ECDC INTERIM RISK ASSESSMENT

Pandemic (H1N1) 2009 influenza
21 August 2009

Table of contents
Executive summary………………………………………………………………………………………………………………………..2
Source, date and type of request……………………………………………………………………………………………………….3
Specific question……………………………………………………………………………………………………………………………3
Consulted experts………………………………………………………………………………………………………………………….3
Evidence assessment……………………………………………………………………………………………………………………..3
Risk assessment……………………………………………………………………………………………………………………………3
1 Background…………………………………………………………………………………………………………………………….3
2 Important features……………………………………………………………………………………………………………………4
2.1 Basic epidemiology……………………………………………………………………………………………………………..5
2.1.1 Age and sex………………………………………………………………………………………………………………..5
2.2 Disease characteristics…………………………………………………………………………………………………………6
2.2.1. Modes of transmission…………………………………………………………………………………………………..6
2.2.2 Spectrum of disease – clinical features………………………………………………………………………………6
2.2.3 Asymptomatic cases………………………………………………………………………………………………………6
2.2.4 Ease of transmission – effective reproductive number…………………………………………………………..6
2.2.5 Clinical attack rate…………………………………………………………………………………………………………7
2.2.6 Hospitalisation rate……………………………………………………………………………………………………….8
2.2.7 Case fatality rate (CFR)………………………………………………………………………………………………….8
2.2.8 Planning assumptions…………………………………………………………………………………………………….9
2.2.9 Risk groups for hospitalisation and severe disease……………………………………………………………….9
2.3 Features of the virus…………………………………………………………………………………………………………..10
2.3.1 Genetic stability…………………………………………………………………………………………………………10
2.3.2 Susceptibility to antivirals and antiviral resistance………………………………………………………………10
2.3.3 Pathogenicity of the virus……………………………………………………………………………………………..10
2.3.4 Immunity………………………………………………………………………………………………………………….10
2.4 Severity………………………………………………………………………………………………………………………….11
2.4.1 Potential worsening of severity………………………………………………………………………………………11
3 Areas of particular uncertainty……………………………………………………………………………………………………11
3.1 Mix of influenza viruses that will be circulating this coming autumn and winter in Europe………………….11
3.2 Likely timing and pattern of spread of the virus in Europe in the summer, autumn and winter……………12
3.3 Shedding of viruses and infectivity…………………………………………………………………………………………12
3.4 Proportion of hospitalised cases requiring intensive care and respiratory support…………………………….12
3.5 Relative and attributable risk of more severe disease…………………………………………………………………12
3.6 Pathological processes underlying severe disease……………………………………………………………………..12
Next steps for ECDC…………………………………………………………………………………………………………………….12
Date of next planned update………………………………………………………………………………………………………….12
References…………………………………………………………………………………………………………………………………13