Live and inactivated influenza vaccines induce similar humoral responses, but only live vaccines induce diverse T-cell responses in young children.

BACKGROUND: Two doses of either trivalent live attenuated or inactivated influenza vaccines (LAIV and TIV, respectively) are approved for young children (≥ 24 months old for LAIV and ≥ 6 months old for TIV) and induce protective antibody responses. However, whether combinations of LAIV and TIV are safe and equally immunogenic is unknown. Furthermore, LAIV is more protective than TIV in children for unclear reasons. METHODS: Children 6-35 months old were administered, 1 month apart, 2 doses of either TIV or LAIV, or combinations of LAIV and TIV in both prime/boost sequences. Influenza-specific antibodies were measured by hemagglutination inhibition (HAI), and T cells were studied in flow cytometric and functional assays. Highly conserved M1, M2, and NP peptides predicted to be presented by common HLA class I and II were used to stimulate interferon-γ enzyme-linked immunospot responses. RESULTS: All LAIV and/or TIV combinations were well tolerated and induced similar HAI responses. In contrast, only regimens containing LAIV induced influenza-specific CD4(+), CD8(+), and γδ T cells, including T cells specific for highly conserved influenza peptides. CONCLUSIONS: Prime/boost combinations of LAIV and TIV in young children were safe and induced similar protective antibodies. Only LAIV induced CD4(+), CD8(+), and γδ T cells relevant for broadly protective heterosubtypic immunity. CLINICAL TRIALS REGISTRATION: NCT00231907.

Comparative immunogenicity of trivalent influenza vaccine administered by intradermal or intramuscular route in healthy adults.

The present study was undertaken with controls using equal doses ID and IM plus the standard full dose IM to assess the role of route of vaccine in immunogenicity of inactivated influenza vaccine. The study was a prospective, randomized, active-controlled, open label clinical trial conducted in healthy young adult outpatients to compare the effect of route (IM versus ID) on antibody responses to influenza vaccine. Volunteers received 3, 6 or 9 microg of vaccine by ID or IM route; 15 microg IM was also studied. Low doses of vaccine given by either route were almost as immunogenic as the standard 15 microg IM dose of influenza vaccine. ID route was not superior to IM vaccine at inducing antibodies. ID vaccine induced significantly more local inflammatory response than IM vaccine.