Thursday, October 07, 2010

MMWR: US & Global Influenza Activity June – Sept 2010



# 4967



The CDC has released their weekly MMWR and it contains a faint surprise.


Although influenza activity in the US has been low, in a report called  . . .


Influenza Activity --- United States and Worldwide, June 13--September 25, 2010



. . . we learn that the majority of influenza A viruses sub-typed in the the United States over the summer were H3N2  (70%) and not the novel H1N1 virus (30%).



I call this a `faint surprise’ because we’ve had some indicators over the summer that the H3N2 virus might be staging a comeback.



In early August we saw a CDC Health Advisory On H3N2, and repeatedly over the past several months reports from the World Health Organization on global flu activity have mentioned this re-emerging seasonal strain.


Global Flu Trends – Week 36
Concerns Over H3N2 In Vietnam
Global Flu Activity Update


You may recall that at about the same time that the pandemic H1N1 virus began its rapid spread around the world, reports of an antigenically drifted version of seasonal H3N2 appeared as well (see Who Knew? New Flu).


This new strain, dubbed A/Perth/16/2009-like H3N2 virus, failed to compete successfully with the emerging pandemic strain last summer and fall, and only in recent months has begun to show up again.



Analysis of this new H3 virus  suggests that exposure to earlier H3N2 viruses – or even taking last year’s seasonal flu vaccine - is unlikely to convey much (if any) immunity to this new strain.


And H3N2, unlike the recent H1N1 virus, is unlikely to spare the elderly. Historically, years where H3 viruses dominate tend to be more severe flu seasons, particularly for the very old and very young.


The good news is that this year’s seasonal flu vaccine contains antigens against this new strain.


Of course, influenza is notoriously unpredictable, so we won’t know how this flu season will play out until it is over.


A few excerpts from this week’s report (reformatted for readability).  But follow this link to read it in its entirety.




From June 13 to September 25, influenza A (H3), 2009 influenza A (H1N1), and influenza B were identified worldwide. Seasonal influenza A (H1) viruses were reported only rarely.


Reports by the WHO Global Influenza Surveillance Network (2) showed that influenza B was the viral type most commonly identified until early July, when 2009 influenza A (H1N1) became the predominant strain.


Since late August, influenza A (H3) has been the viral subtype most commonly identified.


The WHO Global Surveillance Network reported that the predominant viral type or subtype identified from Asia (44% of analyzed specimens) was H3, followed by 2009 influenza A (H1N1) (32% of specimens) and influenza B (13%), but this varied by country.


In Africa, influenza B (44%) viruses were the most common, followed by influenza A (H3) (31%).


In South America, 2009 influenza A (H1N1) (32% of specimens) and influenza B (32%) predominated.


In North America, 69% of isolates were identified as influenza A (H3) viruses. The 2009 influenza A (H1N1) virus predominated in Oceania (63% of specimens). In Europe a small number of cases were reported, and most specimens (59%) were influenza B, followed by 2009 influenza A (H1N1) (19%).