Timing Classification of United States Influenza Epidemics from 1999-2014 and the Association of Epidemic Timing Classification with Influenza-Associated Mortality Open Access

Abstract

Epidemics of influenza occur annually in the United States, causing 3,000- 49,000 deaths, and costing an estimated $16 billion each year. The seasonality of Northern Hemisphere influenza epidemics is well established, but the classification of influenza season timing is not defined. This study attempted to describe three influenza epidemic timing metrics: epidemic start, epidemic peak, and epidemic start to peak interval, for the classification of season timing using United States virologic surveillance data from 1999-2014 (n=13). Epidemic start and epidemic peak were identified and seasons were classified as early, average, or late. Epidemic start to peak interval was identified and seasons were classified as short, average, or long. Using the developed timing metrics, a cross-sectional study of virologic and mortality surveillance data assessed the association of influenza season timing with pneumonia & influenza (P&I) associated mortality through Poisson regression adjusted for predominant influenza A subtype. Among the three timing metrics, epidemic start to peak interval had the strongest association with P&I associated mortality. Short epidemic start to peak interval seasons were associated with a 10% increase in mortality rate compared to average epidemic start to peak interval seasons (RRShortInterval=1.10 95% CI=1.10, 1.11). Long epidemic start to peak interval seasons were associated with a 10% decrease in mortality rate compared average epidemic start to peak interval seasons (RRLongInterval=0.90 95% CI=0.90, 0.91). Given the P&I mortality rate during an average epidemic start to peak interval season of 20.1 deaths per 100,000, and the current United States population of 318.9 million, a short epidemic start to peak interval season could indicate 6,400 more deaths. The findings suggest that classification of epidemic start to peak interval is associated with influenza-associated mortality, but as classification requires the influenza season to reach peak incidence, does not allow for early intervention. Further research is needed to identify the mechanism underlying the association of epidemic start to peak interval and P&I associated mortality to provide a method to predict influenza season mortality. A method of early prediction could allow public health and clinical stakeholders to target prevention and intervention efforts.

Table of Contents

BACKGROUND------------------------------------------------------------------------------------- 1

LITERATURE REVIEW------------------------------------------------------------------------------4

Biology of Influenza Virus--------------------------------------------------------------------------4

Clinical Manifestation of Influenza Virus-----------------------------------------------------------5

Epidemiology of Influenza Virus--------------------------------------------------------------------7

Seasonality of Influenza Epidemics in the United States--------------------------------------------8

Domestic Influenza Surveillance--------------------------------------------------------------------9

MATERIALS AND METHODS-----------------------------------------------------------------------12

Dataset---------------------------------------------------------------------------------------------12

Epidemic Timing Classification---------------------------------------------------------------------14

Influenza A Subtype Variable Description---------------------------------------------------------- 15

Analysis Plan---------------------------------------------------------------------------------------16

RESULTS--------------------------------------------------------------------------------------------17

DISCUSSION----------------------------------------------------------------------------------------21

STRENGTHS AND LIMITATIONS--------------------------------------------------------------------25

FUTURE DIRECTION--------------------------------------------------------------------------------27

REFERENCES----------------------------------------------------------------------------------------28

TABLES----------------------------------------------------------------------------------------------31

About this Master's Thesis

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School	Rollins School of Public Health
Department	Epidemiology
Degree	MPH
Submission	Master's Thesis
Language	English
Research Field	Health Sciences, Epidemiology
Keyword	Epidemic Influenza Mortality
Committee Chair / Thesis Advisor	McGowan Jr., John E, Emory University
Committee Members	Brammer, Lynnette, Centers for Disease Control and Prevention
Partnering Agencies	CDC

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