#14,037
Late April is generally a quiet time for influenza activity anywhere in the world. While flu circulates - albeit at low levels - year-round in the tropics, the Northern Hemisphere is usually done with their winter flu season by late March, and the Southern Hemisphere doesn't generally crank up before June, or sometimes July.
Always the contrarian, influenza A continues its streak of deviating from the expected.Two summers ago we saw Hong Kong - which generally sees its biggest flu season between January and April (often with a smaller late summer `second peak') - saw an unusually severe summer flu season - which began in early May - and claimed hundreds of lives over the following 90 days (Hong Kong's Summer Flu Appears To Have Peaked).
A similar `out-of-season' flu epidemic was hitting Taiwan, Macau, and Southern China at the same time (see Taiwan's Summer H3N2 Epidemic Continues Near Peak and Macao, Hong Kong & Guangdong Province All Reporting Heavy Flu Activity).The strangeness continued during the 2017-2018 Northern Hemisphere, which saw Asia and parts of Europe dominated by Influenza B, while the United States experienced the worst H3N2 flu season - and death toll - since the 1968 pandemic (see CDC: More Than 900,000 Hospitalizations & 80,000 Deaths In Last Winter's Flu Season).
While milder, the current (2018-19) U.S. flu season (which started off as a milder H1N1, and switched to a harsher H3N2) was the longest lasting flu season in a decade. At the same time, Australia - which normally reports very little flu during their summer (Dec - May) - has seen a surprising number of flu cases in early 2019 (see Australia: An Early Surge In Flu Cases).
Yesterday the World Health Organization published their latest global influenza report - current through April 14th - which found most of the Northern Hemisphere's waning flu activity centered in the United States and Saudi Arabia, and notes the early start to Australia's flu season.
Influenza update - 340
29 April 2019 - Update number 340, based on data up to 14 April 2019
Information in this report is categorized by influenza transmission zones, which are geographical groups of countries, areas or territories with similar influenza transmission patterns. For more information on influenza transmission zones, see the link below:
Summary
- In the temperate zone of the northern hemisphere influenza activity decreased overall.
- In North America, influenza activity continued to decrease with influenza A(H3N2) the dominant virus, followed by influenza B.
- In Europe, influenza activity decreased across the continent. Both influenza A viruses co-circulated; influenza A(H3N2) was the most frequently identified subtype.
- In North Africa, influenza detections were low across reporting countries.
- In Western Asia, influenza activity appeared to decrease overall, with exception of Saudi Arabia where activity remained elevated.
- In East Asia, influenza activity was reported in some countries, with influenza B viruses most frequently detected, followed by influenza A(H3N2). A second wave of influenza activity was reported in the Republic of Korea.
- In Southern Asia, influenza activity was low overall.
- In the Caribbean, Central American countries, and the tropical countries of South America, influenza and RSV activity were low in general.
- In West and Middle Africa, influenza activity was low across reporting countries. Influenza activity continued to be reported from Eastern Africa although in decreasing trend with predominantly influenza A(H3N2) followed by B detections.
- In the temperate zones of the southern hemisphere, influenza detections increased in southern Australia and South Africa. The influenza activity in South America remained at inter-seasonal levels.
- Worldwide, seasonal influenza A viruses accounted for the majority of detections.
National Influenza Centres (NICs) and other national influenza laboratories from 124 countries, areas or territories reported data to FluNet for the time period from 01 April 2019 to 14 April 2019 (data as of 2019-04-26 03:51:00 UTC). The WHO GISRS laboratories tested more than 137187 specimens during that time period. A total of 20772 were positive for influenza viruses, of which 17422 (83.9%) were typed as influenza A and 3350 (16.1%) as influenza B.
Of the sub-typed influenza A viruses, 1917 (32.8%) were influenza A(H1N1)pdm09 and 3922 (67.2%) were influenza A(H3N2). Of the characterized B viruses, 108 (8.3%) belonged to the B-Yamagata lineage and 1196 (91.7%) to the B-Victoria lineage.
Of particular note, the percentages of Influenza A viruses characterized as either H1N1pdm vs H3N2 very nearly swapped places since the March 5th report. Today, 2/3rds are now H3N2.
How all of this plays out over the next few months in Australia, or what dominates next fall in the Northern Hemisphere, is far from clear. But it does remind us to expect flu to do the unexpected.
Often with very little warning.
