Doctors' visits and fatalities: How this year compares
CDC uses the Influenza-Like Illness (ILI) activity indicator—which is based on the percent of outpatient visits in a state that are due to ILI—to assess the spread and intensity of influenza activity in the United States.
The agency's data show that 3.4% of patient visits nationwide were due to ILI in week three (W3) of this flu season (i.e. the week ending Jan. 18, 2014). By comparison, 4.3% of patient visits were due to ILI in W3 2013. This might suggest that the current flu season is not so bad, but it's important to remember that last year's flu season was quite severe, sending waves of patients to hospitals.
The 2012-2013 flu season: Inside a flu-stricken ED
Looking at a broader set of data paints a different picture—one that suggests that we are dealing with a more-active-than-usual flu season. From 2004 to 2014, the percentage of outpatient visits due to ILI in W3 ranged from 1.4% to 4.3% and averaged about 2.6%.
But outpatient visits do not tell the whole story: Mortality also is an important way of assessing the severity of a flu season. Although, CDC does not track comprehensive, national data on adult flu mortalities, it does monitor adult deaths caused by both pneumonia and influenza (P&I) in 122 U.S. cities participating in the agency's Cities Mortality Reporting System.
In W3 2014, 8.1% of U.S. deaths were linked to pneumonia and influenza (P&I). That's slightly more than the percentage for W3 2011 and 2012, but it's significantly lower than in 2013, when 9.8% of deaths were linked to P&I. And looking at the data point for all the flu seasons between 2004 and 2014, it's only slightly above average for this time of year.
A unique flu season
This year's flu season may not be more severe than last year's in numbers, but it is unique.
The age distribution of flu cases is quite different this year because H1N1 (yes, that H1N1) is the primary strain of influenza, accounting for more than 60% of cases. An unusually high percentage of flu cases are in adults that are usually considered the least vulnerable to the flu. In W3 2014, 29% of ILI outpatient visits were for patients ages 25 to 49, compared to 24% in W3 2013, 20% in W3 2012, and 19% in W3 2011.
It's also important to note that the severity of the 2013-2014 flu season appears to vary significantly based on region. In some areas, this flu season appears to be worse than last year's.
In Dallas, for example, 28 P&I deaths were reported in W3, and 59 so far this year. That's actually up from the 55 P&I deaths reported in the first three weeks of 2013, and it's up significantly from the 37 P&I deaths in the first three weeks of 2012. The deadly outbreak in North Texas appears to have driven many to get late flu shots, and some pharmacies are now reporting shortages.
Similarly, California hospitals are struggling to manage severe flu outbreaks that have claimed the lives of as many as 146 adults under age 65, according to state epidemiologists. By comparison, California had reported only nine flu-related deaths in that age group at this time last year.
The takeaway: This year's flu season appears to be more active than usual, and its unique makeup means that many people who are traditionally not considered vulnerable to influenza should beware its spread.