Population-based hospitalization burden of influenza a virus subtypes and antigenic drift variants in children in Hong Kong (2004-2011).
OBJECTIVE
We aim to document and analyze influenza hospitalization burden in light of antigenic changes in circulating influenza viruses in Hong Kong.


METHODS
The pediatric age-specific rates of influenza A hospitalization in Hong Kong for 2004-2011 which encompassed the emergence of H1N1pdm09 were extrapolated from admissions to 2 hospitals that together catered for 72.5% of all pediatric admissions on Hong Kong Island. Influenza A was detected by immunofluorescence, culture and/or PCR on nasopharyngeal aspirates.


RESULTS
Influenza A caused high rates of hospitalization in children with year to year fluctuations. The highest hospitalization burden was seen with H1N1pdm09 in 2009. Additional factors affecting hospitalization were the proportion of viral circulation among different subtypes, and antigenic drifts. Taking these into effect, an H3N2 dominated year was not always associated with more hospitalizations than a 'seasonal' H1N1 year. Hospitalization burden was higher in seasons when drifted viruses of H1N1 or H3N2 dominated. No hospitalization was documented in infants <6 months of age during years when an undrifted virus circulated (2006 for H1N1 and 2008 for H3N2) but significant hospitalization was observed with a drifted or shifted virus (2004, 2005, 2007 and 2010 for H3N2, and 2009 for H1N1pdm09).


CONCLUSIONS
We documented a consistently high pediatric hospitalization burden of influenza A. Knowledge of antigenic changes and their proportion of circulation aids in the interpretation of impact of the subtypes. Year-to-year variation in hospitalization rates in young infants appeared to correlate with antigenic variation, lending support to the role of protection from maternal antibodies.
Department of Pediatrics & Adolescent Medicine, The University of Hong Kong, Pokfulam, Hong Kong.
Susan S
SS
Chiu
Public Health Laboratory Services Branch, Centre for Health Protection, Department of Health, Kowloon, Hong Kong.
Janice Y C
JY
Lo
Department of Microbiology, The University of Hong Kong, Pokfulam, Hong Kong.
Kwok-Hung
KH
Chan
Department of Pediatrics & Adolescent Medicine, The University of Hong Kong, Pokfulam, Hong Kong.
Eunice L Y
EL
Chan
Department of Pediatrics and Adolescent Medicine, Pamela Youde Nethersole Eastern Hospital, Chai Wan, Hong Kong.
Lok-Yee
LY
So
Division of Epidemiology and Biostatistics, School of Public Health, The University of Hong Kong, Pokfulam, Hong Kong.
Peng
P
Wu
Division of Epidemiology and Biostatistics, School of Public Health, The University of Hong Kong, Pokfulam, Hong Kong.
Benjamin J
BJ
Cowling
Department of Pediatrics & Adolescent Medicine, The University of Hong Kong, Pokfulam, Hong Kong.
Robin
R
Chen
Centre of Influenza Research, School of Public Health, The University of Hong Kong, Pokfulam, Hong Kong.
J S Malik
JS
Peiris
D000818:Animals; D000956:Antigens, Viral / Q000276:immunology*; D001483:Base Sequence; D002460:Cell Line; D002648:Child; D017931:DNA Primers; D006723:Hong Kong; D006760:Hospitalization; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000276:immunology / Q000302:isolation & purification*; D007251:Influenza, Human / Q000628:therapy* / Q000821:virology; D020133:Reverse Transcriptase Polymerase Chain Reaction
D000956:Antigens, Viral; D017931:DNA Primers