increase in respiratory disease activity was observed as of weeks 37–38. This is related to increases in ARI consultation rates that are typical for the end of the summer season, when other respiratory infections, not necessarily influenza, play a role. Nevertheless, in week 40/2009, at the start of the new influenza season, 17.5% of all specimens from sentinel surveillance tested positive for influenza; in Belgium, Ireland, Israel, Slovenia, Spain and the United Kingdom (Northern Ireland) this proportion was over 25%, ranging from 27.6% in Northern Ireland to 42.9% in Israel. This suggests that, in addition to normal increases in respiratory disease activity now occurring, the presence of the pandemic (H1N1) 2009 virus in the European Region has led to earlier-than-normal influenza activity during the 2009/2010 influenza season.

Prevalence of pandemic (H1N1) 2009 viruses, antigenic characteristics and antiviral susceptibility

For all countries reporting influenza detection, pandemic (H1N1) 2009 was the dominant virus. In the summer, 97% (N = 22 952) of influenza virus detections were influenza type A and 3% were influenza B. Of the influenza A viruses, 18% were not subtyped. Of the influenza A viruses that were subtyped, 96.5%% were pandemic A (H1); 1.2% were seasonal A (H1); and 2.3% were A (H3). Based on the antigenic and/or genetic characterization of viruses, by week 39/2009, a total of 1116 viruses were characterized as pandemic A (H1N1), A/California/7/2009-like. This is the strain currently recommended by WHO for pandemic vaccine preparation, as well as the 2010 southern hemisphere vaccine.

Five countries reported testing the antiviral susceptibility of pandemic (H1N1) 2009 viruses since week 16. All viruses tested for resistance to oseltamivir and zanamivir (599/599) were found to be sensitive to these neuraminidase inhibitors. In addition, the United Kingdom has also reported that, of 1562 viruses analysed for the marker commonly associated with resistance to oseltamivir in seasonal influenza (H275Y), only 2 were found to carry this marker and both were sensitive to zanamivir. (13)

Impact of the pandemic

On 16 July, WHO asked countries to start reporting on the impact of the pandemic as judged by pressure on health care services. A total of 31 countries responded. Albania, Ireland, Kyrgyzstan, Malta, Norway and the United Kingdom (Wales) reported moderate impact at some point between weeks 16 and 39, and the 25 other countries reported low impact.

Severe disease surveillance

Reported deaths and cases of severe disease are being collected in three ways, from:

• national data on pandemic hospitalizations, ICU cases and deaths;
• severe disease and deaths monitored in sentinel severe acute respiratory infection (SARI) surveillance systems; and
• national death registry data.

These data can also be entered on the EuroFlu platform. WHO/Europe is assisting countries to collect the data, and is analysing those provided.