Efficacy of typhoid conjugate vaccine: final analysis of a 4-year, phase 3, randomised controlled trial in Malawian children.

BACKGROUND: Randomised controlled trials of typhoid conjugate vaccines among children in Africa and Asia have shown high short-term efficacy. Data on the durability of protection beyond 2 years are sparse. We present the final analysis of a randomised controlled trial in Malawi, encompassing more than 4 years of follow-up, with the aim of investigating vaccine efficacy over time and by age group. METHODS: In this phase 3, double-blind, randomised controlled efficacy trial in Blantyre, Malawi, healthy children aged 9 months to 12 years were randomly assigned (1:1) by an unmasked statistician to receive a single dose of Vi polysaccharide conjugated to tetanus toxoid vaccine (Vi-TT) or meningococcal capsular group A conjugate (MenA) vaccine. Children had to have no previous history of typhoid vaccination and reside in the study areas for inclusion and were recruited from government schools and health centres. Participants, their parents or guardians, and the study team were masked to vaccine allocation. Nurses administering vaccines were unmasked. We did surveillance for febrile illness from vaccination until follow-up completion. The primary outcome was first occurrence of blood culture-confirmed typhoid fever. Eligible children who were randomly assigned and vaccinated were included in the intention-to-treat analyses. This trial is registered at ClinicalTrials.gov, NCT03299426. FINDINGS: Between Feb 21, 2018, and Sept 27, 2018, 28 130 children were vaccinated; 14 069 were assigned to receive Vi-TT and 14 061 to receive MenA. After a median follow-up of 4·3 years (IQR 4·2-4·5), 24 (39·7 cases per 100 000 person-years) children in the Vi-TT group and 110 (182·7 cases per 100 000 person-years) children in the MenA group were diagnosed with a first episode of blood culture-confirmed typhoid fever. In the intention-to-treat population, efficacy of Vi-TT was 78·3% (95% CI 66·3-86·1), and 163 (129-222) children needed to be vaccinated to prevent one case. Efficacies by age group were 70·6% (6·4-93·0) for children aged 9 months to 2 years; 79·6% (45·8-93·9) for children aged 2-4 years; and 79·3% (63·5-89·0) for children aged 5-12 years. INTERPRETATION: A single dose of Vi-TT is durably efficacious for at least 4 years among children aged 9 months to 12 years and shows efficacy in all age groups, including children younger than 2 years. These results support current WHO recommendations in typhoid-endemic areas for mass campaigns among children aged 9 months to 15 years, followed by routine introduction in the first 2 years of life. FUNDING: Bill & Melinda Gates Foundation.

The safety and immunogenicity of a bivalent conjugate vaccine against Salmonella enterica Typhi and Paratyphi A in healthy Indian adults: a phase 1, randomised, active-controlled, double-blind trial.

BACKGROUND: Enteric fever caused by Salmonella enterica Typhi and Salmonella Paratyphi A is an important public health problem, especially in low-income and middle-income countries with limited access to safe water and sanitation. We present results from, to our knowledge, the first ever human study of a bivalent paratyphoid A-typhoid conjugate vaccine (Sii-PTCV). METHODS: In this double-blind phase 1 study, 60 healthy Indian adults were randomly assigned (1:1) to receive a single intramuscular dose of either Sii-PTCV or typhoid conjugate vaccine (Typbar-TCV). Safety was assessed by observing solicited adverse events for 1 week, unsolicited events for 1 month, and serious adverse events (SAEs) over 6 months. Immunogenicity at 1 month and 6 months was assessed by measuring anti-capsular polysaccharide antigen Vi (anti-Vi) IgG and IgA against Salmonella Typhi and anti-lipopolysaccharide (LPS) IgG against Salmonella Paratyphi A by ELISA, and functional antibodies using serum bactericidal assay (SBA) against Salmonella Paratyphi A. This study is registered with Clinical Trial Registry-India (CTRI/2022/06/043608) and is completed. FINDINGS: 60 participants were enrolled. Of these 60 participants, 57 (95%) participants were male and three (5%) participants were female. Solicited adverse events were observed in 27 (90%) of 30 participants who received Sii-PTCV and 26 (87%) of 30 participants who received Typbar-TCV. The most common local solicited event was pain in 27 (90%) participants who received Sii-PTCV and in 23 (77%) participants who received Typbar-TCV. The most common solicited systemic event was myalgia in five (17%) participants who received Sii-PTCV, whereas four (13%) participants who received Typbar-TCV had myalgia and four (13%) had headache. No vaccine-related unsolicited adverse events or SAEs were reported. The seroconversion rates on day 29 were 96·7% (95% CI 82·8-99·9) with Sii-PTCV and 100·0% (88·4-100·0) with Typbar-TCV for anti-Vi IgG; 93·3% (77·9-99·2) with Sii-PTCV and 100·0% (88·4-100·0) with Typbar-TCV for anti-Vi IgA; 100·0% (88·4-100·0) with Sii-PTCV and 3·3% (0·1-17·2) with Typbar-TCV for anti-LPS (paratyphoid); and 93·3% (77·9-99·2) with Sii-PTCV and 0% (0·0-11·6) with Typbar-TCV for SBA titres (paratyphoid). Paratyphoid anti-LPS immune responses were sustained at day 181. INTERPRETATION: Sii-PTCV was safe and immunogenic for both typhoid and paratyphoid antigens indicating its potential for providing comprehensive protection against enteric fever. FUNDING: Serum Institute of India.

