Pandemic (H1N1) 2009 in the WHO European Region
Chronology and surveillance strategies
On 28 April 2009, confirmed cases of pandemic (H1N1) 2009 influenza were reported in three European Member States.
By 11 October 2009, 49 of the 53 countries had reported over 61 000 detections of pandemic (H1N1) 2009 virus. The actual number of cases was significantly higher, however, as many countries had stopped counting individual cases. As domestic infections with pandemic (H1N1) 2009 increased, countries changed their surveillance strategies, moving from testing all pandemic cases to relying on monitoring, largely by well-established sentinel and other routine surveillance systems for respiratory disease. Several countries announced this change in early July: for example, Ireland on 16 July (week 29) and the Netherlands on 24 July (week 30). The sudden decrease in positive samples reported in weeks 32 (Ireland) and 33 (the Netherlands) most likely reflects this change in testing policy. This is in line with the updated WHO guidance on pandemic surveillance, which no longer required countries to confirm and report all cases after its publication on 16 July 2009.
In addition, countries experiencing domestic transmission adapted their control measures. For example, countries that had implemented policies to contain spread switched to mitigation strategies. For example, in the early stages of the pandemic, the United Kingdom had provided antiviral drugs to people who were symptomatic, as well as to their close contacts, as a measure to delay spread. On 2 July, as infection had become widespread, the country switched its approach, providing antiviral treatment to patients based on clinical diagnosis only.
Investigations of initial cases
Countries experiencing their first pandemic cases performed detailed epidemiological, clinical and virological investigations and reported the data to WHO. WHO/Europe analysed the data provided up to 16 July, which include records of nearly 9000 patients. The main findings are as follows.
- The vast majority of cases were mild, with symptoms typical of ILI.
- By the beginning of June 2009, an increasing proportion of cases with no travel history were reported.
- By 16 July, the main group affected was aged 10–19, likely due to a bias in reporting of cases associated with school outbreaks, particularly in the United Kingdom.
- About 10% of cases were hospitalized, although this should not be regarded as a proxy of severity of disease, as many cases were hospitalized for other reasons, such as containment.
- Complications were practically absent from the data, but 0.4% of confirmed cases reportedly had pneumonia.
- 3% of confirmed cases had a precondition (e.g. cancer, diabetes, HIV, heart disease, seizures, lung disease, pregnancy, malnutrition).
- 2% of confirmed cases reported having been vaccinated against seasonal influenza: this low coverage may be due to the age bias in the data.
This analysis does not include information on the 193 laboratory-confirmed deaths in 18 countries reported to WHO/Europe. Of these deaths, 84 were reported in the United Kingdom, 42 in Spain, 28 in Israel, 6 in France, 4 each in Ireland, the Netherlands and Norway, and 3 or fewer in each of 11 other countries (Greece, Belgium, Bulgaria, Denmark, Germany, Hungary, Italy, Malta, Luxembourg, Portugal and Sweden.
