Typhoid Vaccine Acceleration Consortium Malawi: A Phase III, Randomized, Double-blind, Controlled Trial of the Clinical Efficacy of Typhoid Conjugate Vaccine Among Children in Blantyre, Malawi

Modelling Salmonella Typhi in high-density urban Blantyre neighbourhood, Malawi, using point pattern methods

Salmonella Typhi is a human-restricted pathogen that is transmitted by the faecal-oral route and causative organism of typhoid fever. Using health facility data from 2016 to 2020, this study focuses on modelling the spatial variation in typhoid risk in Ndirande township in Blantyre. To pursue this objective, we developed a marked inhomogeneous Poisson process model that allows us to incorporate both individual-level and environmental risk factors. The results from our analysis indicate that typhoid cases are spatially clustered, with the incidence decreasing by 54% for a unit increase in the water, sanitation, and hygiene (WASH) score. Typhoid intensity was also higher in children aged below 18 years than in adults. However, our results did not show evidence of a strong temporal variation in typhoid incidence. We also discuss the inferential benefits of using point pattern models to characterise the spatial variation in typhoid risk and outline possible extensions of the proposed modelling framework.

Typhoid intestinal perforation in Francophone Africa, a scoping review

Typhoid intestinal perforation (TIP) is a leading cause of peritonitis and indication for emergency surgery in Africa, with reported mortality rates up to 30% in pediatric patients. Currently, data on TIP in Western databases are primarily from countries that speak English, likely due to non-English publication and citation biases. Despite the high burden of infectious diseases in Francophone Africa, data from these countries regarding TIP remain limited. This study aims to highlight the incidence and morbidity of TIP in Francophone African countries using an extended search algorithm. We conducted a scoping review using the PubMed, EMBASE, and SCOPUS databases with the keywords "peritonitis", "non-traumatic ileal perforation", and "typhoid" in Francophone African countries. Additionally, we contacted surgeons in Africa and concurrently used citation chasing to obtain data not found in western databases. In total, 32 studies from 12 countries were identified and included in this review. A total of 22 publications were in French. Patient median age was 20 years and TIP caused a median of 35% of acute peritonitis cases. Mortality rates ranged from 6-37% (median: 16%). Rate of complications ranged from 15-92% (median: 46%). Ileostomy creation as a treatment for TIP varied between hospitals (0-79%), with the highest rates reported in Niger. In Francophone Africa, TIP is associated with high morbidity and mortality, most commonly in children and young adults. Interventions, including improved sanitation and the introduction of typhoid conjugate vaccines into routine vaccination programs, have the potential to significantly decrease typhoid fever and its complications.

Evaluating the relationship between ciprofloxacin prescription and non-susceptibility in Salmonella Typhi in Blantyre, Malawi: an observational study

BACKGROUND

Ciprofloxacin is the first-line drug for treating typhoid fever in many countries in Africa with a high disease burden, but the emergence of non-susceptibility poses a challenge to public health programmes. Through enhanced surveillance as part of vaccine evaluation, we investigated the occurrence and potential determinants of ciprofloxacin non-susceptibility in Blantyre, Malawi.

METHODS

We conducted systematic surveillance of typhoid fever cases and antibiotic prescription in two health centres in Blantyre, Malawi, between Oct 1, 2016, and Oct 31, 2019, as part of the STRATAA and TyVAC studies. In addition, blood cultures were taken from eligible patients presenting at Queen Elizabeth Central Hospital, Blantyre, as part of routine diagnosis. Inclusion criteria were measured or reported fever, or clinical suspicion of sepsis. Microbiologically, we identified Salmonella enterica serotype Typhi (S Typhi) isolates with a ciprofloxacin non-susceptible phenotype from blood cultures, and used whole-genome sequencing to identify drug-resistance mutations and phylogenetic relationships. We constructed generalised linear regression models to investigate associations between the number of ciprofloxacin prescriptions given per month to study participants and the proportion of S Typhi isolates with quinolone resistance-determining region (QRDR) mutations in the following month.

