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Northern Hemisphere Flu Activity Declines

 

 

 

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The latest figures from the CDC and the World Health Organization show that the remarkably mild flu season of 2011-12 continues to wind down in most regions of the northern hemisphere. 

 

The virological data also suggests that flu the strains in circulation continue their slow transition away from last year’s vaccine.

 


From the WHO’s most recent Influenza Update we get the following assessment:

 

At the beginning of the influenza season, most viruses tested were antigenically closely related to those found in the current trivalent seasonal vaccine.

 

However, by mid-season, divergence was noted in both the USA and Europe in the A(H3N2) viruses tested and significant numbers of A(H3N2) viruses tested in recent months have shown reduced cross reactivity with the vaccine viruses.

Influenza B virus detections have been both from the Victoria and Yamagata lineages with the former slightly more common in China and parts of
Europe.

 

It is this recent shift in the antigenic properties of the H3N2 virus, and the resurgence of the Yamagata B lineage, that prompted changes to the flu vaccine formula that will be delivered next fall (see WHO: Northern Hemisphere 2012-2013 Flu Vaccine Composition).

 

The WHO also takes note of a small increase in the number of oseltamivir resistant flu viruses detected late in the season.

 

Resistance to neuraminidase inhibitors has been low or undetectable throughout most of the season; however, a slight increase in levels of resistance to oseltamivir has been reported in influenza A(H1N1)pdm09 isolates in the USA. Most (11/16) of these oseltamivir resistant cases have been from the state of Texas, where influenza A(H1N1)pdm09 has been the most common virus circulating.

 

The CDC’s  FluView has additional details on these resistant cases, but first a brief summary of the U.S. flu season moving towards a close.

 

2011-2012 Influenza Season Week 18 ending May 5, 2012

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

Synopsis:

During week 18 (April 29-May 5, 2012), influenza activity declined nationally and in most regions, but remained elevated in some areas of the United States.

  • U.S. Virologic Surveillance: Of the 2,118 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, 291 (13.7%) were positive for influenza.
  • Pneumonia and Influenza (P&I) Mortality Surveillance: The proportion of deaths attributed to P&I was below the epidemic threshold.
  • Influenza-associated Pediatric Mortality: Two influenza-associated pediatric deaths were reported. One was associated with an influenza B virus and 1 was associated with an influenza A virus for which the subtype was not determined.
  • Outpatient Illness Surveillance: The proportion of outpatient visits for influenza-like illness (ILI) was 1.4%, which is below the national baseline of 2.4%. All regions reported ILI below region-specific baseline levels. Two states experienced low ILI activity; New York City and 48 states experienced minimal ILI activity, and the District of Columbia had insufficient data to calculate ILI activity.
  • Geographic Spread of Influenza: Two states reported widespread geographic activity; 8 states reported regional influenza activity; 12 states reported local activity; the District of Columbia, Puerto Rico, and 28 states reported sporadic activity, and Guam and the U.S. Virgin Islands reported no influenza activity.

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So far this season, 16 oseltamivir-resistant 2009 H1N1 viruses have been detected nationally. Three patients were using oseltamivir for 1 day or more at the time of specimen collection. Thirteen had no exposure to oseltamivir; out of those 13 patients, 2 had family members using oseltamivir. (Resistance of influenza A viruses to antiviral drugs can occur spontaneously or emerge during the course of antiviral treatment or antiviral exposure).

 

Eleven of the 16 oseltamivir-resistant viruses were collected from January to April 2012 and are from Texas, where a total of 421 2009 H1N1 specimens have been tested for oseltamivir resistance. Oseltamivir resistance remains quite low nationally and in Texas, even though the percentage of oseltamivir-resistant 2009 H1N1 viruses in Texas (2.6%) is higher than the national percentage. CDC continues to recommend the use of oseltamivir or zanamivir as early as possible for patients with confirmed or suspected influenza who have severe, complicated, or progressive illness; who require hospitalization; or who are at greater risk for influenza-related complications. Use of the adamantanes is not recommended.

 

 

Perhaps the most encouraging of all the flu graphics we’ve seen this year depicts the vast reduction in pediatric flu deaths over this past flu season.  To date, there have been 22 reported pediatric flu fatalities, well under the average.

 

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And finally, a snapshot of influenza-like activity reported nationwide show very little is occurring.

 

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Influenza activity is now expected to pick up in the southern hemisphere, where their flu season peaks normally in June or July.  

 

We’ll be watching activity in Australia, New Zealand, and South Africa with considerable interest over the next few months to see what trends may be developing.

Related Post:

FluView
CDC
Influenza Activity

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