Challenge of conducting a placebo-controlled randomized efficacy study for influenza vaccine in a season with low attack rate and a mismatched vaccine B strain: a concrete example.
BACKGROUND
Our aim was to determine the efficacy of a trivalent inactivated split virus influenza vaccine (TIV) against culture-confirmed influenza A and/or B in adults 18 to 64 years of age during the 2005/2006 season in the Czech Republic.


METHODS
6203 subjects were randomized to receive TIV (N = 4137) or placebo (N = 2066). The sample size was based on an assumed attack rate of 4% which provided 90% power to reject the hypothesis that vaccine efficacy (VE) was > or = 45%. Cases of influenza like illness (defined as fever (oral temperature > or =37.8 degrees C) plus cough and/or sore throat) were identified both by active (biweekly phone contact) and passive (self reporting) surveillance and nasal and throat swabs were collected from subjects for viral culture.


RESULTS
TIV was well tolerated and induced a good immune response. The 2005/2006 influenza season was exceptionally mild in the study area, as it was throughout Europe, and only 46 culture-confirmed cases were found in the study cohort (10 influenza A and 36 influenza B). Furthermore among the B isolates, 35 were identified as B/Hong Kong 330/2001-like (B/Victoria/2/87 lineage) which is antigenically unrelated to the vaccine B strain (B/Yamagata/16/88 lineage). The attack rate in the vaccine group (0.7%) was not statistically significantly different from the attack rate in the placebo group (0.9%).


CONCLUSIONS
Due to the atypical nature of the influenza season during this study we were unable to assess TIV efficacy. This experience illustrates the challenge of conducting a prospective influenza vaccine efficacy trial during a single season when influenza attack rates and drift in circulating strains or B virus lineage match can be difficult to estimate in advance.


BACKGROUND
Clinical trial registery: NCT00197223.
Beran
Jirí
J
The Vaccination and Travel Medicine Center, Hradec Kralove, Czech Republic. jiri.beran@vakcinace.cz
Wertzova
Veronika
V
Honegr
Karel
K
Kaliskova
Eva
E
Havlickova
Martina
M
Havlik
Jirí
J
Jirincova
Helena
H
Van Belle
Pascale
P
Jain
Varsha
V
Innis
Bruce
B
Devaster
Jeanne-Marie
JM
D000293:Adolescent; D000328:Adult; D000914:Antibodies, Viral / Q000276:immunology; D018153:Czech Republic / Q000453:epidemiology; D004311:Double-Blind Method; D005260:Female; D019267:Hemagglutinin Glycoproteins, Influenza Virus / Q000276:immunology; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000276:immunology; D053122:Influenza A Virus, H3N2 Subtype / Q000276:immunology; D009981:Influenza B virus / Q000276:immunology; D007252:Influenza Vaccines / Q000008:administration & dosage / Q000276:immunology / Q000627:therapeutic use*; D007251:Influenza, Human / Q000453:epidemiology* / Q000517:prevention & control*; D008297:Male; D015164:Vaccines, Inactivated / Q000008:administration & dosage / Q000276:immunology / Q000627:therapeutic use; D055815:Young Adult
D000914:Antibodies, Viral; D019267:Hemagglutinin Glycoproteins, Influenza Virus; D007252:Influenza Vaccines; D015164:Vaccines, Inactivated