After getting a late start, our Northern Hemisphere's flu season is showing signs of winding down. While our flu season was pretty mild in North America, in Russia and parts of Eastern Europe, we saw reports of very heavy flu activity.
In the tropics influenza transmits year-round, albeit at lower levels than seen at higher latitudes (see Influenza Burden and Transmission in the Tropics).
The expected start of the Southern hemisphere's flu season is still a month or two away, but as I mentioned a couple of weeks ago in Unseasonable Seasonal Flu In São Paulo), Brazil continues to report unusual flu activity for what is technically their summer.
Something today's WHO global flu surveillance report mentions: In Brazil, influenza activity was above expected levels for this time of year with influenza A(H1N1)pdm09 virus predominating.
While it is difficult to gauge the impact of off-season H1N1 flu in Brazil, it has become a significant subject of media reports, with headlines like H1N1: overuse of Tamiflu reduces the drug stocks in Sao Paulo and H1N1 cases in Brazil rises to 1,012, with 153 deaths in the year getting a lot of attention the past 24 hours.
We'll have to wait to see if H1N1 becomes a public health issue, along with the Zika virus, during the upcoming Winter Olympics in Rio.
As for the rest of the world, the WHO provides the following summary.
18 April 2016, - Update number 261, based on data up to 03 April, 2016
In the Northern Hemisphere influenza activity was decreasing, while still elevated in some areas, due in part to an increase of influenza B activity. In the Southern Hemisphere influenza activity was reported to be slightly increasing.
- In North America, decreasing but sustained influenza activity was reported with influenza A(H1N1)pdm09 virus predominating.
- In Europe in general a decreasing trend of influenza activity was observed. In Northern Europe, overall influenza activity decreased but remained at moderate levels. A shift towards circulation of influenza B virus was detected in parts of Europe.
- Northern Temperate Asia continued to report ongoing and elevated levels of influenza activity with increasing proportions of influenza B virus.
- In Central America and the Caribbean, low influenza activity was reported in most countries except in Jamaica where elevated severe acute respiratory infection (SARI) activity associated with influenza A(H1N1)pdm09 virus infection was reported.
- In tropical South America, low but increasing circulation of influenza A(H1N1)pdm09 virus was reported. In Brazil, influenza activity was above expected levels for this time of year with influenza A(H1N1)pdm09 virus predominating. Colombia reported high circulation of respiratory syncytial virus (RSV).
- In Temperate South America, influenza activity slightly increased but remained at low level. An increase in influenza-like illness (ILI) and SARI rates were reported in Argentina, Chile and Paraguay.
- In Oceania and South Africa influenza virus activity remained low.
- National Influenza Centres (NICs) and other national influenza laboratories from 92 countries, areas or territories reported data to FluNet for the time period from 21 March 2016 to 03 April 2016 (data as of 2016-04-15 03:54:50 UTC).The WHO GISRS laboratories tested more than 101187 specimens during that time period. 24302 were positive for influenza viruses, of which 13251 (54.5%) were typed as influenza A and 11051 (45.5%) as influenza B. Of the sub-typed influenza A viruses, 4895 (85.8%) were influenza A(H1N1)pdm09 and 811 (14.2%) were influenza A(H3N2). Of the characterized B viruses, 473 (19.6%) belonged to the B-Yamagata lineage and 1936 (80.4%) to the B-Victoria lineage.
Detailed influenza update