Vaccine-induced T cell responses control Orthoflavivirus challenge infection without neutralizing antibodies in humans.

T cells have been identified as correlates of protection in viral infections. However, the level of vaccine-induced T cells needed and the extent to which they alone can control acute viral infection in humans remain uncertain. Here we conducted a double-blind, randomized controlled trial involving vaccination and challenge in 33 adult human volunteers, using the live-attenuated yellow fever (YF17D) and chimeric Japanese encephalitis-YF17D (JE/YF17D) vaccines. Both Orthoflavivirus vaccines share T cell epitopes but have different neutralizing antibody epitopes. The primary objective was to assess the extent to which vaccine-induced T cell responses, independent of neutralizing antibodies, were able to reduce post-challenge viral RNAaemia levels. Secondary objectives included an assessment of surrogate measures of viral control, including post-challenge antibody titres and symptomatic outcomes. YF17D vaccinees had reduced levels of JE/YF17D challenge viraemia, compared with those without previous YF17D vaccination (mean log(area under the curve genome copies per ml): 2.23 versus 3.22; P = 0.039). Concomitantly, YF17D vaccinees had lower post-JE/YF17D challenge antibody titres that reduced JE virus plaque number by 50%, or PRNT (mean log(PRNT titre): 1.87 versus 2.5; P < 0.0001) and symptomatic rates (6% (n = 1/16) versus 53% (n = 9/17), P = 0.007). There were no unexpected safety events. Importantly, after challenge infection, several vaccinees had undetectable viraemia and no seroconversion, even in the absence of neutralizing antibodies. Indeed, high vaccine-induced T cell responses, specifically against the capsid protein, were associated with a level of viral control conventionally interpreted as sterilizing immunity. Our findings reveal the importance of T cells in controlling acute viral infection and suggests a potential correlate of protection against orthoflaviviral infections. ClinicalTrials.gov registration: NCT05568953 .