FINDINGS

From 46 989 blood cultures from Queen Elizabeth Central Hospital, 502 S Typhi isolates were obtained, 30 (6%) of which had either decreased ciprofloxacin susceptibility, or ciprofloxacin resistance. From 11 295 blood cultures from STRATAA and TyVAC studies, 241 microbiologically confirmed cases of typhoid fever were identified, and 198 isolates from 195 participants sequenced (mean age 12·8 years [SD 10·2], 53% female, 47% male). Between Oct 1, 2016, and Aug 31, 2019, of 177 typhoid fever cases confirmed by whole-genome sequencing, four (2%) were caused by S Typhi with QRDR mutations, compared with six (33%) of 18 cases between Sept 1 and Oct 31, 2019. This increase was associated with a preceding spike in ciprofloxacin prescriptions. Every additional prescription of ciprofloxacin given to study participants in the preceding month was associated with a 4·2% increase (95% CI 1·8-7·0) in the relative risk of isolating S Typhi with a QRDR mutation (p=0·0008). Phylogenetic analysis showed that S Typhi isolates with QRDR mutations from September and October, 2019, belonged to two distinct subclades encoding two different QRDR mutations, and were closely related (4-10 single-nucleotide polymorphisms) to susceptible S Typhi endemic to Blantyre.

INTERPRETATION

We postulate a causal relationship between increased ciprofloxacin prescriptions and an increase in fluoroquinolone non-susceptibility in S Typhi. Decreasing ciprofloxacin use by improving typhoid diagnostics, and reducing typhoid fever cases through the use of an efficacious vaccine, could help to limit the emergence of resistance.

FUNDING

Wellcome Trust, Bill & Melinda Gates Foundation, and National Institute for Health and Care Research (UK).

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.

Typhoid fever

Typhoid fever is an invasive bacterial disease associated with bloodstream infection that causes a high burden of disease in Africa and Asia. Typhoid primarily affects individuals ranging from infants through to young adults. The causative organism, Salmonella enterica subsp. enterica serovar Typhi is transmitted via the faecal-oral route, crossing the intestinal epithelium and disseminating to systemic and intracellular sites, causing an undifferentiated febrile illness. Blood culture remains the practical reference standard for diagnosis of typhoid fever, where culture testing is available, but novel diagnostic modalities are an important priority under investigation. Since 2017, remarkable progress has been made in defining the global burden of both typhoid fever and antimicrobial resistance; in understanding disease pathogenesis and immunological protection through the use of controlled human infection; and in advancing effective vaccination programmes through strategic multipartner collaboration and targeted clinical trials in multiple high-incidence priority settings. This Primer thus offers a timely update of progress and perspective on future priorities for the global scientific community.

Vi polysaccharide and conjugated vaccines afford similar early, IgM or IgG-independent control of infection but boosting with conjugated Vi vaccines sustains the efficacy of immune responses

Introduction

Vaccination with Vi capsular polysaccharide (Vi-PS) or protein-Vi typhoid conjugate vaccine (TCV) can protect adults against Salmonella Typhi infections. TCVs offer better protection than Vi-PS in infants and may offer better protection in adults. Potential reasons for why TCV may be superior in adults are not fully understood.

Methods and results

Here, we immunized wild-type (WT) mice and mice deficient in IgG or IgM with Vi-PS or TCVs (Vi conjugated to tetanus toxoid or CRM197) for up to seven months, with and without subsequent challenge with Vi-expressing Salmonella Typhimurium. Unexpectedly, IgM or IgG alone were similarly able to reduce bacterial burdens in tissues, and this was observed in response to conjugated or unconjugated Vi vaccines and was independent of antibody being of high affinity. Only in the longer-term after immunization (>5 months) were differences observed in tissue bacterial burdens of mice immunized with Vi-PS or TCV. These differences related to the maintenance of antibody responses at higher levels in mice boosted with TCV, with the rate of fall in IgG titres induced to Vi-PS being greater than for TCV.

Discussion

Therefore, Vi-specific IgM or IgG are independently capable of protecting from infection and any superior protection from vaccination with TCV in adults may relate to responses being able to persist better rather than from differences in the antibody isotypes induced. These findings suggest that enhancing our understanding of how responses to vaccines are maintained may inform on how to maximize protection afforded by conjugate vaccines against encapsulated pathogens such as S. Typhi.

Typhoid conjugate vaccine effectiveness in Malawi: evaluation of a test-negative design using randomised, controlled clinical trial data

BACKGROUND

Typhoid conjugate vaccines are being introduced in low-income and middle-income countries to prevent typhoid illness in children. Vaccine effectiveness studies assess vaccine performance after introduction. The test-negative design is a commonly used method to estimate vaccine effectiveness that has not been applied to typhoid vaccines because of concerns over blood culture insensitivity. The overall aim of the study was to evaluate the appropriateness of using a test-negative design to assess typhoid Vi polysaccharide-tetanus toxoid conjugate vaccine (Vi-TT) effectiveness using a gold standard randomised controlled trial database.

