Influenza vaccine effectiveness in the 2011-2012 season: protection against each circulating virus and the effect of prior vaccination on estimates.
BACKGROUND
Each year, the US Influenza Vaccine Effectiveness Network examines the effectiveness of influenza vaccines in preventing medically attended acute respiratory illnesses caused by influenza.


METHODS
Patients with acute respiratory illnesses of ≤ 7 days' duration were enrolled at ambulatory care facilities in 5 communities. Specimens were collected and tested for influenza by real-time reverse-transcriptase polymerase chain reaction. Receipt of influenza vaccine was defined based on documented evidence of vaccination in medical records or immunization registries. Vaccine effectiveness was estimated in adjusted logistic regression models by comparing the vaccination coverage in those who tested positive for influenza with those who tested negative.


RESULTS
The 2011-2012 season was mild and peaked late, with circulation of both type A viruses and both lineages of type B. Overall adjusted vaccine effectiveness was 47% (95% confidence interval [CI], 36-56) in preventing medically attended influenza; vaccine effectiveness was 65% (95% CI, 44-79) against type A (H1N1) pdm09 but only 39% (95% CI, 23-52) against type A (H3N2). Estimates of vaccine effectiveness against both type B lineages were similar (overall, 58%; 95% CI, 35-73). An apparent negative effect of prior year vaccination on current year effectiveness estimates was noted, particularly for A (H3N2) outcomes.


CONCLUSIONS
Vaccine effectiveness in the 2011-2012 season was modest overall, with lower effectiveness against the predominant A (H3N2) virus. This may be related to antigenic drift, but past history of vaccination might also play a role.
Department of Epidemiology, University of Michigan School of Public Health, Ann Arbor.
Suzanne E
SE
Ohmit
Mark G
MG
Thompson
Joshua G
JG
Petrie
Swathi N
SN
Thaker
Michael L
ML
Jackson
Edward A
EA
Belongia
Richard K
RK
Zimmerman
Manjusha
M
Gaglani
Lois
L
Lamerato
Sarah M
SM
Spencer
Lisa
L
Jackson
Jennifer K
JK
Meece
Mary Patricia
MP
Nowalk
Juhee
J
Song
Marcus
M
Zervos
Po-Yung
PY
Cheng
Charles R
CR
Rinaldo
Lydia
L
Clipper
David K
DK
Shay
Pedro
P
Piedra
Arnold S
AS
Monto
D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D007223:Infant; D007252:Influenza Vaccines / Q000008:administration & dosage* / Q000276:immunology*; D007251:Influenza, Human / Q000453:epidemiology* / Q000517:prevention & control* / Q000821:virology; D008297:Male; D008875:Middle Aged; D009975:Orthomyxoviridae / Q000145:classification / Q000302:isolation & purification; D060888:Real-Time Polymerase Chain Reaction; D020133:Reverse Transcriptase Polymerase Chain Reaction; D016896:Treatment Outcome; D014481:United States / Q000453:epidemiology; D014611:Vaccination / Q000379:methods*; D055815:Young Adult
D007252:Influenza Vaccines
ambulatory care; influenza; medically attended influenza; vaccine effectiveness