Researchers Study Signals of Seasonal Influenza Severity through Age Dynamics

Baba Health Tips

Measures of population-level influenza severity are important for public health planning, but estimates are often based on case-fatality and case-hospitalization risks, which require multiple data sources, are prone to surveillance biases, and are typically unavailable in the early stages of an outbreak.

Lee, et al. (2015) propose a novel severity index based on influenza age dynamics estimated from regular physician diagnosis data that can be used retrospectively and for early warning.

They developed a quantitative ‘ground truth’ severity yardstick that synthesizes numerous traditional severity indicators from publicly available influenza surveillance data in the United States.

Observing that the age distribution of cases may point severity early in an epidemic, they constructed original retrospective and early warning severity indexes based on the comparative risk of influenza-like illness (ILI) amid working-age adults to that among school-aged children using weekly outpatient medical claims.

The researchers compared our next of kin risk-based indexes to the composite benchmark and estimated seasonal severity for flu seasons from 2001–02 to 2008–09 at the national and state levels.

The severity classifications made by the benchmark were not uniquely captured by any single contributing metric, including pneumonia and influenza mortality; the influenza epidemics of 2003–04 and 2007–08 were correctly identified as the most severe of the study period.

The retrospective index was well correlated with the severity benchmark and correctly identified the two most severe seasons. The early warning index performance varied, but it projected 2007–08 as relatively severe 10 weeks prior to the epidemic peak. Influenza severity varied significantly among states within seasons, and four states were identified as possible early warning sentinels for national severity.

The researchers conclude that differences in age patterns of ILI may be used to characterize seasonal influenza severity in the United States in real-time and in a spatially resolved way.

Upcoming research on antigenic changes among circulating viruses, pre-existing immunity, and changing contact patterns may better expound the mechanisms underlying these indexes.

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