The World Health Organisation (WHO) has advised countries in the Northern Hemisphere as well as countries in tropical climates where H1N1 was initially slow to spread to prepare for a second wave of pandemic spread. They have also cautioned temperate parts of the southern hemisphere to remain vigilant.

In addition to pointing to evidence that H1N1 is now the dominant influenza strain in most parts of the world, the WHO has pointed out that there appear to be some important differences between the nature of illness reported during the pandemic and observations during seasonal epidemics of influenza.

Differences include:

• The age groups affected by H1N1 are younger, particularly for those cases experiencing severe or fatal illness. Most deaths have occurred in adults under the age of 50. This age distribution is notably different to that observed in epidemics of seasonal influenza where approximately 90% of severe and fatal cases occur in people over the age of 65.
• H1N1 has a greater propensity to directly infect the lungs, causing severe respiratory failure in severe cases. In the southern hemisphere this has reportedly imposed a significant burden on intensive care units.

Groups identified as being particularly vulnerable include: pregnant women and their offspring. An increased risk during pregnancy has been established across most countries. Individuals suffering from particular medical conditions including respiratory disease (in particular asthma), cardiovascular disease, diabetes and immunosuppression. The obese. The WHO points out that obesity itself is now a global epidemic.