An oral norovirus vaccine generates mucosal immunity and reduces viral shedding in a phase 2 placebo-controlled challenge study

There are currently no licensed vaccines for norovirus, a leading cause of epidemic and endemic gastroenteritis worldwide. Clinical advancement of promising vaccine candidates from phase 2 studies to phase 3 field trials has been hampered by the lack of robust immunological correlates of protection. Here, we conducted a phase 2b randomized, placebo-controlled vaccination and challenge study to assess the safety, efficacy, immunogenicity, and correlates of protection of VXA-G1.1-NN, an oral tablet norovirus vaccine. VXA-G1.1-NN was safe and well tolerated, conferred protection against norovirus GI.1 challenge, and reduced viral shedding in stool and emesis. Norovirus VP1-specific serum immunoglobulin A (IgA), IgG, and functional blocking antibody titers increased substantially after oral vaccination. Moreover, oral immunization stimulated VP1-specific IgA antibodies in nasal lining fluid, saliva, and fecal samples. Serum and mucosal antibody responses 7 days after vaccination were correlated with the induction of antibody-secreting, α4β7⁺ mucosal-homing B cells. Machine learning analyses of vaccine-stimulated immune components identified serum functional blocking antibody and fecal IgA as robust correlates of protection. These results demonstrate the potential of VXA-G1.1-NN as a safe and effective oral norovirus vaccine and reveal critical immunological features underpinning vaccine efficacy.