Protective efficacy against pandemic influenza of seasonal influenza vaccination in children in Hong Kong: a randomized controlled trial.
BACKGROUND
The efficacy of seasonal influenza vaccination against 2009 pandemic influenza A(H1N1) remains unclear.


METHODS
One child aged 6-17 years in each of 796 households was randomized to receive 2009-2010 seasonal trivalent inactivated influenza vaccine (TIV) or saline placebo between August 2009 and February 2010. Households were followed up with serology, symptom diaries, and collection of respiratory specimens during illnesses. The primary outcomes were influenza infection confirmed by reverse-transcription polymerase chain reaction (RT-PCR) or a ≥4-fold rise in serum antibody titer measured by hemagglutination inhibition assay.


RESULTS
Receipt of TIV led to 8-13-fold mean geometric rises in antibody titers against seasonal A and B viruses, but only 1.5-fold mean geometric rises against the pandemic A(H1N1) virus that was not included in the vaccine. Children who received TIV had a reduced risk of seasonal influenza B confirmed by RT-PCR, with a vaccine efficacy estimate of 66% (95% confidence interval [CI], 31%-83%). Children who received TIV also a had reduced risk of pandemic influenza A(H1N1) indicated by serology, with a vaccine efficacy estimate of 47% (95% CI, 15%-67%).


CONCLUSIONS
Seasonal TIV prevented pandemic influenza A(H1N1) and influenza B infections in children. Pandemic A(H1N1) circulated at the time of vaccination and for a short time afterward with no substantial seasonal influenza activity during that period. The potential mechanism for seasonal TIV to provide protection, possibly short lived, for children against pandemic A(H1N1) infection despite poor cross-reactive serologic response deserves further investigation. Clinical Trials Registration. NCT00792051.
School of Public Health.
Benjamin J
BJ
Cowling
Sophia
S
Ng
Edward S K
ES
Ma
Vicky J
VJ
Fang
Hau Chi
HC
So
Winnie
W
Wai
Calvin K Y
CK
Cheng
Jessica Y
JY
Wong
Kwok-Hung
KH
Chan
Dennis K M
DK
Ip
Susan S
SS
Chiu
J S Malik
JS
Peiris
Gabriel M
GM
Leung
D000293:Adolescent; D002648:Child; D005260:Female; D006723:Hong Kong / Q000453:epidemiology; D006801:Humans; D015994:Incidence; D053118:Influenza A Virus, H1N1 Subtype / Q000276:immunology*; D009981:Influenza B virus / Q000276:immunology*; D007252:Influenza Vaccines / Q000008:administration & dosage* / Q000276:immunology; D007251:Influenza, Human / Q000453:epidemiology / Q000276:immunology / Q000517:prevention & control* / Q000821:virology; D008297:Male; D058873:Pandemics; D010919:Placebos; D012307:Risk Factors; D015164:Vaccines, Inactivated / Q000008:administration & dosage / Q000276:immunology
D007252:Influenza Vaccines; D010919:Placebos; D015164:Vaccines, Inactivated