Background. Enhanced influenza vaccines are needed to provide improved protection for elderly individuals. The intradermal vaccination route was hypothesized to provide immunogenicity superior to that provided by the intramuscular vaccination route.

Methods. In a multicenter, randomized study, 1107 volunteers >60 years of age received intradermal trivalent inactivated influenza vaccine containing 15 or 21 µg of hemagglutinin per strain or intramuscular control vaccine. Intradermal vaccines used a novel microinjection system designed to ensure easy, convenient, consistent vaccination. The primary end points of the study were the strain-specific hemagglutination inhibition geometric mean titers (GMTs) noted 21 days after vaccination. Groups were compared using noninferiority and superiority analyses.

Results. For each strain, the GMTs noted in association with each intradermal vaccine were superior to those noted with the intramuscular control (adjusted P < .0001). Seroprotection rates, seroconversion rates, and mean titer increases were also superior for intradermally administered vaccine in all but one of the analyses undertaken. Systemic reactogenicity was comparable between routes. Local injection site reactions, particularly erythema but not pain, were more commonly associated with intradermal vaccination.

Conclusions. For the first time, the intradermal vaccination route has been used to elicit immune responses significantly superior to those noted in association with the conventional intramuscular vaccination route. This was done using an easy-to-use, reliable microinjection system. This superior response is expected to enhance annual protection against influenza in this vulnerable population.

Trial registration. Clinicaltrials.gov registry number: NCT00296829.