Enteric fever, primarily caused by Salmonella enterica Typhi and Salmonella enterica Paratyphi A (SPA), is endemic mainly in South Asia, disproportionately affecting school-age children. Although typhoid conjugate vaccines (TCVs) are effective and implemented in many countries, no licensed vaccine exists against paratyphoid A. Bivalent vaccines targeting both S. Typhi and SPA may address this gap. Although field efficacy trials are not considered feasible, controlled human infection models (CHIMs) offer an alternative pathway for evaluating vaccine efficacy. This will be the first efficacy study of a bivalent vaccine against typhoid and paratyphoid A using a paratyphoid CHIM.

This is a phase II multicentre, double-blind, randomised controlled trial assessing the efficacy and immunogenicity of a bivalent conjugate vaccine candidate, Serum Institute of India Typhoid Conjugate Vaccine (Bivalent) (SII-TCV(B)), against SPA using a CHIM in healthy UK adults aged 18–55 years. A total of 192 participants will be randomised 1:1 to receive either SII-TCV(B) or a licensed Vi-polysaccharide typhoid vaccine (Vi-PS). All participants will be orally challenged with S. Paratyphi A (strain NVGH308) 28 days postvaccination. Participants will be monitored closely for 14 days and treated at 14 days postchallenge or promptly on diagnosis, according to prespecified criteria. The primary objective is to evaluate vaccine efficacy of SII-TCV(B) against paratyphoid infection using a CHIM. The coprimary immunogenicity objective is to assess non-inferiority of the typhoid IgG response compared with a licensed Vi-PS control.

The study has received ethical approval from the Berkshire Research Ethics Committee (24/SC/0309) and regulatory approval from the UK Medicines and Healthcare products Regulatory Agency. Results will be disseminated via peer-reviewed publications and scientific meetings.

ISRCTN65855590.