WHO says Southern Hemisphere flu activity remains fairly low
Influenza activity remains elevated in South America, has started to decline in southern Africa, and is still below seasonal threshold levels in Australia and New Zealand, the World Health Organization (WHO) said in its latest update.
Samples testing positive for influenza in Chile and Paraguay have increased, the WHO said, with the H3N2 strain dominating in Chile. Flu has declined in Brazil, however, and remains low in Argentina. Flu activity has also declined in Colombia and Peru but remains elevated. 2009 H1N1 is the predominant strain in southern Africa as activity drops there.
Flu activity remains low in Australia and New Zealand, but influenza-like illness in West Australia is rising and appears to have crossed the seasonal threshold. H1N1 is the predominant strain in Oceania, which includes Australia and New Zealand.
Northern Hemisphere flu activity remains at inter-seasonal levels, the WHO said.
Analysis of FluNet isolates from the most recent week of data shows that 88% of viruses are influenza A. Of flu A viruses subtyped, 76% were H1N1.
Aug 6 WHO update
Study shows mismatch between flu vaccine, circulating strains common
In an illustration of how commonly mismatch occurs between flu vaccine strains and circulating flu viruses, Hong Kong investigators used 17 years' of data to determine that a close match occurs only 11% to 21% of the time, depending on the strain and vaccine.
The researchers analyzed data from 1996 through 2012 in Hong Kong, focusing on H3N2 (one of the two major "A" strains) and influenza B, because changes in vaccine composition were more frequent for those viruses. They sequenced an average of 30 clinical samples of influenza H3N2 and 30 of influenza B per year, except for 2009, when influenza H1N1 predominated. They divided the 17-year period into 34 flu seasons—winter and summer for each year.
The researchers found that Northern Hemisphere vaccine strains were closely matched with circulating strains for 7 (20.6%) of 34 seasons for H3N2 and 5 (14.7%) of 34 seasons for influenza B. For the Southern Hemisphere vaccine, the rates were 14.7% for H3N2 and 11.1% for influenza B.
Strain drift among seasons was 41.2% for H3N2 and 35.3% for influenza B. In addition, biannual vaccination strategy—administering Northern Hemisphere vaccines in November followed by Southern Hemisphere vaccines in May—did not improve match rates.
The authors conclude, "A specific strategy is urgently needed to select and produce influenza vaccines targeting the tropical and subtropical regions."
Aug 7 Emerg Infect Dis study