Immunogenicity and safety of intradermal versus intramuscular route of influenza immunization in infants less than 6 months of age: A randomized controlled trial

We aimed to explore intradermal influenza vaccination in infants <6 months. One hundred twenty-six infants 2-3 months of age were randomized to receive either two doses, 1 month apart, of 0.25 ml of a trivalent inactivated influenza vaccine (7.5 μg of hemagglutinin per strain) via the intramuscular (IM) route or 0.1 ml of the same vaccine (3 μg of hemagglutinin per strain) via the intradermal (ID) route. The vaccine was well tolerated. Only four infants had hemagglutination inhibition (HAI) titer <40 against ≥1 vaccine-covered antigen pre-vaccination. There was no difference in fold-rise of HAI titer response between those in the IM or ID group. We documented maintenance of HAI titers above seroprotective levels against all three vaccine antigens in 97.6% of subjects regardless of vaccination methods over a time of waning maternal antibodies.