Effectiveness of inactivated influenza vaccines varied substantially with antigenic match from the 2004-2005 season to the 2006-2007 season.
BACKGROUND
We estimated the effectiveness of inactivated influenza vaccines for the prevention of laboratory-confirmed, medically attended influenza during 3 seasons with variable antigenic match between vaccine and patient strains.


METHODS
Patients were enrolled during or after a clinical encounter for acute respiratory illness. Influenza infection was confirmed by culture or reverse-transcriptase polymerase chain reaction. Case-control analyses were performed that used data from patients who were ill without influenza (hereafter, "test-negative control subjects") and data from asymptomatic control subjects from the population (hereafter, "traditional control subjects"). Vaccine effectiveness (VE) was estimated as [100 x (1-adjusted odds ratio)]. Influenza isolates were antigenically characterized.


RESULTS
Influenza was detected in 167 (20%) of 818 patients in 2004-2005, in 51 (14%) of 356 in 2005-2006, and in 102 (11%) of 932 in 2006-2007. Analyses that used data from test-negative control subjects showed that VE was 10% (95% confidence interval [CI], -36% to 40%) in 2004-2005, 21% (95% CI, -52% to 59%) in 2005-2006, and 52% (95% CI, 22% to 70%) in 2006-2007. Using data from traditional control subjects, VE for those seasons was estimated to be 5% (95% CI, -52% to 40%), 11% (95% CI, -96% to 59%), and 37% (95% CI, -10% to 64%), respectively; confidence intervals included 0. The percentage of viruses that were antigenically matched to vaccine strains was 5% (3 of 62) in 2004-2005, 5% (2 of 42) in 2005-2006, and 91% (85 of 93) in 2006-2007.


CONCLUSIONS
Influenza VE varied substantially across 3 seasons and was highest when antigenic match was optimal. VE estimates that used data from test-negative control subjects were consistently higher than those that used data from traditional control subjects.
Belongia
Edward A
EA
Epidemiology Research Center, Marshfield Clinic Research Foundation, Marshfield, Wisconsin 54449, USA. belongia.edward@marshfieldclinic.org
Kieke
Burney A
BA
Donahue
James G
JG
Greenlee
Robert T
RT
Balish
Amanda
A
Foust
Angie
A
Lindstrom
Stephen
S
Shay
David K
DK
D000293:Adolescent; D000328:Adult; D000368:Aged; D016022:Case-Control Studies; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000145:classification / Q000235:genetics / Q000276:immunology / Q000302:isolation & purification; D053122:Influenza A Virus, H3N2 Subtype / Q000145:classification / Q000235:genetics / Q000276:immunology / Q000302:isolation & purification; D009980:Influenza A virus / Q000145:classification / Q000235:genetics / Q000276:immunology / Q000302:isolation & purification; D009981:Influenza B virus / Q000145:classification / Q000235:genetics / Q000276:immunology / Q000302:isolation & purification; D007252:Influenza Vaccines / Q000008:administration & dosage* / Q000276:immunology; D007251:Influenza, Human / Q000453:epidemiology* / Q000517:prevention & control / Q000821:virology*; D008297:Male; D008875:Middle Aged; D011159:Population Surveillance; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012621:Seasons; D016896:Treatment Outcome; D015164:Vaccines, Inactivated / Q000008:administration & dosage* / Q000276:immunology; D014922:Wisconsin / Q000453:epidemiology; D055815:Young Adult
D007252:Influenza Vaccines; D015164:Vaccines, Inactivated