Intramuscular administration of influenza hemagglutinin in one hind leg elicits similar protective immune responses as administration in two hind legs.

The objective of this study was to determine if distributing vaccine antigen across two intramuscular vaccination sites, rather than a single site could elicit more efficient protective immune responses. Most commercial injectable vaccines are administered unilaterally in a single location. The hypothesis of this study was vaccination in two distinct intramuscular sites would produce a more robust immune response compared to a single injection site. Mice vaccinated with a bilateral vaccination strategy may elicit broader antigen dissemination leading to increased germinal center activation and higher protective immune responses. Mice were vaccinated in the left and/or right hind leg quadricep muscle with recombinant H1 and/or H3 computationally optimized broadly reactive antigen (COBRA) hemagglutinin influenza protein and then challenged with either an H1N1 or H3N2 influenza virus. Hemagglutination inhibition (HAI) assays were performed on sera collected after final vaccination. There were no significant statistical differences in HAI titers between mice receiving a single HA protein (monovalent) in one versus two legs. The lowest antibody response was observed in mice receiving the bivalent mixture in a single injection site. Similarly, B cells collected from the spleen or draining lymph nodes had anti-HA IgG levels that were not significantly improved following bilateral vaccination compared to single-site vaccination. Furthermore, there were no statistically significant differences in weight loss or lung viral titers following influenza virus challenge in vaccinated mice, regardless of the vaccine regimen. These results indicate that administering intramuscular vaccinations in two locations does not augment immune responses compared to a single site administration.