The Identification of Enteric Fever-Specific Antigens for Population-Based Serosurveillance.

BACKGROUND: Enteric fever, caused by Salmonella enterica serovars Typhi and Paratyphi A, is a major public health problem in low- and middle-income countries. Moderate sensitivity and scalability of current methods likely underestimate enteric fever burden. Determining the serological responses to organism-specific antigens may improve incidence measures. METHODS: Plasma samples were collected from blood culture-confirmed enteric fever patients, blood culture-negative febrile patients over the course of 3 months, and afebrile community controls. A panel of 17 Salmonella Typhi and Paratyphi A antigens was purified and used to determine antigen-specific antibody responses by indirect ELISAs. RESULTS: The antigen-specific longitudinal antibody responses were comparable between enteric fever patients, patients with blood culture-negative febrile controls, and afebrile community controls for most antigens. However, we found that IgG responses against STY1479 (YncE), STY1886 (CdtB), STY1498 (HlyE), and the serovar-specific O2 and O9 antigens were greatly elevated over a 3-month follow up period in S. Typhi/S. Paratyphi A patients compared to controls, suggesting seroconversion. CONCLUSIONS: We identified a set of antigens as good candidates to demonstrate enteric fever exposure. These targets can be used in combination to develop more sensitive and scalable approaches to enteric fever surveillance and generate invaluable epidemiological data for informing vaccine policies. CLINICAL TRIAL REGISTRATION: ISRCTN63006567.

Environmental Surveillance for Salmonella Typhi and its Association With Typhoid Fever Incidence in India and Malawi.

BACKGROUND: Environmental surveillance (ES) for Salmonella Typhi potentially offers a low-cost tool to identify communities with a high burden of typhoid fever. METHODS: We developed standardized protocols for typhoid ES, including sampling site selection, validation, characterization; grab or trap sample collection, concentration; and quantitative PCR targeting Salmonella genes (ttr, staG, and tviB) and a marker of human fecal contamination (HF183). ES was implemented over 12 months in a historically high typhoid fever incidence setting (Vellore, India) and a lower incidence setting (Blantyre, Malawi) during 2021-2022. RESULTS: S. Typhi prevalence in ES samples was higher in Vellore compared with Blantyre; 39/520 (7.5%; 95% confidence interval [CI], 4.4%-12.4%) vs 11/533 (2.1%; 95% CI, 1.1%-4.0%) in grab and 79/517 (15.3%; 95% CI, 9.8%-23.0%) vs 23/594 (3.9%; 95% CI, 1.9%-7.9%) in trap samples. Detection was clustered by ES site and correlated with site catchment population in Vellore but not Blantyre. Incidence of culture-confirmed typhoid in local hospitals was low during the study and zero some months in Vellore despite S. Typhi detection in ES. CONCLUSIONS: ES describes the prevalence and distribution of S. Typhi even in the absence of typhoid cases and could inform vaccine introduction. Expanded implementation and comparison with clinical and serological surveillance will further establish its public health utility.

Safety and Efficacy of a Typhoid Conjugate Vaccine in Malawian Children.

