Flu activity widespread in parts of Southern Hemisphere

Aug 6, 2012 (CIDRAP News) – Influenza activity is widespread across most of Australia and is running above baseline levels in New Zealand, though activity patterns and circulating virus strains vary among Southern Hemisphere countries, the World Health Organization (WHO) said in its latest global flu update.

Compared with the past two seasons, Australian officials are reporting an earlier increase in the percentage of doctor's visits for flu like illness, with rates that are higher than recent seasonal peaks, according to the WHO's Aug 3 update.

Influenza A (H3N2) is the predominant strain in Australia, with most of the influenza B activity occurring in the Northern Territory, Queensland, and Western Australia. Virus subtyping by the WHO's collaborating center in Australia has found that nearly all of the H3N2 viruses are a more recent strain that's not included in the Southern Hemisphere's seasonal flu vaccine.

In October 2011, a WHO advisory group recommended using the previous season's same three strains in the Southern Hemisphere's 2012 seasonal flu vaccine, including Perth/16/2009 for the H3N2. However, the advisors' review of virologic surveillance in the following months led them in February to recommend a different H3N2 strain, Victoria/361/2011, for the 2012-13 Northern Hemisphere flu season.

Despite the H3N2 strain mismatch, the WHO said the vaccine will still offer significant protection.

In New Zealand, unlike Australia, the circulating H3N2 is the Perth-like strain included in the vaccine, according to the report.

Medical visits for flu-like illness in News Zealand are above baseline for the third straight week, with H3N2 responsible for about two-thirds of positive samples.

Meanwhile, flu activity varied in other Southern Hemisphere locations, the WHO said. Chile reported a decrease in flu activity for the first time since early July. Paraguay reported an increase in flu activity, while Argentina continued to report few flu detections. In South Africa, flu activity has been declining since the middle of June.

Brazil, which has reported increasing flu activity for the past several weeks, is reporting fewer severe infections since the middle of July, along with fewer overall detections, which may suggest that the season has peaked, according to the report. Ecuador reported a steady rise in the number of influenza B detections, while Bolivia's flu markers showed a decreasing trend.

Some of Asia's tropical areas, such as southern China and Singapore, are reporting recent rises in flu activity. That part of China has seen a persistent rise in the number doctor's visits for flulike illness over the past 4 weeks, as well as an increase in the percentage of respiratory samples testing positive for flu over the past 6 to 8 weeks. Of the 102 H3N2 viruses that the National Influenza Center of China characterized in the third week of July, all were related to the Perth-like strain.

All reporting countries in the Northern Hemisphere's temperate regions indicate flu at interseasonal levels. The WHO made note of new variant H3N2 (H3N2v) cases reported from Indiana, which were detailed in a recent issue of Morbidity and Morality Weekly Report (MMWR).

Three US states have reported a spate of new H3N2v cases over the past several days, mostly in people who visited or were involved in county fairs, and the US Centers for Disease Control and Prevention (CDC) said on Aug 3 that the virus may be circulating widely in US pigs. CDC experts added that the infections are clinically similar to seasonal flu.

Some tropical countries in the Americas reported active flu transmission, including El Salvador, which has reported a steady increase in 2009 H1N1 detections over the past 10 weeks. In the Caribbean, Cuba and Jamaica continued to report influenza B infections, which have been increasing for the past 6 weeks, the WHO report said.

See also:

Aug 3 WHO global flu update

Aug 3 CIDRAP News story "CDC watching variant H3N2 as cases climb"

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