Individual Impact 23

have been found in recently circulating, highly pathogenic H5N1 viruses (Taubenberger 2005).

At present, H5N1 avian influenza remains largely a disease of birds. The species barrier is significant: despite the infection of tens of millions of poultry over large geographical areas for more than two years, fewer than 200 human cases have been confirmed by a laboratory (WHO 200601). Human cases, first documented in 1997 (Yuen 1998), coincided with outbreaks of highly pathogenic H5N1 avian influenza in poultry. Very limited human-to-human transmission of the H5N1 strain was documented in healthcare workers and family members with contact (Katz 1999, Buxton Bridges 2000). Although H5 antibodies were detected in these groups, indicating infection with the virus, no cases of severe disease occurred.

There are little data to show to what extent asymptomatic infection or mild clinical disease occur following infection with highly pathogenic avian H5N1 strains. If asymptomatic infections were frequent, the 55 % fatality rate of severe human H5N1 disease reported as of 21 March 2006 (WHO 20060321) would of course be less alarming. However, these episodes may be the exception, at least in some settings. In a study conducted in a Cambodian village with H5N1 outbreaks in poultry and 4 fatal human cases, testing of blood samples from 351 villagers found no additional infections, although many villagers had had significant exposure to infected poultry (ProMED 20060322.0893 and Buchy, personal communication).

Until now, the disease has predominantly affected children and young adults. Of 116 patients for whom demographical data had been published on the WHO Website from December 2003 until 9 February 2006, 50 % were 16 years old or younger, 75 % were younger than 30 years, and 90 % were younger than 40 years old (Promed 20060211.0463). The reason for this age distribution (exposure risk, disease reporting bias, intrinsic host issues, etc.) is unclear. Likewise, it is not known whether, and to what extent, genetic composition plays a role in the susceptibility and resistance to infection with H5N1 influenza virus (Promed 20060216.0512).

The next pandemic is expected to cause clinical disease in 2 billion people. Bestcase scenarios, modelled on the mild pandemic of 1968, anticipate between 2 million and 7.4 million cases (WHO 2005b). However, if we translate the death toll associated with the 1918 influenza virus to the current population, there could be 180 million to 360 million deaths globally (Osterholm 2005).

Individual Impact

The fate of an individual during an influenza outbreak, be it epidemic or pandemic, is variable. It is estimated that about half of those infected have no clinical symptoms or signs. Among the others, clinical presentation varies from afebrile respiratory symptoms mimicking the common cold, to febrile illnesses ranging in severity from mild to debilitating (Hoffmann 2006a), and may cause disorders affecting the lung, heart, brain, liver, kidneys, and muscles (Nicholson 2003).

The clinical course is influenced by the patient’s age, the degree of pre-existing immunity, properties of the virus, smoking, co-morbidities, immunosuppression, and pregnancy (Nicholson 2003). Death mostly occurs as a consequence of primary viral pneumonia or of secondary respiratory bacterial infections, especially in pa-