Monitoring patterns and levels of worldwide activity
Pandemic (H1N1) 2009 briefing note 22
21 JULY 2010 | GENEVA - As part of regular monitoring of H1N1 pandemic influenza, the WHO is in close dialogue with public health experts in countries worldwide, specifically to determine whether H1N1 activity has returned to levels and patterns normally seen for seasonal flu.
Worldwide, pandemic influenza activity has remained low over the past few months, and there has been little evidence of outbreaks outside the normal influenza season in most countries since the Northern Hemisphere winter. Temperate regions of the Northern Hemisphere are not presently reporting any outbreaks of influenza following one or two waves of the pandemic, and are adjusting their public health responses accordingly.
The Southern Hemisphere winter, which runs from May to September, is the typical influenza season for this part of the world. In most countries in the temperate zone of the Southern Hemisphere the level of influenza reported at present is low, and countries are reporting a mix of influenza strains comprising pandemic influenza A (H1N1) 2009, H3N2, and influenza B viruses. Where the pandemic H1N1 strain is prevalent, some severe illness and deaths have been reported.
Active transmission of pandemic influenza virus still persists in localized areas of the tropics, particularly in South and Southeast Asia, the Caribbean, Central America and West Africa.
Given the diverse pattern of influenza activity in the tropics and that the influenza season in the Southern Hemisphere is still ongoing, it is too early to determine if these countries have transitioned to levels and patterns expected for seasonal influenza. WHO remains in regular dialogue with countries affected to assess whether the pandemic influenza activity has transitioned to a seasonal pattern.