Technical Consultation on the Severity of Disease Caused by the new influenza A (H1N1) Virus Infections

World Health Organization convened, via teleconference, a technical consultation to assess current knowledge about the severity of disease caused by influenza A(H1N1) infection and the implications this might have near the start of a pandemic. WHO also sought data on the clinical spectrum and patterns of the disease and how this compares with what is seen during epidemics of seasonal influenza.

Information was provided by epidemiologists, clinicians, and virologists from Canada, Mexico, Spain, the United Kingdom, and the United States of America. These countries have the largest experience, to date, in aggressive surveillance for the disease and the detection and investigation of cases. In addition, international experts advised WHO during the consultation.

Although the data were preliminary and many key questions cannot be answered at present, this conference provided a stronger understanding about this illness than was previously known. The larger number of cases in Canada, Mexico and the United States provided the most comprehensive initial picture of patterns of illness and spread to date.


In all of the countries most cases have experienced a mild influenza-like illness. However, severe illness has been reported in Mexico and the United States, especially in persons who are at risk for complications of seasonal influenza, such as the very young, women who are pregnant and persons with underlying medical conditions but also in some healthy young adults. It is too early to get a reliable estimate of the case fatality ratio.

The principal reason for hospitalization of cases in Mexico and the United States is severe respiratory disease. In the Mexican experience, secondary bacterial pneumonia has occurred among hospitalized cases.

Mexico experienced a large number of persons over a short period of time seeking care and hospitalization for respiratory disease. However, health care systems of other affected countries have not had a similar experience to date.

While all age groups have been affected, most cases are occurring in younger age groups (median age in mid-20's). Some caution must be exercised in interpreting this observation as it may reflect age patterns of persons travelling to Mexico and the occurrence of outbreaks in school settings.

Some, but not all, countries reported cases that had diarrhoea, in addition to typical flu-like symptoms, such as fever, cough, runny nose, headache, and malaise. This initial finding about diarrhoea requires further confirmation, and studies to determine if virus is shed in the faeces. If this is found, it could have significance for countries or situations in which there is inadequate sanitation.

In addition, some confirmed cases did not have fever and it is likely that there are asymptomatic and very mild cases of infection -- as occurs with seasonal epidemics of influenza.

The virus appears to be easily transmitted from person to person. In the United States and Mexico, community transmission has been widespread. Based on early data, the secondary attack rate is estimated to be around 22% by some experts, and as high as 30% by others.

At present, the virus is susceptible to oseltamivir (Tamiflu) and zanamivir (Relenza). In laboratory studies, there is no evidence so far that the virus has markers for human virulence that have been described for the 1918 H1N1 pandemic virus and avian influenza H5N1 viruses.

There is no convincing laboratory or clinical evidence that vaccination against normal seasonal influenza confers protection against the new H1N1 virus, but this requires more study.

In summary, there is now early information about the H1N1 virus' behaviour and disease patterns, coming primarily from Canada, Mexico and the United States. Cases in most other countries have been too few to get a full picture of the virus and its impact in those setting. The situation is expected to evolve over time and bears careful watching.

Although illness to date has been mainly mild, as the number of cases and the geographic spread of the virus increase, increased numbers of severe illness and deaths are expected -- as we see with influenza epidemics each year.

WHO is grateful to countries for their rapid investigation of cases and their willingness to share these early findings internationally.