Mucosal boosting increases protective efficacy of an influenza vaccine in mice.

Influenza virus infection is a significant cause of global mortality. However, the development of influenza vaccines that induce robust immune responses at the site of respiratory mucosal exposure has proven challenging. Here, we assessed immune responses and protective efficacy of a rhesus adenovirus serotype 52 (RhAd52) vectored influenza vaccine encoding the hemagglutinin (HA) glycoprotein from A/California/07/2009 administrated by systemic or mucosal routes of immunization. We observed robust and durable systemic and mucosal immunity, including IgA and tissue resident memory T cells in the respiratory mucosa, in mice that received the vaccine intranasally or intratracheally. In contrast, only systemic immune responses were observed in mice that received the vaccine intramuscularly. Moreover, a single intranasal or intratracheal dose of RhAd52-HA provided near complete protection against a lethal challenge with a mouse-adapted influenza virus strain, whereas intramuscular immunization with RhAd52-HA and mRNA-HA provided less robust protection. Our data demonstrate the importance of mucosal immunity for enhancing vaccine protection against influenza.