Table 3. Estimates of the attack rates and severity of the pandemic A(H1N1)pdm09 influenza in different regions.
| Region and time | Data type | Model | Attack rate | Severity |
|---|---|---|---|---|
| Finland 2009/2010 and 2010/2011 (the current study) | Laboratory-based surveillance of cases over time; coverage of vaccination over time | Dynamic | 5.9% and 3.0% during the two seasons (17% and 3.5% in age group 10-14 years) | Hospitalization/infection ratio 0.7% (0.4% in age group 5-14 years); intensive care/hospitalization ratio 8% |
| Finland 2009/2010 and 2010/2011 (the same data as in the current study) [2] | Laboratory-based attack rates per season; coverage of vaccination | Static | 3.9% and 1% during the two seasons (11% and 2.4% in age group 10-14 years) | Hospitalization/infection ratio 1.1% (0.3% in age group 5-14 years); intensive care/hospitalization ratio 10% |
| London, two outbreaks, August 2009 and Sep-Dec 2009 [6] | Laboratory-based surveillance of cases over time; incidence of influenza-like illness over time; seroconversion rates | Dynamic | 9% and 10% during the outbreaks (22% and 30% in age group 5-14 years) | Not estimated |
| Several regions, 2009/2010 [3] | Pre- and post-pandemic sera | Static | 24% in 2009/2010 (meta-analysis); 46% in age group 5-19 years | Symptomatic disease/infection ratio 1/3, fatal cases/infection ratio 0.02% |
| UK, three waves: summer 2009, autumn and winter 2009/2010, autumn and winter 2010/2011 [4] | Laboratory-based surveillance of cases per wave; incidence of influenza-like illness; serological surveys | Static | 5%, 10% and 15% in three waves. 10%, 20% and 10% in age group 5-14 years | Hospitalization/infection ratio 0.2%; intensive care/infection ratio 0.03% |
| Netherlands, a single season in autumn-winter 2009 [5] | Laboratory-based surveillance; serological surveys pre- and post season 2009/2010 | Static | 8%, with 35% in age group 5-19 | Hospitalization/infection ratio 0.14%; intensive care/infection ratio 0.017% |
The attack rate refers to the (estimated) proportion of infections occurring during one epidemic season. Definitions of severity vary according to study, based on different types of data. For convenience, the estimates from the current study are shown on the first row.