BACKGROUND: Typhoid fever caused by multidrug-resistant H58 Salmonella Typhi is an increasing public health threat in sub-Saharan Africa. METHODS: We conducted a phase 3, double-blind trial in Blantyre, Malawi, to assess the efficacy of Vi polysaccharide typhoid conjugate vaccine (Vi-TCV). We randomly assigned children who were between 9 months and 12 years of age, in a 1:1 ratio, to receive a single dose of Vi-TCV or meningococcal capsular group A conjugate (MenA) vaccine. The primary outcome was typhoid fever confirmed by blood culture. We report vaccine efficacy and safety outcomes after 18 to 24 months of follow-up. RESULTS: The intention-to-treat analysis included 28,130 children, of whom 14,069 were assigned to receive Vi-TCV and 14,061 were assigned to receive the MenA vaccine. Blood culture-confirmed typhoid fever occurred in 12 children in the Vi-TCV group (46.9 cases per 100,000 person-years) and in 62 children in the MenA group (243.2 cases per 100,000 person-years). Overall, the efficacy of Vi-TCV was 80.7% (95% confidence interval [CI], 64.2 to 89.6) in the intention-to-treat analysis and 83.7% (95% CI, 68.1 to 91.6) in the per-protocol analysis. In total, 130 serious adverse events occurred in the first 6 months after vaccination (52 in the Vi-TCV group and 78 in the MenA group), including 6 deaths (all in the MenA group). No serious adverse events were considered by the investigators to be related to vaccination. CONCLUSIONS: Among Malawian children 9 months to 12 years of age, administration of Vi-TCV resulted in a lower incidence of blood culture-confirmed typhoid fever than the MenA vaccine. (Funded by the Bill and Melinda Gates Foundation; ClinicalTrials.gov number, NCT03299426.).

A TonB dependent transporter YncD of Salmonella enterica Serovar Typhi possesses vaccine potential.

Multiple TonB dependent transporters (TBDTs) contribute to bacterial virulence due to the importance roles that their substrates play in bacterial growth, and possess vaccine potential. A putative TBDT, YncD, had been identified as one of in vivo induced antigens during human infection of typhoid fever, and is required for the pathogenicity of Salmonella enterica Serovar Typhi. The present study was aimed to determine the function and immunogenicity of YncD. Homologous recombination method was used to construct an yncD-deletion mutant and cirA-iroN-fepA-deletion mutant from the wild-type S. Typhi Ty2. The growth of mutants and the wild-type strain were assessed in iron-deficient medium, as well as in human macrophage cells. Recombinant YncD protein was expressed and purified using Ni-NTA affinity chromatography and anion exchange. A mouse model was then used to evaluate the immunogenicity and protection efficacy of the recombinant YncD. Antibody levels, serum bactericidal efficiency, passive immune protection, opsonophagocysis were assayed to analyse the immunoprotection mechanism of the recombinant YncD. Our results showed that YncD is associated with the iron-uptake of S. Typhi. The yncD-deletion mutant displayed impaired growth in iron-deficient medium, comparable to that the cirA-iroN-fepA-deletion mutant did. The mutation of yncD markedly decreased bacterial growth within human macrophage cells. Moreover, subcutaneous immunization of mice with recombinant YncD elicited high levels of specific anti-YncD IgG, IgG1 and IgG2a, which protected the immunized mice against the intraperitoneal challenge of S. Typhi, and decreased bacterial burdens in the livers and spleens of the infected mice. Passive immunization using the immunized sera also efficiently protected the mice from the challenge of S. Typhi. Moreover, the immunized sera enhanced in vitro bactericidal activity of complement, and opsonophagocytosis. Our results showed that YncD displays a role in the iron-uptake of S. Typhi and possesses immunogenicity.

A dedicated C-6 ß-hydroxyacyltransferase required for biosynthesis of the glycolipid anchor for Vi antigen capsule in typhoidal Salmonella.

Vi antigen is an extracellular polysaccharide produced by Salmonella enterica Typhi, Citrobacter freundii, and some soil bacteria belonging to the Burkholderiales. In Salmonella Typhi, Vi-antigen capsule protects the bacterium against host defenses, and the glycan is used in a current glycoconjugate vaccine to protect against typhoid. Vi antigen is a glycolipid assembled in the cytoplasm and translocated to the cell surface by an export complex driven by an ABC transporter. In Salmonella Typhi, efficient export and cell-surface retention of the capsule layer depend on a reducing terminal acylated-HexNAc moiety. Although the precise structure and biosynthesis of the acylated terminus has not been resolved, it distinguishes Vi antigen from other known glycolipid substrates for bacterial ABC transporters. The genetic locus for Vi antigen-biosynthesis encodes a single acyltransferase candidate (VexE), which is implicated in the acylation process. Here, we determined the structure of the VexE in vitro reaction product by mass spectrometry and NMR spectroscopy to reveal that VexE catalyzes ß-hydroxyacyl-ACP dependent acylation of the activated sugar precursor, uridine-5'-diphospho-GlcNAc, at C-6 to form UDP-6-O-[ß-hydroxymyristoyl]-α-d-GlcNAc. VexE belongs to the lysophosphatidyl acyltransferase family, and comparison of an Alphafold VexE model to solved lysophosphatidyl acyltransferase structures, together with modeling enzyme:substrate complexes, led us to predict an enzyme mechanism. This study provides new insight into Vi terminal structure, offers a new model substrate to investigate the mechanism of glycolipid ABC transporters, and adds biochemical understanding for a novel reaction used in the synthesis of an important bacterial virulence factor.

