Influenza viruses in circulation, 2009: Multiple viral subtypes (influenza A subtypes, pandemic H1N1, and influenza B) circulated throughout 2009 in both the Southern and Northern Hemispheres (Figures WO-12 and WO-13).

Southern Hemisphere influenza season: Several countries experienced multiple viral epidemics that included RSV, parainfluenza, and seasonal influenza (both H3N2 and H1N1); in some cases, 2009-H1N1 influenza A overpowered co-infecting viruses to become the predominant respiratory infection. The following figures, depicting annual influenza trends in Chile (Figure WO-14), Australia (Figure WO-15), Hong Kong (Figure WO-16), Cambodia (Figure WO-17), Kenya (Figure WO-18), South Africa (Figure WO-19), and New Zealand (Figure WO-20), illustrate the significance of 2009-H1N1 influenza A in the Southern Hemisphere 2009 influenza season.

As of September 11, 2009, as the Southern Hemisphere influenza season waned, the following trends in ILI were apparent:

• Tropical regions: A mixed picture, with some countries showing a decline in activity; others, sustained or increased activity.

• Temperate regions, Southern Hemisphere: Australia and temperate regions of South America had passed the peak of their winter influenza epidemic; some activity remained due to RSV.

• Temperate regions, Northern Hemisphere: In Japan, influenza activity exceeded the seasonal epidemic threshold, indicating an early beginning to the annual influenza season. Flu activity was also on the increase in Sweden and several regions of the Russian Federation, but most countries in Europe and Central and Western Asia reported declining activity.

SOURCES: Cox (2009); Fukuda (2009); Shortridge (2009).