Salmonella enterica serovar Typhi and Salmonella enterica serovar Paratyphi A cause typhoid and paratyphoid fever respectively, which are collectively referred to as enteric fever. Paratyphoid fever may be caused by Salmonella serovars Paratyphi A, B or C, with S. Paratyphi A currently predominating. ¹ Salmonella serovars Typhi and Paratyphi A are restricted to human hosts, and transmission occurs through the faecal-oral route via the ingestion of faecally-contaminated food and water. ^{2 , 3} The disease incubation period is typically 7 to 14 days, and commonly presents with fever, malaise, and mild gastrointestinal symptoms, often indistinguishable from other febrile illnesses. ^{3 , 4}

Globally, in 2017 there were an estimated 11-27 million cases of enteric fever, and >120,000 associated deaths; 75% of illnesses and >85% of deaths are caused by Salmonella Typhi. ⁵ There is, however, evidence that Salmonella Paratyphi A is responsible for a growing proportion of enteric fever, and in endemic countries such as India or Nepal, paratyphoid fever can account for >40% of enteric fever. ^{6 – 8} Moreover, the unavailability of reliable routine diagnostic capacity and weak surveillance systems likely result in an underestimation of the true contribution of S. Paratyphi A to the burden of enteric fever. ^{9 , 10}

Geographically, typhoid fever is found worldwide and presents a major public health problem to countries in Asia and sub-Saharan Africa, whereas paratyphoid fever is concentrated in, but is not limited to, countries in Asia and the Middle East. Both typhoid and paratyphoid fever are associated with unsanitary living conditions and lack of access to microbiologically safe water and food. While high population density in urban slums is a known risk factor for both typhoid and paratyphoid fever, typhoid fever in Africa is also frequently found in rural, low-population density sites across the continent. ^{11 , 12} There are substantial variations on the age distribution of both typhoid and paratyphoid fever across geographies and over time; however, enteric fever appears to be common among young children, and a substantial burden of disease remains until early adulthood. While the highest typhoid fever incidence rates are often found among children under five years of age, paratyphoid fever appears to peak later in life, often between 5 and 15 years of age. ⁵

Outcomes for enteric fever are improved with timely and appropriate antimicrobial therapy. However, antimicrobial resistance (AMR) is a growing concern. Resistance to traditional first-line treatment - ampicillin, chloramphenicol, and cotrimoxazole - among typhoidal Salmonella strains has been reported since the 1950s. ¹³ Resistance to all three antimicrobials, or multidrug resistance (MDR), has been documented since the 1980s. ^{14 , 15} The subsequent use of nalidixic acid and ciprofloxacin as alternative treatments for resistant enteric fever led to the emergence of fluoroquinolone non-susceptibility (FQNS) and ciprofloxacin resistance. Extensively drug resistant (XDR) strains are resistant to traditional first-line antimicrobials, fluoroquinolones, and third-generation cephalosporines: these emerged in Pakistan in 2016 and have spread worldwide through international travel. ^{16 – 19} While antimicrobial resistance patterns including MDR vary by place and serotype, both S. Typhi and S. Paratyphi A are currently reported to present FQNS at a prevalence of >90%, while XDR strains have been identified only in S. serovar Typhi to date.

Access to microbiologically safer water and food, sanitation, and hygiene (WASH) practices remains important measures for the prevention of enteric fever. ²¹ However, in the absence of the resources required for the provision and development of such infrastructures, vaccines are a more viable alternative. Safe and effective vaccines against typhoid fever exist, and their programmatic use has been recommended by the World Health Organization (WHO) since 2008. The most recent WHO position paper, updated in 2018, recommends the use of typhoid conjugate vaccines (TCV) in the infant immunization schedule from 6 months of age, and promotes TCV introduction into the routine programmatic schedule at 9 months of age or in the second year of life in endemic countries with high disease burdens or with high AMR prevalence. ²² Single dose TCV has demonstrated high protective efficacy in paediatric and young adult populations in endemic areas, ranging from 79-85%. ^{23 – 28} However, evidence suggests a progressive decline of protection over time, particularly, among those who received the vaccine before the age of 2 years, and the need for a booster dose is currently under review. ^{29 , 30} While TCVs have been instrumental for typhoid fever control in highly endemic countries, and have successfully contributed to the containment of outbreaks caused by MDR and XDR strains, ^{18 , 28} the absence of a vaccine to prevent paratyphoid fever remains a critical gap in enteric fever prevention strategies. Current vaccine development efforts are focused on bivalent conjugate vaccine candidates that combine the antigen O:2, which is present in S. Paratyphi A, linked to a carrier protein with an existing TCV construct. ^{31 , 32} Some of these products are soon to start efficacy evaluations, which should corroborate existing evidence that antibody levels against the O:2 antigen correlate to serum bactericidal activity. ^{31 – 33} In addition, a live-attenuated vaccine candidate using the CVD 1902 strain of S. Paratyphi A was proven immunogenic in a phase 1 study, and it is currently being investigated for efficacy using a controlled human infection model (CHIM). ^{34 , 35}