Investigation of core machinery for biosynthesis of Vi antigen capsular polysaccharides in Gram-negative bacteria.

Salmonella enterica serovar Typhi causes typhoid fever. It possesses a Vi antigen capsular polysaccharide coat that is important for virulence and is the basis of a current glycoconjugate vaccine. Vi antigen is also produced by environmental Bordetella isolates, while mammal-adapted Bordetella species (such as Bordetella bronchiseptica) produce a capsule of undetermined structure that cross-reacts with antibodies recognizing Vi antigen. The Vi antigen backbone is composed of poly-α-(1→4)-linked N-acetylgalactosaminuronic acid, modified with O-acetyl residues that are necessary for vaccine efficacy. Despite its biological and biotechnological importance, some central aspects of Vi antigen production are poorly understood. Here we demonstrate that TviE and TviD, two proteins encoded in the viaB (Vi antigen production) locus, interact and are the Vi antigen polymerase and O-acetyltransferase, respectively. Structural modeling and site-directed mutagenesis reveal that TviE is a GT4-family glycosyltransferase. While TviD has no identifiable homologs beyond Vi antigen systems in other bacteria, structural modeling suggests that it belongs to the large SGNH hydrolase family, which contains other O-acetyltransferases. Although TviD possesses an atypical catalytic triad, its O-acetyltransferase function was verified by antibody reactivity and 13C NMR data for tviD-mutant polysaccharide. The B. bronchiseptica genetic locus predicts a mode of synthesis distinct from classical S. enterica Vi antigen production, but which still involves TviD and TviE homologs that are both active in a reconstituted S. Typhi system. These findings provide new insight into Vi antigen production and foundational information for the glycoengineering of Vi antigen production in heterologous bacteria.

Early transcriptional responses to human enteric fever challenge.

Enteric fever, caused by oral infection with typhoidal Salmonella serovars, presents as a non-specific febrile illness preceded by an incubation period of 5 days or more. The enteric fever human challenge model provides a unique opportunity to investigate the innate immune response during this incubation period, and how this response is altered by vaccination with the Vi polysaccharide or conjugate vaccine. We find that on the same day as ingestion of typhoidal Salmonella, there is already evidence of an immune response, with 199 genes upregulated in the peripheral blood transcriptome 12 hours post-challenge (false discovery rate <0.05). Gene sets relating to neutrophils, monocytes, and innate immunity were over-represented (false discovery rate <0.05). Estimating cell proportions from gene expression data suggested a possible increase in activated monocytes 12 hours post-challenge (P = 0.036, paired Wilcoxon signed-rank test). Furthermore, plasma TNF-α rose following exposure (P = 0.011, paired Wilcoxon signed-rank test). There were no significant differences in gene expression (false discovery rate <0.05) in the 12 hours response between those who did and did not subsequently develop clinical or blood culture confirmed enteric fever or between vaccination groups. Together, these results demonstrate early perturbation of the peripheral blood transcriptome after enteric fever challenge and provide initial insight into early mechanisms of protection.

Programmatic Effectiveness of a Pediatric Typhoid Conjugate Vaccine Campaign in Navi Mumbai, India.

BACKGROUND: The World Health Organization recommends vaccines for prevention and control of typhoid fever, especially where antimicrobial-resistant typhoid circulates. In 2018, the Navi Mumbai Municipal Corporation (NMMC) implemented a typhoid conjugate vaccine (TCV) campaign. The campaign targeted all children aged 9 months through 14 years within NMMC boundaries (approximately 320 000 children) over 2 vaccination phases. The phase 1 campaign occurred from 14 July 2018 through 25 August 2018 (71% coverage, approximately 113 420 children). We evaluated the phase 1 campaign's programmatic effectiveness in reducing typhoid cases at the community level. METHODS: We established prospective, blood culture-based surveillance at 6 hospitals in Navi Mumbai and offered blood cultures to children who presented with fever ≥3 days. We used a cluster-randomized (by administrative boundary) test-negative design to estimate the effectiveness of the vaccination campaign on pediatric typhoid cases. We matched test-positive, culture-confirmed typhoid cases with up to 3 test-negative, culture-negative controls by age and date of blood culture and assessed community vaccine campaign phase as an exposure using conditional logistic regression. RESULTS: Between 1 September 2018 and 31 March 2021, we identified 81 typhoid cases and matched these with 238 controls. Cases were 0.44 times as likely to live in vaccine campaign communities (programmatic effectiveness, 56%; 95% confidence interval [CI], 25% to 74%; P = .002). Cases aged ≥5 years were 0.37 times as likely (95% CI, .19 to .70; P = .002) and cases during the first year of surveillance were 0.30 times as likely (95% CI, .14 to .64; P = .002) to live in vaccine campaign communities. CONCLUSIONS: Our findings support the use of TCV mass vaccination campaigns as effective population-based tools to combat typhoid fever.

