Friday, January 17, 2014

FluView Week 2: The Shifting Demographics Of Seasonal Flu


FluView Week 2



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Although seasonal flu comes around each year, every flu season has a distinct `personality’ as the various flu strains struggle for dominance, and accumulated antigenic drift aids in their ability to evade community immunity. 


Some flu seasons are relatively mild (like 2011-2012), while others are more severe (like 2012-2013).  And the preferential target of these viruses shift from year-to-year as well, something we saw during 2009 H1N1 pandemic, which had its greatest affect on those under the age of 65.


This year – for the first time in 3 years – we are seeing the pH1N1 virus (introduced in the 2009 pandemic, but now a seasonal strain) the major player in North America, and as that strain tends to hit younger people harder.  As a result we’ve seen a good deal of media coverage of young adults hospitalized and some even dying from the flu. 


Credit FluView Week 2

The above chart shows how much less this year’s flu is affecting those over 65 (green) than it did last year, with those aged between 18 and 64 representing the bulk of this year’s hospitalizations.


Despite this distressing age shift towards younger patients, the absolute numbers for week 2 of this year are less severe than they were this time last year.


First, a brief summary of week 2’s activities from the CDC, then I’ll have some side-by-side comparisons of this week vs 1 years ago.


2013-2014 Influenza Season Week 2 ending January 11, 2014

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


During week 2 (January 5-11, 2014), influenza activity remained high in the United States.

  • Viral Surveillance: Of 10,841 specimens tested and reported during week 2 by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories, 2,721 (25.1%) were positive for influenza.
  • Pneumonia and Influenza Mortality: The proportion of deaths attributed to pneumonia and influenza (P&I) was above the epidemic threshold.
  • Influenza-Associated Pediatric Deaths: Ten influenza-associated pediatric deaths were reported.
  • Influenza-associated Hospitalizations: A cumulative rate for the season of 13.8 laboratory-confirmed influenza-associated hospitalizations per 100,000 population was reported.
  • Outpatient Illness Surveillance: The proportion of outpatient visits for influenza-like illness (ILI) was 3.6%, above the national baseline of 2.0%. All 10 regions reported ILI above region-specific baseline levels. Fourteen states experienced high ILI activity; 12 states experienced moderate ILI activity; eight states and New York City experienced low ILI activity; 16 states experienced minimal ILI activity, and the District of Columbia had insufficient data.
  • Geographic Spread of Influenza: The geographic spread of influenza in 40 states was reported as widespread; nine states and Guam reported regional influenza activity; the District of Columbia and Puerto Rico reported local influenza activity; one state reported sporadic influenza activity, and the U.S. Virgin Islands did not report.

With the caveat that different flu seasons peak at different times, and that there may be a good deal more flu ahead before the end of spring, I’ve compared this year’s flu season through week two with last year’s in the charts below.



The P&I Mortality rate has – for the first time this flu season – exceeded the epidemic threshold.  It is, however, considerably lower than it was this time last year (and last year reached a 10 year peak in late January).




Pediatric deaths are often slow in being reported, and right now, the 2014 season is lagging only slightly behind last year’s number for week 2.  In an average year we generally see around 75-125 pediatric deaths reported.


The most dramatic side-by-side comparisons comes from hospitalizations from influenza by age group. 


While those under the age of 65 are seeing roughly the same sort of numbers we saw in 2013, those over the age of 65 are dramatically lower than from last year’s flu season.


The take-away from all of this is that influenza is not a trivial illness, and even during an average year, it can claim thousands of lives.  Some years – like this year – we see younger people more affected.  We won’t know how this year’s flu season will stack up against previous years until the season is over, and the number are tallied.  


Regardless of the strain of flu in circulation, you are much better off avoiding infection rather than treating it. So while it may only provide moderate protection, getting the flu shot each year is cheap insurance. 


That, and following good flu hygiene practices (covering coughs, washing hands frequently, staying home when sick, avoiding close contact with those who are sick),  are your best defense against our yearly flu epidemic.