H3N2 a player in early Northern Hemisphere flu season

Oct 13, 2010 (CIDRAP News) – More signals suggest the Southern Hemisphere's flu season is winding down, while activity is low at the start of the Northern Hemisphere's flu season, except in China, which is seeing moderate H3N2 circulation.

Some parts of the world are still reporting some 2009 H1N1 transmission, according to the most recent World Health Organization (WHO) update. Many, though, are also finding influenza A (H3N2), with most of it the Perth-like strain included in this year's seasonal flu vaccines.

US officials have said they expect to see more H3N2 circulation this year and have noted that flu seasons can be worse when H3N2 is the dominant strain.

Australia, which recently experienced a late-season surge of 2009 H1N1 influenza, is reporting decreasing activity according to several markers last week, including hospital and intensive care unit admissions, the WHO said in an Oct 8 report posted online yesterday. Virus type varied by country, with the 2009 H1N1 virus detected most often in Australia and New Zealand and influenza B predominating in South Africa.

Chile, like Australia, also reported a late-season rise in flu activity, which is now declining, the WHO said. In some regions of Chile, flu activity exceeded last year's pandemic levels, though the national level this year was lower overall. H2N2 was the strain detected most frequently, with low levels of 2009 H1N1 and influenza B.

The WHO said flu activity in tropical countries has varied widely, and although flu is decreasing in most countries, it is increasing in Southeast Asian countries such as Cambodia and Thailand.

The type of flu circulating in the tropical countries has also shown a lot of variation, even among neighboring countries, with cocirculation common in many locations. For example, in Cambodia, H3N2 is the most frequently detected virus, but in neighboring Thailand, most outbreaks involve 2009 H1N1. In Costa Rica and Honduras, H3N2 has been the most common virus detected, but influenza B has been dominant in Nicaragua.

China is reporting moderate H3N2 circulation, with tests showing much lower levels of influenza B activity, the WHO said. Northern China is experiencing an increase in the number of outpatient visits for flu-like illness, and Hong Kong officials said flu activity is decreasing some but still remains high, with most of it the H3N2 virus.

Flu activity is starting to decline in India, where the 2009 H1N1 virus was dominant, and in Bangladesh, where H3N2 was more common. India's health ministry, in an Oct 11 update for the week ending Oct 10, showed that cases declined for the fourth week in a row.

Saudi Arabia recently reported three deaths from the 2009 H1N1 virus, and a health ministry spokesman said the overall level of flu, including the H1N1 strain, is normal for this time of year, Agence France-Presse (AFP) reported yesterday. Pilgrims from Muslim countries are arriving in Saudi Arabia in advance of early November hajj observances in Mecca and Medina. Many countries have advised hajj travelers to receive the seasonal flu vaccine in advance of their pilgrimages.

Mexico reported a spike in flu-like illnesses in August, especially the southern part of the country, though reports leveled off in September. Most flu detections in the country have been H3N2, with a subset characterized as the Perth-like strain, which is included in flu vaccines for the Southern and Northern Hemisphere flu vaccines.

Elsewhere in North America, Canada and the United States reported low levels of flu activity, with US officials noting sporadic detections of H3N2 and influenza B.

See also:

Oct 8 WHO influenza update

Oct 11 Indian health ministry flu report

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