Immunogenicity and Safety of Intradermal Influenza Vaccination in Healthy Older Adults

Background. Influenza vaccine immunogenicity is suboptimal in older persons. Intradermal (ID) vaccination may be a promising alternative to intramuscular (IM) vaccination. Methods. This randomized trial compared the immunogenicity of 60% dose ID influenza vaccination to standard IM vaccination of full-dose or 60% dose vaccine. Pre- and postvaccination measurements in the hemagglutination inhibition antibody titer were compared. Participants who received reduced-dose vaccine were revaccinated with full-dose IM vaccine. Results. 257 healthy adults aged >= 65 years received 1 of the following trivalent inactivated influenza vaccines: standard-dose (15 mu g each of 3 hemagglutinin vaccine antigens in 0.5 mL) IM injection, reduced-dose (9 mu g, 0.3 mL) IM injection, reduced-dose ( 9 mu g, 0.3 mL) ID injection, or 2 reduced-dose (4.5 mu g, 0.15 mL) ID injections. Respective seroprotection rates were 65.6%, 57.8%, 68.9%, and 67.2% against A/H1N1; 76.6%, 75.0%, 75.4%, and 75.0% against A/H3N2; and 26.6%, 17.2%, 16.4%, and 25.0% against influenza B. Subsequent full-dose IM vaccination of participants randomized to reduced-dose vaccine by either IM or ID routes did not improve seroprotection rates. Local reactions of redness, swelling, and itching were significantly more frequent among recipients of ID injections. Conclusion. Influenza vaccine at 60% dose by either IM or ID route elicited antibody responses generally similar to full-dose IM vaccination among healthy elderly persons (ClinicalTrials. gov identifier: NCT00504231).