Incidence of medically attended influenza infection and cases averted by vaccination, 2011/2012 and 2012/2013 influenza seasons.
BACKGROUND
We estimated the burden of outpatient influenza and cases prevented by vaccination during the 2011/2012 and 2012/2013 influenza seasons using data from the United States Influenza Vaccine Effectiveness (US Flu VE) Network.


METHODS
We defined source populations of persons who could seek care for acute respiratory illness (ARI) at each of the five US Flu VE Network sites. We identified all members of the source population who were tested for influenza during US Flu VE influenza surveillance. Each influenza-positive subject received a sampling weight based on the proportion of source population members who were tested for influenza, stratified by site, age, and other factors. We used the sampling weights to estimate the cumulative incidence of medically attended influenza in the source populations. We estimated cases averted by vaccination using estimates of cumulative incidence, vaccine coverage, and vaccine effectiveness.


RESULTS
Cumulative incidence of medically attended influenza ranged from 0.8% to 2.8% across sites during 2011/2012 and from 2.6% to 6.5% during the 2012/2013 season. Stratified by age, incidence ranged from 1.2% among adults 50 years of age and older in 2011/2012 to 10.9% among children 6 months to 8 years of age in 2012/2013. Cases averted by vaccination ranged from 4 to 41 per 1000 vaccinees, depending on the study site and year.


CONCLUSIONS
The incidence of medically attended influenza varies greatly by year and even by geographic region within the same year. The number of cases averted by vaccination varies greatly based on overall incidence and on vaccine coverage.
Group Health Research Institute, Seattle, WA, United States. Electronic address: jackson.ml@ghc.org.
Michael L
ML
Jackson
Group Health Research Institute, Seattle, WA, United States.
Lisa A
LA
Jackson
Marshfield Clinic Research Foundation, Marshfield, WI, United States.
Burney
B
Kieke
Marshfield Clinic Research Foundation, Marshfield, WI, United States.
David
D
McClure
Baylor Scott & White Health and Texas A&M University Health Science Center College of Medicine, Temple, TX, United States.
Manjusha
M
Gaglani
Baylor Scott & White Health and Texas A&M University Health Science Center College of Medicine, Temple, TX, United States.
Kempapura
K
Murthy
University of Michigan School of Public Health, Ann Arbor, MI, United States.
Ryan
R
Malosh
University of Michigan School of Public Health, Ann Arbor, MI, United States.
Arnold
A
Monto
University of Pittsburgh, Pittsburgh, PA, United States.
Richard K
RK
Zimmerman
Centers for Disease Control and Prevention, Atlanta, GA, United States; Batelle, Atlanta, GA, United States.
Ivo M
IM
Foppa
Centers for Disease Control and Prevention, Atlanta, GA, United States.
Brendan
B
Flannery
Centers for Disease Control and Prevention, Atlanta, GA, United States.
Mark G
MG
Thompson
D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D015994:Incidence; D007223:Infant; D007252:Influenza Vaccines / Q000008:administration & dosage* / Q000276:immunology*; D007251:Influenza, Human / Q000453:epidemiology* / Q000517:prevention & control*; D008297:Male; D008875:Middle Aged; D009819:Office Visits; D019371:Topography, Medical; D014481:United States / Q000453:epidemiology; D055815:Young Adult
D007252:Influenza Vaccines
Incidence; Influenza vaccines; Influenza, human