Intradermal Flu Vaccine at Reduced Dose Elicits Similar Response as Full-Dose IM Injection in Older Adults

U.S. investigators randomized 257 healthy communitydwelling adults ages 65 and older to receive one of four different trivalent inactivated influenza vaccine regimens:

• standard dose (15 mcg of each of three hemagglutinin vaccine antigens in 0.5 mL) by intramuscular (IM) injection;
• reduced-dose vaccine (9 mcg in 0.3 mL) by IM injection;
• reduced-dose vaccine (9 mcg in 0.3 mL) by intradermal (ID) injection; or
• two splitreduced-dose ID injections(4.5 mcg in 0.15 mL each).

The respective seroprotection rates were 65.6 percent, 57.8 percent, 68.9 percent and 67.2 percent against the A/H1N1 strain; 76.6 percent, 75.0 percent, 75.4 percent and 75.0 percent against the A/H3N2 strain; and 26.6 percent, 17.2 percent, 16.4 percent and 25.0 percent against the B strain. Subsequent fulldose IM vaccination of participants randomized to reduceddose vaccine by either the IM or ID routes failed to improve seroprotection rates. Local reactions, including redness, swelling and itching, were significantly more frequent among recipients of ID injections.

The authors concluded that delivery of influenza vaccine at 60 percent of the standard dose by either IM or ID route elicited antibody responses generally similar to full-dose IM vaccination among healthy elderly persons.