Potential of a novel flagellin epitope as a broad-spectrum vaccine candidate against enteric fever.

Relentless emergence of antibiotic resistant Salmonella strains, coupled with the drawbacks associated with currently available vaccines against enteric fever, warrants an urgent need to look for new vaccine candidates. Out of the multiple virulence factors harbored by Salmonella, flagella are regarded as one of the most important targets of innate as well as adaptive immune response. Individual Salmonella serotypes alternate between expression of two different antigenic forms encoded by fliC and fljB genes, respectively thereby employing this as a strategy to escape the host immune response. In the present study, using various immunoinformatic approaches, a flagellin epitope, present in both antigenic forms of typhoidal Salmonellae has been targeted. Following B-cell epitope and B-cell derived T-cell epitope prediction and interaction studies with major histocompatibility complexes using molecular docking, a peptide epitope was selected. Further, it was screened for its presence in majority of typhoidal serovars along with other useful attributes, in silico. Thereafter, safety studies were performed with the synthesized peptide. Subsequently, immunization studies were carried out using S. Typhi as well as S. Paratyphi A induced murine peritonitis model. Active immunization with peptide-BSA conjugate resulted in 75% and 80% mice survival following lethal challenge with S. Typhi and S. Paratyphi A respectively, along with a significant IgG antibody titer, thereby highlighting its immunogenic potential. Reduced bacterial burden in vital organs along with improved histoarchitecture and cytokine levels further substantiated the protective efficacy of the proposed candidate. Passive immunization studies with the candidate verified the protective efficacy of the generated antibodies against lethal challenge of bacteria in mice. Given the endemic nature of enteric fever and the antigenic variability observed in Salmonella serotypes, present study highlights the importance of using a vaccine candidate, which, along with generating a strong immune response, also exhibits a broad coverage against both, S. Typhi as well as S. Paratyphi A strains.

Typhoid Fever Surveillance, Incidence Estimates, and Progress Toward Typhoid Conjugate Vaccine Introduction - Worldwide, 2018-2022.

Typhoid fever, an acute febrile illness caused by Salmonella enterica serovar Typhi (S. Typhi), is endemic in many low- and middle-income countries† (1). In 2015, an estimated 11-21 million typhoid fever cases and 148,000-161,000 associated deaths occurred worldwide (2). Effective prevention strategies include improved access to and use of infrastructure supporting safe water, sanitation, and hygiene (WASH); health education; and vaccination (1). The World Health Organization (WHO) recommends programmatic use of typhoid conjugate vaccines for typhoid fever control and prioritization of vaccine introduction in countries with the highest typhoid fever incidence or high prevalence of antimicrobial-resistant S. Typhi (1). This report describes typhoid fever surveillance, incidence estimates, and the status of typhoid conjugate vaccine introduction during 2018-2022. Because routine surveillance for typhoid fever has low sensitivity, population-based studies have guided estimates of case counts and incidence in 10 countries since 2016 (3-6). In 2019, an updated modeling study estimated that 9.2 million (95% CI = 5.9-14.1) typhoid fever cases and 110,000 (95% CI = 53,000-191,000) deaths occurred worldwide, with the highest estimated incidence in the WHO South-East Asian (306 cases per 100,000 persons), Eastern Mediterranean (187), and African (111) regions (7). Since 2018, five countries (Liberia, Nepal, Pakistan, Samoa [based on self-assessment], and Zimbabwe) with estimated high typhoid fever incidence (≥100 cases per 100,000 population per year) (8), high antimicrobial resistance prevalence, or recent outbreaks introduced typhoid conjugate vaccines into their routine immunization programs (2). To guide vaccine introduction decisions, countries should consider all available information, including surveillance of laboratory-confirmed cases, population-based and modeling studies, and outbreak reports. Establishing and strengthening typhoid fever surveillance will be important to measure vaccine impact.

Associations of water, sanitation, and hygiene with typhoid fever in case-control studies: a systematic review and meta-analysis.

