Who Leads Influenza Epidemics? Age-specific differences in the occurrence of seasonal and pandemic influenza A in Thailand (2003-2011) Pubblico

Abstract

Background: Identifying the age group responsible for driving the spread of influenza epidemics could help inform mitigation strategies to combat seasonal and pandemic influenza. This study evaluated the hypothesis that older, rather than younger, children led seasonal influenza A epidemics in Thailand.

Methods: Using data from a surveillance system for acute lower respiratory tract illness in hospitalized patients and influenza-like illness in outpatients in Thailand, we describe influenza A seasonality in Sa Kaeo and Nakhon Phanom provinces. An epidemic was defined as any time period with more than 3 (Sa Kaeo) or 5 (Nakhon Phanom) cases for two out of 3 consecutive weeks. Survival analyses and log rank statistics were applied to test the hypothesis that 10-19 year olds lead influenza epidemics in Thailand when compared to all other age groups.

Results: Between 2003 and 2011, we identified 22 epidemics: 4 in outpatient data in Sa Kaeo, 8 in inpatient data in Sa Kaeo and 10 in inpatient data in Nakhon Phanom. Overall, log-rank differences between age groups were small during seasonal, pre-pandemic influenza. In outpatients, rank orderings indicated that 10-19 year olds led influenza A epidemics during pre-pandemic years (2003-2005), although this finding was not statistically significant (p=0.07). Similar findings were observed in hospitalized inpatients during pre-pandemic years (2005-2008): 10-19 year olds led epidemics in Sa Kaeo (p=0.22) and Nakhon Phanom (p=0.07). When the H1N1 pandemic strain was circulating (2009-2011), ages shifted to older individuals and orderings suggest that 30-64 year olds in Sa Kaeo (p=0.05) and 20-29 year olds in Nakhon Phanom (p<0.0001) led influenza epidemics. For pandemic years, there was a greater difference in the age groups that led influenza epidemics relative to young school-aged children.

Conclusions: In Thailand, there was little difference in the age group that led seasonal influenza A epidemics. In age-shifted pandemic years, older adults led the epidemics, suggesting that school closings may be less effective at limiting influenza transmission during outbreaks caused by a novel influenza virus. Given the small differences between age groups, vaccination remains the most important prevention approach in efforts to limit influenza A transmission at the community level.

Table of Contents

I. LITERATURE REVIEW (p1)

II. MANUSCRIPT (p15)
LIST OF TABLES (p31)
Table I: Frequency of influenza A laboratory test results and distribution of age groups among influenza A positive outpatients and hospitalized inpatients seen for pneumonia-related illness in Sa Kaeo and Nakhon Phanom, Thailand (2003-2011) (p31)
Table II: Wilcoxon test results for equality across epidemics, of age-specific timing of influenza A incidence for outpatients and hospitalized inpatients (p32)
Table III-A: Log rank survival analysis results assessing the combined age-specific timing to incidence of influenza A for outpatients and hospitalized inpatients in Sa Kaeo and Nakhon Phanom, Thailand (2003-2011) (p33)
Table III-B: Province specific log rank survival analysis results assessing the combined age-specific timing to incidence of influenza A for hospitalized inpatients in Sa Kaeo and Nakhon Phanom, Thailand (2005-2011) (p34)
LIST OF FIGURES (p35)
Figure I: Locations of Sa Kaeo and Nakhon Phanom provinces in Thailand (p35)
Figure II: Defining influenza A epidemics in Thailand (p36)
Figure III: Seasonality of influenza A positive outpatients in Sa Kaeo Province, Thailand (2003-2005) (p37)
Figure IV: Seasonality of influenza A positive hospitalized inpatients in Sa Kaeo and Nakhon Phanom provinces, Thailand (2005-2011) (p38)

III. PUBLIC HEALTH IMPLICATIONS (p43)

IV. APPENDICES (p47)
APPENDIX A: Letter of invitation and Emory University Internal Review board Exemption (p48)
APPENDIX B: List of influenza A epidemics in Sa Kaeo and Nakhon Phanom, Thailand (2003-2011) (p49)
APPENDIX C: Tables depicting Cox proportional hazard model results (p50)

About this Master's Thesis

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School	Rollins School of Public Health
Department	Epidemiology
Subfield / Discipline	Global Epidemiology - MPH & MSPH
Degree	MPH
Submission	Master's Thesis
Language	English
Research Field	Health Sciences, Public Health
Parola chiave	Thailand influenza
Committee Chair / Thesis Advisor	Klugman, Keith P, Emory University
Committee Members	Olsen, Sonja J, Thailand Ministry of Public Health - CDC Collaboration Center
Partnering Agencies	Emory University schools, faculty or affiliated programs CDC

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