Efficacy, Safety, and Immunogenicity of the Shigella sonnei 1790GAHB GMMA Candidate Vaccine: Results from a Phase 2b Randomized, Placebo-Controlled Challenge Study in Adults

Abstract

Background: Shigellosis accounts for substantial morbidity and mortality worldwide and is the second most common cause of moderate and severe diarrhoea in children, after rotavirus. Methods: This phase 2b study (NCT03527173) evaluated vaccine efficacy (VE) against shigellosis, safety, and immunogenicity of a Shigella sonnei GMMA candidate vaccine (1790GAHB) in adults, using a S. sonnei 53G controlled human infection model (CHIM). Participants (randomized 1:1) received two doses of 1790GAHB or placebo (GAHB-Placebo), at day (D) 1 and D29, and an oral challenge of S. sonnei 53G at D57. VE was evaluated using several clinical endpoints, reflecting different case definitions of shigellosis. The success criterion was defined as a lower limit of the 90% confidence interval >0 for the primary endpoint. Findings: Thirty-six and 35 participants received 1790GAHB or placebo, respectively; 33 and 29 received the bacterial challenge, 15 and 12 participants developed shigellosis. Efficacy was not demonstrated for any endpoint. Solicited/unsolicited adverse events were more frequent in 1790GAHB versus placebo recipients post-vaccination. Anti-S. sonnei lipopolysaccharide (LPS) immunoglobulin G (IgG) responses increased at D29 and remained stable through D57 in the 1790GAHB group; no increase was shown in the placebo group. Interpretation: 1790GAHB had an acceptable safety profile and induced anti-LPS IgG responses but did not demonstrate clinical efficacy against shigellosis. Baseline and pre-challenge antibody levels were higher in participants who did not develop shigellosis post-challenge, thus suggesting a role of anti-LPS IgG antibodies in clinical protection. Although a precise threshold for clinical protection remains unknown, antibody levels derived from convalescent sera have demonstrated protection from shigellosis in field studies; however, this concept did not translate to the current CHIM. For further development of the vaccine, an increased content of S. sonnei O-antigen is likely needed to enhance anti-LPS immune responses. Trial Registration: NCT03527173 Funding Statement: GlaxoSmithKline Biologicals SA, the Bill and Melinda Gates Foundation. Declaration of Interests: AP, LBM, RR, GLC, PF, IDR, FM, FN, EM, JA, AA, OR and VC are employees of the GSK group of companies. AP, LBM, EM, and JA hold shares in the GSK group of companies. JA, LBM, AS, GLC, AGWN, PF, and IDR report grants from the Bill and Melinda Gates Foundation and the Wellcome Trust during the conduct of the study. LBM reports grants from the Bill and Melinda Gates foundation and the Wellcome Trust, outside the submitted work. AS, AGWN, and JPG were employees of the GSK group of companies at the time of study conduct. AA report personal fees from GSK Vaccines Institute for Global Health during the conduct of the study. AS hold shares in the GSK group of companies. LBM and AS are inventors of patents owned by the GSK group of companies and relevant to Shigella vaccine. AES, KAC, LC, MMcN, and RWK report grants from the GSK group of companies during the conduct of the trial. MMcN reports grants from NIH, outside the submitted work. MD, SP, and RWF have nothing to disclose. Ethics Approval Statement: The study was conducted in accordance with all applicable regulatory requirements, International Conference on Harmonisation - Good Clinical Practice guidelines, and the Declaration of Helsinki. The protocol and study-related documents were reviewed and approved by the institutional review board at the site. All participants provided electronic consent through REDcap system (www.projectredcap.org), except for one who provided written consent because the REDcap system was not working.