Vaccine developers and manufacturers rely on WHO guidance for research and development, regulation and prequalification pathways for novel vaccines. Currently, no such guidance has been released for paratyphoid vaccines, and immune correlates of protection for a paratyphoid vaccine are yet to be established. It is widely acknowledged that phase 3 efficacy trials for a Salmonella Paratyphi A-containing vaccines are unlikely to be logistically and economically feasible due to the lower prevalence of paratyphoid fever in comparison to typhoid fever. Hence, the WHO’s Product Development for Vaccines Advisory Committee (PDVAC) has endorsed a regulatory pathway for bivalent conjugate vaccines that would rely on data obtained from a CHIM in an adult population in a non-endemic country, paired with a phase 3 safety and immunogenicity trial in a target population, and the commitment from manufacturers to conduct post marketing effectiveness studies. ^{36 , 37}

Following a consultative process with vaccine experts, developers, manufacturers, and policy-makers, the WHO has developed a technical research and development roadmap, and defined the preferred product characteristics (PPC) for Salmonella Paratyphi A-containing vaccines, aiming at guiding the work of vaccine developers, manufacturers, and funding bodies in regards to vaccine development and regulatory data requirements, ensuring that critical questions are addressed in a manner to facilitate regulatory processes of national and international bodies, and support robust policy decision-making after the products are available. ^{38 – 41} These documents provide considerations for bivalent conjugate and live-attenuated products, and highlight the current research gaps that might hinder decision-making in regards to the use and implementation of these vaccines in both: endemic settings where paratyphoid fever constitutes currently a public health concern, and non-endemic settings where Salmonella Paratyphi A could be introduced and fill in the niche that TCV could open by targeting Salmonella Typhi.

Due to the lower incidence of paratyphoid fever relative to typhoid fever and its geographic distribution, a monovalent Salmonella Paratyphi A vaccine is unlikely to be commercially viable. ^{42 , 43} However, there is growing interest in addressing the public health burden of paratyphoid fever through the development of a bivalent S. Typhi/Paratyphi A vaccine. ^{44 – 46}

The development of the bivalent Salmonella Typhi/Paratyphi A vaccines PPC and R&D roadmap responded to the need expressed by vaccine developers and manufacturers for WHO normative guidance on the characteristics of a successful vaccine that would have credence with decision-makers in settings endemic for paratyphoid fever, facilitate the regulatory and licensure procedures, and identify the research needs to ensure essential data and information gaps can be addressed during vaccine development. To this end, the WHO convened a Technical Advisory Group on Salmonella Vaccines (TAG-SV), a diverse group of experts with academic, vaccine development and regulatory background who advise the WHO in matters related to the research and development of Salmonella vaccines.

The development of the PPC and R&D roadmap followed WHO’s established procedures for the development of normative documents. An initial baseline situation analysis was conducted through a literature review to identify current Salmonella Paratyphi A-containing vaccines in the development pipeline, and to assess where critical data gaps that could hinder their progression to licensure, implementation, and public health decision-making were found, and how to address these in an equitable manner. The TAG-SV conducted an iterative consultative process, structured as a collaborative effort, through technical consultations, and these were supplemented with contributions from external observers from regulatory agencies and manufacturing companies when required. Both the PPC and R&D roadmap documents underwent an open, public consultation, which received input from academic experts, industry partners, and regulatory agencies, and were reviewed and endorsed by PDVAC in December 2024.