BACKGROUND: Water, sanitation, and hygiene (WASH) play a pivotal role in controlling typhoid fever, as it is primarily transmitted through oral-fecal pathways. Given our constrained resources, staying current with the most recent research is crucial. This ensures we remain informed about practical insights regarding effective typhoid fever control strategies across various WASH components. We conducted a systematic review and meta-analysis of case-control studies to estimate the associations of water, sanitation, and hygiene exposures with typhoid fever. METHODS: We updated the previous review conducted by Brockett et al. We included new findings published between June 2018 and October 2022 in Web of Science, Embase, and PubMed. We used the Risk of Bias in Non-Randomized Studies of Interventions (ROBINS-I) tool for risk of bias (ROB) assessment. We classified WASH exposures according to the classification provided by the WHO/UNICEF Joint Monitoring Programme for Water Supply, Sanitation, and Hygiene (JMP) update in 2015. We conducted the meta-analyses by only including studies that did not have a critical ROB in both Bayesian and frequentist random-effects models. RESULTS: We identified 8 new studies and analyzed 27 studies in total. Our analyses showed that while the general insights on the protective (or harmful) impact of improved (or unimproved) WASH remain the same, the pooled estimates of OR differed. Pooled estimates of limited hygiene (OR = 2.26, 95% CrI: 1.38 to 3.64), untreated water (OR = 1.96, 95% CrI: 1.28 to 3.27) and surface water (OR = 2.14, 95% CrI: 1.03 to 4.06) showed 3% increase, 18% decrease, and 16% increase, respectively, from the existing estimates. On the other hand, improved WASH reduced the odds of typhoid fever with pooled estimates for improved water source (OR = 0.54, 95% CrI: 0.31 to 1.08), basic hygiene (OR = 0.6, 95% CrI: 0.38 to 0.97) and treated water (OR = 0.54, 95% CrI: 0.36 to 0.8) showing 26% decrease, 15% increase, and 8% decrease, respectively, from the existing estimates. CONCLUSIONS: The updated pooled estimates of ORs for the association of WASH with typhoid fever showed clear changes from the existing estimates. Our study affirms that relatively low-cost WASH strategies such as basic hygiene or water treatment can be an effective tool to provide protection against typhoid fever in addition to other resource-intensive ways to improve WASH. TRIAL REGISTRATION: PROSPERO 2021 CRD42021271881.

Cost-effectiveness analysis of typhoid conjugate vaccines in an outbreak setting: a modeling study.

BACKGROUND: Several prolonged typhoid fever epidemics have been reported since 2010 throughout eastern and southern Africa, including Malawi, caused by multidrug-resistant Salmonella Typhi. The World Health Organization recommends the use of typhoid conjugate vaccines (TCVs) in outbreak settings; however, current data are limited on how and when TCVs might be introduced in response to outbreaks. METHODOLOGY: We developed a stochastic model of typhoid transmission fitted to data from Queen Elizabeth Central Hospital in Blantyre, Malawi from January 1996 to February 2015. We used the model to evaluate the cost-effectiveness of vaccination strategies over a 10-year time horizon in three scenarios: (1) when an outbreak is likely to occur; (2) when an outbreak is unlikely to occur within the next ten years; and (3) when an outbreak has already occurred and is unlikely to occur again. We considered three vaccination strategies compared to the status quo of no vaccination: (a) preventative routine vaccination at 9 months of age; (b) preventative routine vaccination plus a catch-up campaign to 15 years of age; and (c) reactive vaccination with a catch-up campaign to age 15 (for Scenario 1). We also explored variations in outbreak definitions, delays in implementation of reactive vaccination, and the timing of preventive vaccination relative to the outbreak. RESULTS: Assuming an outbreak occurs within 10 years, we estimated that the various vaccination strategies would prevent a median of 15-60% of disability-adjusted life-years (DALYs). Reactive vaccination was the preferred strategy for WTP values of $0-300 per DALY averted. For WTP values > $300, introduction of preventative routine TCV immunization with a catch-up campaign was the preferred strategy. Routine vaccination with a catch-up campaign was cost-effective for WTP values above $890 per DALY averted if no outbreak occurs and > $140 per DALY averted if implemented after the outbreak has already occurred. CONCLUSIONS: Countries for which the spread of antimicrobial resistance is likely to lead to outbreaks of typhoid fever should consider TCV introduction. Reactive vaccination can be a cost-effective strategy, but only if delays in vaccine deployment are minimal; otherwise, introduction of preventive routine immunization with a catch-up campaign is the preferred strategy.

Immunisation used for offensive and defensive purposes during the Second World War.

