Intradermal influenza vaccination of healthy adults using a new microinjection system: a 3-year randomised controlled safety and immunogenicity trial

Background: Intradermal vaccination provides direct and potentially more efficient access to the immune system via specialised dendritic cells and draining lymphatic vessels. We investigated the immunogenicity and safety during 3 successive years of different dosages of a trivalent, inactivated, split-virion vaccine against seasonal influenza given intradermally using a microinjection system compared with an intramuscular control vaccine. Methods: In a randomised, partially blinded, controlled study, healthy volunteers (1150 aged 18 to 57 years at enrolment) received three annual vaccinations of intradermal or intramuscular vaccine. In Year 1, subjects were randomised to one of three groups: 3 mu g or 6 mu g haemagglutinin/strain/dose of inactivated influenza vaccine intradermally, or a licensed inactivated influenza vaccine intramuscularly containing 15 mu g/strain/dose. In Year 2 subjects were randomised again to one of two groups: 9 mu g/strain/dose intradermally or 15 mu g intramuscularly. In Year 3 subjects were randomised a third time to one of two groups: 9 mu g intradermally or 15 mu g intramuscularly. Randomisation lists in Year 1 were stratified for site. Randomisation lists in Years 2 and 3 were stratified for site and by vaccine received in previous years to ensure the inclusion of a comparable number of subjects in a vaccine group at each centre each year. Immunogenicity was assessed 21 days after each vaccination. Safety was assessed throughout the study. Results: In Years 2 and 3, 9 mu g intradermal was comparably immunogenic to 15 mu g intramuscular for all strains, and both vaccines met European requirements for annual licensing of influenza vaccines. The 3 mu g and 6 mu g intradermal formulations were less immunogenic than intramuscular 15 mu g. Safety of the intradermal and intramuscular vaccinations was comparable in each year of the study. Injection site erythema and swelling was more common with the intradermal route. Conclusion: An influenza vaccine with 9 mu g of haemagglutinin/strain given using an intradermal microinjection system showed comparable immunogenic and safety profiles to a licensed intramuscular vaccine, and presents a promising alternative to intramuscular vaccination for influenza for adults younger than 60 years.