There are currently two bivalent conjugate products in development. Both products link the Salmonella Paratyphi A O:2 antigen to a carrier protein, and combine it with a conjugate construct of Salmonella Typhi Vi-polysaccharide. The product most advanced in the development pipeline, the Serum Institute India bivalent paratyphoid A-typhoid conjugate vaccine (SII-PTCV) uses tetanus toxin (TT) as the protein carrier for the Vi-polysaccharide from Salmonella Typhi, and diphtheria toxoid (DT) for the O:2 antigen. The vaccine recently completed a phase 1 study in India that included 60 adults aged 18-45 years who received a single dose of either SII-PTCV, or the monovalent typhoid conjugate vaccine Typbar-TCV. Participants were followed for up to 181 days. The vaccine had a good safety profile, with mostly mild adverse events being reported and no significant differences between the intervention and comparator groups. The vaccine was immunogenic against Vi with a 97.7% and 93.3% seroconversion for anti-Vi IgG and anti-Vi IgA, respectively, in the intervention group 29 days post-vaccination, which was comparable to that observed among Typbar-TCV recipients and was sustained to day 181. The SII-PTCV vaccine was also immunogenic against O:2 with an observed increase in Serum Bactericidal Activity (SBA) titers post-vaccination and a sustained 100% seroconversion among SII-PTCV vaccine recipients but not in the Typbar-TCV control group. ³²

A second bivalent conjugate product that uses O:2 and Vi-polysaccharide, both linked to the CRM ₁₉₇ carrier protein is currently being developed by the GSK Vaccines Institute for Global Health (GVGH). The O:2-CRM ₁₉₇ construct was shown to be immunogenic in pre-clinical studies in animal models, and reactive against a purposedly selected panel of clinical isolates. This vaccine is now progressing towards completion of phase 1 evaluation. ^{31 , 33}

In addition to conjugate products, an oral live-attenuated bivalent vaccine containing a Salmonella Typhi CVD909 strain, which is the main component of a licensed oral vaccine against typhoid fever – Ty21 - and the CVD1902 strain of Salmonella Paratyphi A is under development by the University of Maryland, in collaboration with Bharat Biotech International and the University of Oxford. An oral live-attenuated monovalent CVD1902 vaccine was tested in healthy volunteers who ingested either a single dose of an increasing number of colony-forming units (CFUs) up to 10 ¹⁰ or a placebo. The study showed that a single dose of at least 10 ⁹ CFUs of CVD1902? was capable of eliciting cell-mediated immune responses that had the potential to be protective against S. Paratyphi A infection. ³⁴ This vaccine has recently completed a CHIM, and preliminary results indicate that a CVD1902-containing oral vaccine is efficacious in protecting against S. Paratyphi A infection. The next step is now to combine the two strains: the typhoid CVD909 and paratyphoid CVD1902. ^{35 , 47}

Following the successful development and introduction of vaccines against typhoid fever, the logical next step is the development of a bivalent vaccine for broader control of enteric fever. Public health authorities across endemic and non-endemic countries have increasingly identified a bivalent vaccine containing a Salmonella Paratyphi A component as a critical need. Concern of increasing proportion of enteric fever cases caused by S. Paratyphi A, fear of a possible serovar replacement following the introduction of TCV, and increasing prevalence of AMR have driven demand. ^{8 , 45 , 48 – 50}

The characteristics and use case for a bivalent Salmonella Typhi/Paratyphi A vaccine are mainly driven by the characteristics and most recent recommendations for TCVs. As outlined in Table 1, the expectation would be for a vaccine that can prevent both typhoid and paratyphoid fever disease and their complications. The vaccine should be suitable to be safely administered from six months of age, and be suitable for administration through the routine infant immunization scheme as well as for its use in campaigns for outbreak control and other events of public health concern, such as an increase in intestinal perforations in a context where microbiologic confirmation of the underlying cause might be constrained. The bivalent vaccine would be expected to confer protection against typhoid fever non-inferior to that observed in TCVs, and be superior to naturally acquired immunity for the Salmonella Paratyphi A component. Regulatory recommendations for such vaccine have already been outlined by PDVAC and WHO, ^{36 , 37} and post marketing evaluations will be a requirement to corroborate safety, efficacy and immunogenicity findings, and to demonstrate non-interference between the two antigens in the bivalent vaccine, and among other vaccine antigens that might be co-administered within the vaccination schedule. ⁴⁰

