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Portrait Of A Flu Season In Decline

 

 

 

# 5542

 

 

The CDC , World Health Organization, and PHAC have all released fresh surveillance reports on the rapidly waning 2010-2011 flu season today.

 

The good news is that influenza in the Northern Hemisphere is all but over for the year.  The bad news is that there are signs that the flu is starting to gather momentum – a bit earlier than usual – in the cooler climes of the Southern Hemisphere.

 

Of course, influenza and other respiratory viruses continue to circulate during the summer months north of the equator as well, just at greatly reduced levels. 

 


We’ll look first at the WHO report, followed by excerpts from the Canadian FluWatch and US FluView reports.

 

 

Influenza update - 06 May 2011

Update number 133

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Summary

• Worldwide influenza activity is currently low.


• Influenza activity across the entire temperate Northern Hemisphere is generally back to baseline or pre-seasonal levels


• As levels of influenza activity decrease, influenza type B has become more commonly detected compared to influenza A viruses across the Northern Hemisphere temperate areas and in much of the tropics.


• Transmission in tropical areas of the world is also generally low with some transmission reported in countries of Sub-Saharan Africa with a mixture of viruses and slight predominance of influenza type B.


• In the temperate areas of the southern hemisphere the influenza season has not yet started.

 

• Nearly all influenza A viruses tested continue to be antigenically similar to those found in the current trivalent vaccine. More than 90% of the influenza type B viruses are also of the lineage found in the vaccine (Victoria lineage); however, a small number of B viruses of the Yamagata lineage are also being reported.

 

 

From Canada’s FluWatch (which lags 1 week behind the CDC reports) we get the following synopsis for week 16:

 

April 17 to 23, 2011 (Week 16)

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Overall Influenza Summary

  • In week 16, influenza activity in Canada continues to decline.
  • The overall percentage of influenza positive specimens continues to decrease.
  • Fewer regions reported localized influenza activity this week compared to the previous week although slightly more outbreaks were reported, all in the Atlantic provinces.
  • The ILI consultation rate decreased, and both adult and paediatric hospitalizations with influenza were similar to the previous week.

 

And from the CDC’s FluView Report we get the following excerpts:

 

2010-2011 Influenza Season Week 17 ending April 30, 2011

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All data are preliminary and may change as more reports are received.

Synopsis:

During week 17 (April 24-30, 2011), influenza activity in the United States continued to decrease.

  • Of the 1,901 specimens tested by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories and reported to CDC/Influenza Division, 69 (3.6%) were positive for influenza.
  • The proportion of deaths attributed to pneumonia and influenza (P&I) was below the epidemic threshold.
  • Three influenza-associated pediatric deaths were reported, bringing the season total to 100. One of these deaths was associated with a 2009 influenza A (H1N1) virus, one was associated with an influenza A virus for which the subtype was not determined, and one was associated with an influenza B virus.
  • The proportion of outpatient visits for influenza-like illness (ILI) was 1.3%, which is below the national baseline of 2.5%. All 10 regions reported ILI below region-specific baselines. All 50 states and New York City experienced minimal ILI activity, and the District of Columbia had insufficient data to calculate an ILI activity level.
  • The geographic spread of influenza in two states was reported as regional; the District of Columbia and seven states reported local influenza activity; Puerto Rico and 33 states reported sporadic influenza activity, and the U.S. Virgin Islands, Guam, and eight states reported no influenza activity.

 

After several months of riding at or above the epidemic threshold, P&I mortality across 122 U.S. Cities has finally dropped below the threshold for week 17.  

 

P&I mortality is often a `trailing indicator’ as deaths may occur after days or weeks of hospitalization, and reporting may lag by a few weeks.

 

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And three more pediatric (under age 18) flu-related fatalities were reported to the CDC, making 100 pediatric deaths since October; slightly more than we’ve seen in non-pandemic flu seasons in recent years.

 

Influenza-Associated Pediatric Mortality

 

 

Numbers, like the 100 pediatric deaths shown above, are likely undercounts. In the coming months researchers will dissect this past flu season and give us a better idea of its impact.  

 

And while the North American flu season is drawing to a close, the CDC and the vaccine manufacturers are already working on next fall’s flu vaccine.

 

For more on how vaccine strains are selected, and answers to many common flu vaccine questions, you may wish to visit:

 

Questions & Answers

Vaccine Selection for the 2011–2012 Influenza Season

Related Post:

Influenza Activity
FluView

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