The best defence against natural or intentional biological agents during armed conflict is usually immunisation, as with typhoid fever, but exceptional circumstances are informative. A large iatrogenic epidemic of hepatitis B occurred in 1942 due to contaminated lots of yellow fever (YF) vaccine used in the US military, even though there was no natural risk of infection. YF vaccine was intended to protect against Japanese Army's use of YF as a biowarfare agent, which did not eventuate. Salmonella typhi was used to infect German soldiers in a Paris café during Christmas 1941 in the knowledge that the café staff but not the soldiers were likely to be immunised against typhoid fever. German Army use of the Weil-Felix reaction to eliminate civilians likely to be typhus infected was subverted by Polish medical officers. They immunised civilians with locally produced Proteus antigens to create false-positive Weil-Felix reactions in order to exempt men from forced labour schemes. Immunisation against biowarfare agents has a mixed record, indicating that vaccines rarely cover well for intelligence gaps.

Cost-effectiveness analysis of typhoid vaccination in Lao PDR.

BACKGROUND: Typhoid vaccination has been shown to be an effective intervention to prevent enteric fever and is under consideration for inclusion in the national immunization program in Lao PDR. METHODS: A cost-utility analysis was performed using an age-structured static decision tree model to estimate the costs and health outcomes of introducing TCV. Vaccination strategies combined with five delivery approaches in different age groups compared to no vaccination were considered from the societal perspective, using the Gavi price of 1.5 USD per dose. The vaccination program was considered to be cost-effective if the incremental cost-effectiveness ratio was less than a threshold of 1 GDP per capita for Lao PDR, equivalent to USD 2,535 in 2020. RESULTS: In the model, we estimated 172.2 cases of enteric fever, with 1.3 deaths and a total treatment cost of USD 7,244, based on a birth cohort of 164,662 births without TCV vaccination that was followed over their lifetime. To implement a TCV vaccination program over the lifetime horizon, the estimated cost of the vaccine and administration costs would be between USD 470,934 and USD 919,186. Implementation of the TCV vaccination program would prevent between 14 and 106 cases and 0.1 to 0.8 deaths. None of the vaccination programs appeared to be cost-effective. CONCLUSIONS: Inclusion of TCV in the national vaccination program in Lao PDR would only be cost-effective if the true typhoid incidence is 25-times higher than our current estimate.

Short Vi-polysaccharide abrogates T-independent immune response and hyporesponsiveness elicited by long Vi-CRM197 conjugate vaccine.

Polysaccharide-protein conjugates have been developed to overcome the T-independent response, hyporesponsiveness to repeated vaccination, and poor immunogenicity in infants of polysaccharides. To address the impact of polysaccharide length, typhoid conjugates made with short- and long-chain fractions of Vi polysaccharide with average sizes of 9.5, 22.8, 42.7, 82.0, and 165 kDa were compared. Long-chain-conjugated Vi (165 kDa) induced a response in both wild-type and T cell-deficient mice, suggesting that it maintains a T-independent response. In marked contrast, short-chain Vi (9.5 to 42.7 kDa) conjugates induced a response in wild-type mice but not in T cell-deficient mice, suggesting that the response is dependent on T cell help. Mechanistically, this was explained in neonatal mice, in which long-chain, but not short-chain, Vi conjugate induced late apoptosis of Vi-specific B cells in spleen and early depletion of Vi-specific B cells in bone marrow, resulting in hyporesponsiveness and lack of long-term persistence of Vi-specific IgG in serum and IgG+ antibody-secreting cells in bone marrow. We conclude that while conjugation of long-chain Vi generates T-dependent antigens, the conjugates also retain T-independent properties, leading to detrimental effects on immune responses. The data reported here may explain some inconsistencies observed in clinical trials and help guide the design of effective conjugate vaccines.

An integrated in silico based subtractive genomics and reverse vaccinology approach for the identification of novel vaccine candidate and chimeric vaccine against XDR Salmonella typhi H58.

Salmonella typhi is notorious for causing enteric fever which is also known as typhoid fever. It emerged as an extreme drug resistant strain that requires urgent attention to prevent its global spread. Statistically, about 11-17 million typhoid illnesses are reported worldwide annually. The only alternative approach for the control of this illness is proper vaccination. However, available typhoid vaccine has certain limitations such as poor long-term efficacy, and non-recommendation for below 6 years children, which opens the avenues for designing new vaccines to overcome such limitations. Computational-based reverse vaccinology along with subtractive genomics analysis is one of the robust approaches used for the prioritization of vaccine candidates through direct screening of genome sequence assemblies. In the current study, we have successfully designed a peptide-based novel antigen chimeric vaccine candidate against the XDR strain of S. typhi H58. The pipeline revealed four peptides from WP_001176621.1 i.e., peptidoglycan-associated lipoprotein Pal and two peptides from WP_000747548.1 i.e., OmpA family lipoprotein as promising target for the induction of immune response against S. typhi. The six epitopes from both proteins were found as immunogenic, antigenic, virulent, highly conserved, nontoxic, and non-allergenic among whole Salmonella H58 proteome. Furthermore, the binding interaction between a chimeric vaccine and human population alleles was unveiled through structure-based studies. So far, these proteins have never been characterized as vaccine targets against S. typhi. The current study proposed that construct V2 could be a significant vaccine candidate against S. typhi H58. However, to ascertain this, future experimental holistic studies are recommended as follow-up.