Table 2 outlines the research gaps identified by the TAG-SV and other experts who contributed to the development of the document. ⁴¹ The development and implementation of TCVs prompted substantial investment from funders, governments, and other stakeholders to address epidemiology and burden of disease data gaps in preparation to TCV introductions into routine schedules and through immunization campaigns. Laboratory and population-based regional surveillance initiatives such as the Severe Typhoid in Africa (SETA) program, the Surveillance for Enteric Fever in Asia Project (SEAP), Surveillance for Enteric Fever in India (SEFI), and the Strategic Typhoid Alliance Across Africa and Asia (STRAATA) aimed at characterising the incidence and outcomes of enteric fever, with a focus on typhoid fever. ^{12 , 51 , 52} While these surveillance initiatives have shed light into the burden and distribution of paratyphoid fever and its contribution to the overall burden of enteric fever, substantial uncertainty remains regarding the true disease incidence – particularly by age group – due to small case numbers and resulting statistical limitations. n age sub-groups. ^{53 – 55} Moreover, while paratyphoid fever is recognised as endemic in Asia and the Middle East, its presence in sub-Saharan Africa is increasingly being documented. ^{56 – 59}

Traditionally, in the absence of laboratory capacity for complete Salmonella identification, the term enteric fever has been equated to typhoid fever, which has resulted in an underappreciation of the role of paratyphoid fever. Furthermore, when typing does not extend to distinguishing among Salmonella Paratyphi A, B, and C, paratyphoid fever may be assumed to be mostly caused by S. Paratyphi A. ^{60 , 61} Building microbiology diagnostic capacity in endemic settings, and developing rapid, easy-to-use, affordable diagnostic tests that can discriminate between invasive Salmonella serovars remain a priority to generate good quality, complete surveillance data suitable for decision-making. Although modelling provides insight, robust primary data remain essential.

One of the main requests from vaccine developers has been for clear guidance on the regulatory processes, and the role of CHIM to evaluate vaccine efficacy in the absence of phase 3 data. The WHO’s PDVAC endorsed an alternative regulatory pathway, which would consider the evaluation of efficacy data from a CHIM in non-endemic populations, paired with an immunogenicity and safety study in no less than 3,000 subjects in an endemic, target population, and provided there is commitment from manufacturers for post marketing evaluation of safety, effectiveness, and non-interference. ³⁶ WHO convened an expert consultation with academics, vaccine developers, and regulators, where all key stakeholders expressed their openness to license bivalent vaccines on those bases. ³⁷

Finally, while a favourable vaccine value profile for bivalent Salmonella Typhi/Paratyphi A vaccines was published in 2023, ⁴² decision-making stakeholders will still need evidence of the cost-benefit and cost-effectiveness of using a bivalent vaccine, and the cost implications in comparison to the use of a monovalent TCV. For this, it is imperative that key capacities are strengthened in-country to ensure nationally-owned and operated sustainable surveillance and reporting systems for enteric fever and other invasive Salmonella diseases, which will, in turn, facilitate the implementation of post marketing evaluations and continuous vigilance of vaccine safety events, and generate evidence-based information to support the implementation of sustainable funding for the use of bivalent vaccines, and market incentives to ensure vaccine supply.

Despite the advances in the control of enteric fever caused by Salmonella Typhi worldwide, the development of a vaccine that addresses enteric fever more broadly by adding protection against Salmonella Paratyphi A remains a priority for endemic countries. Moreover, the WHO’s vision calls for a broadly protective Salmonella vaccine for the prevention of invasive disease caused by both typhoidal and non-typhoidal Salmonella (NTS) serovars. Quadrivalent vaccines covering the four predominant Salmonella serovars: Salmonella Typhi, Paratyphi A, Typhimurium, and Enteritidis, are increasingly recognized as a strategic priority, particularly in the context of minimizing childhood injections. ⁶² The bivalent Salmonella Typhi/Paratyphi A vaccine represents a first step towards that vision and, in conjunction with national surveillance and laboratory capacity strengthening, and ongoing improvements in WASH practices, has the potential to significantly reduce the burden of infection and mitigate the spread of AMR in low-and-middle income countries, where populations are most impacted.