Engineering a Novel Bivalent Oral Vaccine against Enteric Fever.

Enteric fever is a major global healthcare issue caused largely by serovars Typhi and Paratyphi A. The objective of this study was to develop a novel, bivalent oral vaccine capable of protecting against both serovars. Our approach centred on genetically engineering the attenuated Typhi ZH9 strain, which has an excellent safety record in clinical trials, to introduce two Paratyphi A immunogenic elements: flagellin H:a and lipopolysaccharide (LPS) O:2. We first replaced the native Typhi gene encoding flagellin with the highly homologous gene from Paratyphi A using Xer-cise technology. Next, we replaced the Typhi gene encoding tyvelose epimerase with a spacer sequence to enable the sustained expression of O:2 LPS and prevent its conversion to O:9 through tyvelose epimerase activity. The resulting new strain, ZH9PA, incorporated these two genetic changes and exhibited comparable growth kinetics to the parental ZH9 strain. A formulation containing both ZH9 and ZH9PA strains together constitutes a new bivalent vaccine candidate that targets both Typhi and Paratyphi A antigens to address a major global healthcare gap for enteric fever prophylaxis. This vaccine is now being tested in a Phase I clinical trial (NCT04349553).