Prevention of Typhoid by Vi Conjugate Vaccine and Achievable Improvements in Household Water, Sanitation, and Hygiene: Evidence From a Cluster-Randomized Trial in Dhaka, Bangladesh.

BACKGROUND: Typhoid fever contributes to approximately 135 000 deaths annually. Achievable improvements in household water, sanitation, and hygiene (WASH) combined with vaccination using typhoid conjugate vaccines (TCVs) may be an effective preventive strategy. However, little is known about how improved WASH and vaccination interact to lower the risk of typhoid. METHODS: A total of 61 654 urban Bangladeshi children aged 9 months to <16 years, residing in 150 clusters with a baseline population of 205 760 residents, were randomized 1:1 by cluster to Vi-tetanus toxoid TCV or Japanese encephalitis (JE) vaccine. Surveillance for blood culture-confirmed typhoid fever was conducted over 2 years. Existing household WASH status was assessed at baseline as Better or Not Better using previously validated criteria. The reduction in typhoid risk among all residents associated with living in TCV clusters, Better WASH households, or both was evaluated using mixed-effects Poisson regression models. RESULTS: The adjusted reduced risk of typhoid among all residents living in the clusters assigned to TCV was 55% (95% confidence interval [CI], 43%-65%; P < .001), and that of living in Better WASH households, regardless of cluster, was 37% (95% CI, 24%-48%; P < .001). The highest risk of typhoid was observed in persons living in households with Not Better WASH in the JE clusters. In comparison with these persons, those living in households with Better WASH in the TCV clusters had an adjusted reduced risk of 71% (95% CI, 59%-80%; P < .001). CONCLUSIONS: Implementation of TCV programs combined with achievable and culturally acceptable household WASH practices were independently associated with a significant reduction in typhoid risk. CLINICAL TRIALS REGISTRATION: ISRCTN11643110.

Protection by vaccination of children against typhoid fever with a Vi-tetanus toxoid conjugate vaccine in urban Bangladesh: a cluster-randomised trial.

BACKGROUND: Typhoid fever remains a major cause of morbidity and mortality in low-income and middle-income countries. Vi-tetanus toxoid conjugate vaccine (Vi-TT) is recommended by WHO for implementation in high-burden countries, but there is little evidence about its ability to protect against clinical typhoid in such settings. METHODS: We did a participant-masked and observer-masked cluster-randomised trial preceded by a safety pilot phase in an urban endemic setting in Dhaka, Bangladesh. 150 clusters, each with approximately 1350 residents, were randomly assigned (1:1) to either Vi-TT or SA 14-14-2 Japanese encephalitis (JE) vaccine. Children aged 9 months to less than 16 years were invited via parent or guardian to receive a single, parenteral dose of vaccine according to their cluster of residence. The study population was followed for an average of 17·1 months. Total and overall protection by Vi-TT against blood culture-confirmed typhoid were the primary endpoints assessed in the intention-to-treat population of vaccinees or all residents in the clusters. A subset of approximately 4800 participants was assessed with active surveillance for adverse events. The trial is registered at www.isrctn.com, ISRCTN11643110. FINDINGS: 41 344 children were vaccinated in April-May, 2018, with another 20 412 children vaccinated at catch-up vaccination campaigns between September and December, 2018, and April and May, 2019. The incidence of typhoid fever (cases per 100 000 person-years) was 635 in JE vaccinees and 96 in Vi-TT vaccinees (total Vi-TT protection 85%; 97·5% CI 76 to 91, p<0·0001). Total vaccine protection was consistent in different age groups, including children vaccinated at ages under 2 years (81%; 95% CI 39 to 94, p=0·0052). The incidence was 213 among all residents in the JE clusters and 93 in the Vi-TT clusters (overall Vi-TT protection 57%; 97·5% CI 43 to 68, p<0·0001). We did not observe significant indirect vaccine protection by Vi-TT (19%; 95% CI -12 to 41, p=0·20). The vaccines were well tolerated, and no serious adverse events judged to be vaccine-related were observed. INTERPRETATION: Vi-TT provided protection against typhoid fever to children vaccinated between 9 months and less than 16 years. Longer-term follow-up will be needed to assess the duration of protection and the need for booster doses. FUNDING: The study was funded by the Bill & Melinda Gates Foundation.