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State of the flu: How this year's flu is unique

February 4, 2014

    Juliette Mullin, Editor

    Influenza remained at epidemic levels in week four (W4) of 2014, when about 8.8% of all reported deaths in the United States were due to pneumonia and influenza (P&I).

    At the same time, fewer states reported "high" levels of influenza-like illness (ILI) activity compared to the week before, according to the latest CDC data, which covers the week ending on Jan. 25. Moreover, the percentage of outpatient visits for ILI remained about steady, dropping slightly from 3.4% in W3 to 3.3% in W4.

    Nonetheless, some states are seeing increases in flu activity levels (which are determined based on outpatient visits). The maps below offer a look at the flu activity level in each state in W4, and a comparison with W3.

    As I noted last week (and as you can see in the map above), the severity of this year's flu depends heavily on where you live. Although CDC data show that the overall season is less deadly that last year's season, some areas are reporting more deaths this year.

    How bad is the 2013-2014 flu season?

    For example, public health officials in California say the flu season has been unusually severe, claiming the lives of at least 147 young and middle-aged state residents, more than 10 times the number of flu deaths reported at this time last year.

    H1N1 targets unusual flu patients

    Adults between ages 25 and 49 are usually considered the least vulnerable to the flu. This season, influenza is driving an unusually high number of those Americans to doctors' offices. Twenty-eight percent of flu patients were between ages 25 and 49 in W4. That's compared to 23% in W4 2013, 18% in W4 2012, and 19% in W4 2011.

    The unusual age breakdown of this year's flu has been widely attributed to the predominance of 2009 H1N1 strain this season. According to CDC's Michael Young, H1N1 tends to strike younger adults harder than older ones.

    According to CDC viral surveillance, about 56% of positive flu specimens tested in W4 were the 2009 H1N1 strain. By comparison, the strain account for only 20% of flu specimens in W4 2013, 14% of flu specimens in W4 2012, and 21% of flu specimens in W4 2011.

    But then why aren't we bracing for the start of a deadly pandemic, like the one we saw when H1N1 spread across the globe in 2009?

    For one, vaccine manufacturers included 2009 H1N1 in this year's flu vaccine, experts say. Moreover, the H1N1 strain was new in 2009, surfacing for the first time ever in the United States. For a flu strain of this type to become a pandemic, it needs to "find a group of people that has never seen anything like it before. We don't have that population anymore," Young said in a HealthDay interview in December.

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