Pandemic influenza H1N1 has now been reported in over 170 countries and territories worldwide. While the case counts no longer reflect actual disease activity, WHO is actively monitoring the progress of the pandemic through frequent consultations with the WHO Regional Offices and member states. Of particular interest is the situation in temperate countries of the Southern Hemisphere, which are now passing through their winter season. This season, pandemic H1N1 has been the predominant influenza virus in nearly all of the temperate regions of Southern Hemisphere, with South Africa being a notable exception. Australia and countries in the southern part of South America experienced rapid increases in cases of pandemic influenza early in their winter season. These same areas are now starting to report decreases in the numbers of people seeking care and being admitted to hospital. Although the virus is still circulating in these areas as it moves into areas not affected earlier, the overall national trends are downward. South Africa, in contrast, experienced an early influenza season with a seasonal subtype, influenza A (H3N2). As the influenza season in South Africa reached its peak in early to mid June and began to decline, pandemic influenza H1N1 appeared and has now become the dominant subtype seen there as well. In the temperate areas of the northern hemisphere which experienced early outbreaks of pandemic H1N1 influenza, including countries in North America and Europe, the virus continues to spread to new areas and cause intense local outbreaks. However, the overall national trend in cases is downward in the Americas. Tropical regions of the world, which typically experience year round transmission of influenza viruses with peak transmission at different and often multiple times in a year, are now seeing increases in cases, for example in tropical areas of Central and South America and in South and South East Asia. In summary, the overall picture of transmission globally is one of declining transmission in the temperate regions of the Southern Hemisphere with the exception of southern Africa. The season in these areas was characterized by rapid rise and fall of respiratory disease numbers, as is seen in a normal influenza season. The impact and severity of the season in these areas in terms of proportion of cases which developed severe disease and the load imposed on health care infrastructure is still being evaluated but generally appeared slightly worse than a normal influenza season in most places with increased hospitalization requiring respiratory critical care. The northern hemisphere is experiencing continued spread of the virus but declining activity is being observed in areas affected early in the course of the pandemic. Tropical areas of the world are now experiencing increasing numbers of cases at a time when the usual seasonal peaks would occur. As the pandemic H1N1 influenza virus is now the dominant strain in most areas of the world, it can be expected to persist into the coming influenza season in the Northern Hemisphere. Additionally, there is a risk of further spread of virus in highly populated areas as community spread starts occurring in Asia and Africa.
