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WHO Pandemic (H1N1) 2009 briefing note 9

GENEVA — Monitoring of outbreaks from different parts of the world provides sufficient information to make some tentative conclusions about how the influenza pandemic might evolve in the coming months.

WHO is advising countries in the northern hemisphere to prepare for a second wave of pandemic spread. Countries with tropical climates, where the pandemic virus arrived later than elsewhere, also need to prepare for an increasing number of cases.  Countries in temperate parts of the southern hemisphere should remain vigilant. As experience has shown, localized “hot spots” of increasing transmission can continue to occur even when the pandemic has peaked at the national level.

H1N1 now the dominant virus strain

Evidence from multiple outbreak sites demonstrates that the H1N1 pandemic virus has rapidly established itself and is now the dominant influenza strain in most parts of the world. The pandemic will persist in the coming months as the virus continues to move through susceptible populations. Close monitoring of viruses by a WHO network of laboratories shows that viruses from all outbreaks remain virtually identical. Studies have detected no signs that the virus has mutated to a more virulent or lethal form.  Likewise, the clinical picture of pandemic influenza is largely consistent across all countries. The overwhelming majority of patients continue to experience mild illness. Although the virus can cause very severe and fatal illness, also in young and healthy people, the number of such cases remains small.

Not the same as seasonal influenzaCurrent evidence points to some important differences between patterns of illness reported during the pandemic and those seen during seasonal epidemics of influenza. The age groups affected by the pandemic are generally younger. This is true for those most frequently infected, and especially so for those experiencing severe or fatal illness.  To date, most severe cases and deaths have occurred in adults under the age of 50 years, with deaths in the elderly comparatively rare. This age distribution is in stark contrast with seasonal influenza, where around 90% of severe and fatal cases occur in people 65 years of age or older.

Severe respiratory failure

Perhaps most significantly, clinicians from around the world are reporting a very severe form of disease, also in young and otherwise healthy people, which is rarely seen during seasonal influenza infections. In these patients, the virus directly infects the lung, causing severe respiratory failure. Saving these lives depends on highly specialized and demanding care in intensive care units, usually with long and costly stays.  During the winter season in the southern hemisphere, several countries have viewed the need for intensive care as the greatest burden on health services. Some cities in these countries report that nearly 15 percent of hospitalized cases have required intensive care.  Preparedness measures need to anticipate this increased demand on intensive care units, which could be overwhelmed by a sudden surge in the number of severe cases.

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