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How Severe Is an Influenza Season?

Amesh A. Adalja, MD, FACP, FACEP, FIDSA, January 26, 2018

In a recent press briefing, Centers for Disease Control and Prevention (CDC)  influenza division chief Dan Jernigan characterized the current 2017-18 influenza season as likely to be “moderately severe” and, thus far, not worse than that of 2003-04.1 This assessment is based on an important paper he and others at the CDC published in the American Journal of Epidemiology.2 


How to Assess Influenza Severity

In this paper, Biggerstaff and colleagues adapted a methodology known as the Moving Epidemic Method to develop intensity thresholds, which are used by the National Flood Insurance Program to assess flood risk, for key indicators of influenza severity. The indicators used were the percentage of visits to ILINet providers for influenzalike illness, rates of influenza-associated hospitalizations (FluSurv-Net), and the percentage of deaths from influenza or pneumonia (122 Cities Mortality Reporting System and the National Center for Health Statistics mortality surveillance data). 

The authors analyzed data from the 11 influenza seasons spanning 2003 through 2015, excluding the 2009 pandemic year (because of how aberrant a pandemic season is compared to an ordinary season). Intensity thresholds were calculated at the 50%, 90%, and 98% levels for each indicator, and if 2 of the 3 indicators crossed a given threshold, the season was classified as being of low, moderate, high, or very high severity. Additionally, seasons were stratified according to age-related severity for children, adults, and older adults.


2 High-Severity Seasons; Age Differences

Using these tools, 2 seasons were classified as being of high severity, none of very high severity (including the 2009 pandemic), and 7 as of moderate severity. The 2 high-severity seasons were 2003-04 and 2014-15 – both dominated by the H3N2 strain of influenza A. 

Age-specific severity was noted to be high for the 2003-04, 2012-13, and 2014-15 seasons for older adults, while children had 2 seasons classified as very high severity: 2003-04 and the 2009 pandemic. Correlation among the various indicators was noted to be good to excellent.

This paper serves as an important tool when communicating about and assessing the severity of an individual influenza season, allowing proper historical context to be incorporated in the midst of a flu season. The notable differences in how age affects the severity of a given season likely reflects the immunological history of a given age cohort’s experience with influenza – a phenomenon known as original antigenic sin – in which viral strains similar to those experienced in early life pose little problem later in life (as evidenced by the fact that the 2009 pandemic strain was of high severity only for children and not older adults).3 The fact that high-severity seasons are dominated by H3N2 is also a noteworthy finding that confirms the general consensus regarding the pathogenicity of this strain of influenza – which may be an intrinsic characteristic of the virus or a problem with the efficacy of egg-based vaccines targeting this strain4 or a combination of both. 

At the conclusion of the 2017-18 influenza season, assessing its severity using these tools will provide a rigorous picture of how it compares to past seasons and will compare to future seasons.



  1. Centers for Disease Control and Prevention. Transcript for CDC update on widespread flu activity. January 12, 2018. Accessed January 23, 2018.
  2. Biggerstaff M, Kniss K, Jernigan DB, et al. Systematic assessment of multiple routine and near-real time indicators to classify the severity of influenza seasons and pandemics in the United States, 2003-04 through 2015-2016. Am J Epidemiol 2017. Accessed January 23, 2018.
  3. Adalja AA, Henderson DA. Original antigenic sin and pandemic (H1N1) 2009. Emerg Infect Dis 2010;16(6):1028-1029.
  4. Zost SJ, Parkhouse K, Gumina ME, et al. Contemporary H3N2 influenza viruses have a glycosylation site that alters binding of antibodies elicited by egg-adapted vaccine strains. Proc Natl Acad Sci U S A 2017;114(47):12578-12583.