Protective efficacy of seasonal influenza vaccination against seasonal and pandemic influenza virus infection during 2009 in Hong Kong.
BACKGROUND
The relationship between seasonal influenza vaccine and susceptibility to 2009 pandemic A/H1N1 virus infection is not fully understood.


METHODS
One child 6-15 years of age from each of 119 households was randomized to receive 1 dose of inactivated trivalent seasonal influenza vaccine (TIV) or saline placebo in November 2008. Serum samples were collected from study subjects and their household contacts before and 1 month after vaccination (December 2008), after winter (April 2009) and summer influenza (September-October 2009) seasons. Seasonal and pandemic influenza were confirmed by serum hemagglutinination inhibition, viral neutralization titers, and reverse-transcription polymerase chain reaction performed on nasal and throat swab samples collected during illness episodes.


RESULTS
TIV recipients had lower rates of serologically confirmed seasonal A/H1N1 infection (TIV group, 8%; placebo group, 21%; P=.10) and A/H3N2 infection (7% vs 12%; P=A9), but higher rates of pandemic A/H1N1 infection (32% vs 17%; [Formula: see text]). In multivariable analysis, those infected with seasonal influenza A during the study had a lower risk of laboratory-confirmed pandemic A/H1N1 infection (adjusted odds ratio [OR], 0.35; 95% confidence interval [CI], 0.14-0.87), and receipt of seasonal TIV was unassociated with risk of pandemic A/H1N1 infection (adjusted OR, 1.11; 95% CI, 0.54-2.26).


CONCLUSIONS
TIV protected against strain-matched infection in children. Seasonal influenza infection appeared to confer cross-protection against pandemic influenza. Whether prior seasonal influenza vaccination affects the risk of infection with the pandemic strain requires additional study.


BACKGROUND
ClinicalTrials.gov number NCT00792051 .
Infectious Disease Epidemiology Group, School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong Special Administrative Region, China. bcowling@hku.hk
Benjamin J
BJ
Cowling
Sophia
S
Ng
Edward S K
ES
Ma
Calvin K Y
CK
Cheng
Winnie
W
Wai
Vicky J
VJ
Fang
Kwok-Hung
KH
Chan
Dennis K M
DK
Ip
Susan S
SS
Chiu
J S Malik
JS
Peiris
Gabriel M
GM
Leung
D000293:Adolescent; D057134:Antibodies, Neutralizing / Q000097:blood; D000914:Antibodies, Viral / Q000097:blood; D002648:Child; D056738:Cross Protection; D005260:Female; D006385:Hemagglutination Inhibition Tests; D006723:Hong Kong / Q000453:epidemiology; D006801:Humans; D007252:Influenza Vaccines / Q000008:administration & dosage / Q000276:immunology*; D007251:Influenza, Human / Q000453:epidemiology* / Q000517:prevention & control* / Q000821:virology; D008297:Male; D009500:Neutralization Tests; D009666:Nose / Q000821:virology; D009975:Orthomyxoviridae / Q000302:isolation & purification*; D058873:Pandemics; D010614:Pharynx / Q000821:virology; D010919:Placebos / Q000008:administration & dosage; D012367:RNA, Viral / Q000235:genetics / Q000302:isolation & purification; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014611:Vaccination / Q000379:methods*; D015164:Vaccines, Inactivated / Q000008:administration & dosage / Q000276:immunology
D057134:Antibodies, Neutralizing; D000914:Antibodies, Viral; D007252:Influenza Vaccines; D010919:Placebos; D012367:RNA, Viral; D015164:Vaccines, Inactivated