Influenza vaccine effectiveness among children 6 to 59 months of age during 2 influenza seasons: a case-cohort study.
OBJECTIVE
To measure vaccine effectiveness (VE) in preventing influenza-related health care visits among children aged 6 to 59 months during 2 consecutive influenza seasons.


METHODS
Case-cohort study estimating effectiveness of inactivated influenza vaccine in preventing inpatient/outpatient visits (emergency department [ED] and outpatient clinic). We compared vaccination status of laboratory-confirmed influenza cases with a cluster sample of children from a random sample of practices in 3 counties (subcohort) during the 2003-2004 and 2004-2005 seasons.


METHODS
Counties encompassing Rochester, New York, Nashville, Tennessee, and Cincinnati, Ohio.


METHODS
Children aged 6 to 59 months seen in inpatient/ED or outpatient clinic settings for acute respiratory illnesses and community-based subcohort comparison. Main Exposure Influenza vaccination.


METHODS
Influenza vaccination status of cases vs subcohort using time-dependent Cox proportional hazards models to estimate VE in preventing inpatient/ED and outpatient visits.


RESULTS
During the 2003-2004 and 2004-2005 seasons, 165 and 80 inpatient/ED and 74 and 95 outpatient influenza cases were enrolled, while more than 4500 inpatient/ED and more than 600 outpatient subcohorts were evaluated, respectively. In bivariate analyses, cases had lower vaccination rates than subcohorts. However, significant influenza VE could not be demonstrated for any season, age, or setting after adjusting for county, sex, insurance, chronic conditions recommended for influenza vaccination, and timing of influenza vaccination (VE estimates ranged from 7%-52% across settings and seasons for fully vaccinated 6- to 59-month-olds).


CONCLUSIONS
In 2 seasons with suboptimal antigenic match between vaccines and circulating strains, we could not demonstrate VE in preventing influenza-related inpatient/ED or outpatient visits in children younger than 5 years. Further study is needed during years with good vaccine match.
Strong Memorial Hospital, Rochester, NY 14642, USA. peter_szilagyi@urmc.rochester.edu
Peter G
PG
Szilagyi
Gerry
G
Fairbrother
Marie R
MR
Griffin
Richard W
RW
Hornung
Stephanie
S
Donauer
Ardythe
A
Morrow
Mekibib
M
Altaye
Yuwei
Y
Zhu
Sandra
S
Ambrose
Kathryn M
KM
Edwards
Katherine A
KA
Poehling
Geraldine
G
Lofthus
Michol
M
Holloway
Lyn
L
Finelli
Marika
M
Iwane
Mary Allen
MA
Staat
D017677:Age Distribution; D000553:Ambulatory Care / Q000706:statistics & numerical data; D016022:Case-Control Studies; D002675:Child, Preschool; D016000:Cluster Analysis; D015331:Cohort Studies; D003140:Communicable Disease Control / Q000379:methods; D005260:Female; D005500:Follow-Up Studies; D006760:Hospitalization / Q000706:statistics & numerical data; D006801:Humans; D007115:Immunization Schedule; D015994:Incidence; D007223:Infant; D007252:Influenza Vaccines / Q000008:administration & dosage*; D007251:Influenza, Human / Q000453:epidemiology* / Q000517:prevention & control*; D008297:Male; D009819:Office Visits / Q000706:statistics & numerical data; D018570:Risk Assessment; D012621:Seasons; D012680:Sensitivity and Specificity; D017678:Sex Distribution; D017749:Total Quality Management; D014611:Vaccination / Q000706:statistics & numerical data*; D015164:Vaccines, Inactivated / Q000008:administration & dosage
D007252:Influenza Vaccines; D015164:Vaccines, Inactivated