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 microg each of 3 hemagglutinin vaccine antigens in 0.5 mL) IM injection, reduced-dose (9 microg, 0.3 mL) IM injection, reduced-dose (9 microg, 0.3 mL) ID injection, or 2 reduced-dose (4.5 microg, 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).