Incidence of influenza virus infections in children in Hong Kong in a 3-year randomized placebo-controlled vaccine study, 2009-2012.
BACKGROUND
School-aged children suffer high rates of influenza virus infections and associated illnesses each year, and are a major source of transmission in the community. However, information on the cumulative incidence of infection in specific epidemics is scarce, and there are limited studies with sufficient follow-up to identify the strength and duration of protection against reinfection.


METHODS
We randomly allocated children 5-17 years of age to receive trivalent inactivated influenza vaccine (TIV) or placebo from September 2009 through January 2010, and then conducted follow-up for 3 years including regular collection of sera, symptom diaries, and collection of nose and throat swabs during illness episodes in participants or their household members.


RESULTS
Of 796 children initially randomized, 484 continued to participate for all 3 years. In unvaccinated children, cumulative incidence of infection was estimated to be 59% in the first wave of H1N1pdm09 in 2009-2010, and 7%, 14%, 20%, and 31% in subsequent epidemics of H3N2 (2010), H1N1pdm09 (2011), B (2012), and H3N2 (2012), respectively. Infection with H1N1pdm09 in 2009-2010 and H3N2 in 2010 was associated with protection against infection with subsequent epidemics of the same subtype in 2011 and 2012, respectively, but we found no evidence of heterotypic or heterosubtypic protection against infection.


CONCLUSIONS
We identified substantial incidence of influenza virus infections in children in Hong Kong in 5 major epidemics over a 3-year period, and evidence of homosubtypic but not heterosubtypic protection following infection.


BACKGROUND
NCT00792051.
Division of Epidemiology and Biostatistics, School of Public Health.
Benjamin J
BJ
Cowling
Division of Public Health Laboratory Sciences, School of Public Health Centre for Influenza Research.
Ranawaka A P M
RA
Perera
Division of Epidemiology and Biostatistics, School of Public Health.
Vicky J
VJ
Fang
Department of Microbiology.
Kwok-Hung
KH
Chan
Division of Epidemiology and Biostatistics, School of Public Health.
Winnie
W
Wai
Division of Epidemiology and Biostatistics, School of Public Health.
Hau Chi
HC
So
Division of Public Health Laboratory Sciences, School of Public Health Centre for Influenza Research.
Daniel K W
DK
Chu
Division of Epidemiology and Biostatistics, School of Public Health.
Jessica Y
JY
Wong
Division of Epidemiology and Biostatistics, School of Public Health.
Eunice Y
EY
Shiu
Department of Ecology and Evolutionary Biology, Princeton University, New Jersey.
Sophia
S
Ng
Division of Community Medicine and Public Health Practice, School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong Special Administrative Region, China.
Dennis K M
DK
Ip
Division of Public Health Laboratory Sciences, School of Public Health Centre for Influenza Research.
J S Malik
JS
Peiris
Division of Community Medicine and Public Health Practice, School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong Special Administrative Region, China.
Gabriel M
GM
Leung
D000293:Adolescent; D002648:Child; D005260:Female; D006723:Hong Kong / Q000453:epidemiology; D006801:Humans; D015994:Incidence; D007252:Influenza Vaccines / Q000008:administration & dosage* / Q000276:immunology*; D007251:Influenza, Human / Q000453:epidemiology* / Q000517:prevention & control*; D008297:Male; D009975:Orthomyxoviridae; D010919:Placebos / Q000008:administration & dosage; D016896:Treatment Outcome; D015164:Vaccines, Inactivated / Q000008:administration & dosage / Q000276:immunology
D007252:Influenza Vaccines; D010919:Placebos; D015164:Vaccines, Inactivated
immunity; incidence rates; influenza virus; vaccination