METHODS

Using blood culture data from a randomised controlled trial of Vi-TT in Malawi, we simulated a test-negative design to derive vaccine effectiveness estimates using three different approaches and compared these to randomised trial efficacy results. In the randomised trial, 27 882 children aged 9 months to 12 years were randomly assigned (1:1) to receive a single dose of Vi-TT or meningococcal capsular group A conjugate vaccine between Feb 21 and Sept 27, 2018, and were followed up for blood culture-confirmed typhoid fever until Sept 30, 2021.

FINDINGS

For all three test-negative design approaches, vaccine effectiveness estimates (test-negative design A, 80·3% [95% CI 66·2 to 88·5] vs test-negative design B, 80·5% [66·5 to 88·6] vs test-negative design C, 80·4% [66·9 to 88·4]) were almost identical to the randomised trial results (80·4% [95% CI 66·4 to 88·5]). Receipt of Vi-TT did not affect the risk of non-typhoid fever (vaccine efficacy against non-typhoid fever -0·4% [95% CI -4·9 to 3·9] vs -1% [-5·6 to 3·3] vs -2·5% [-6·4 to 1·3] for test-negative design A, test-negative design B, and test-negative design C, respectively).

INTERPRETATION

This study validates the test-negative design core assumption for typhoid vaccine effectiveness estimation and shows the accuracy and precision of the estimates compared with the randomised controlled trial. These results show that the test-negative design is suitable for assessing typhoid conjugate vaccine effectiveness in post-introduction studies using blood culture surveillance.

FUNDING

Bill & Melinda Gates Foundation.

Safety and immunogenicity of a typhoid conjugate vaccine among children aged 9 months to 12 years in Malawi: a nested substudy of a double-blind, randomised controlled trial

BACKGROUND

Typhoid fever is a substantial public health problem in Africa, yet there are few clinical trials of typhoid conjugate vaccine (TCV). We assessed immunogenicity and safety of Typbar TCV in Malawi.

METHODS

This substudy was nested within a phase 3, double-blind, parallel design, randomised controlled trial of TCV in children from Ndirande Health Centre in Ndirande township, Blantyre, Malawi. To be eligible, participants had to be aged between 9 months and 12 years with no known immunosuppression or chronic health conditions, including HIV or severe malnutrition; eligible participants were enrolled into three strata of approximately 200 children (9-11 months, 1-5 years, and 6-12 years), randomly assigned (1:1) to receive TCV or control (meningococcal serogroup A conjugate vaccine [MCV-A]) intramuscularly. Serum was collected before vaccination and at 28 days and 730-1035 days after vaccination to measure anti-Vi antibodies by ELISA. Because of COVID-19, day 730 visits were extended up to 1035 days. This nested substudy evaluated reactogenicity, safety, and immunogenicity by age stratum. Safety outcomes, analysed in the intention-to-treat population, included solicited adverse events within 7 days of vaccination (assessed on 3 separate days) and unsolicited adverse events within 28 days of vaccination. This trial is registered with ClinicalTrials.gov, NCT03299426.

FINDINGS

Between Feb 22 and Sept 6, 2018, 664 participants were screened, and 631 participants were enrolled and randomly assigned (320 to the TCV group and 311 to the MCV-A group). 305 participants in the TCV group and 297 participants in the MCV-A group were vaccinated. Among TCV recipients, anti-Vi IgG geometric mean titres increased more than 500 times from 4·2 ELISA units (EU)/mL (95% CI 4·0-4·4) at baseline to 2383·7 EU/mL (2087·2-2722·3) at day 28, then decreased to 48·0 EU/mL (39·9-57·8) at day 730-1035, remaining more than 11 times higher than baseline. Among MCV-A recipients, anti-Vi IgG titres remained unchanged: 4·3 EU/mL (4·0-4·5) at baseline, 4·4 EU/mL (4·0-4·7) on day 28, and 4·6 EU/mL (4·2-5·0) on day 730-1035. TCV and MCV-A recipients had similar solicited local (eight [3%] of 304, 95% CI 1·3-5·1 and three [1%] of 293, 0·4-3·0) and systemic (27 [9%] of 304, 6·2-12·6 and 27 [9%] of 293, 6·4-13·1) reactogenicity. Related unsolicited adverse events occurred similarly in TCV and MCV-A recipients in eight (3%) of 304 (1·3-5·1) and eight (3%) of 293 (1·4-5·3).

INTERPRETATION

This study provides evidence of TCV safety, tolerability, and immunogenicity up to 730-1035 days in Malawian children aged 9 months to 12 years.

FUNDING

Bill & Melinda Gates Foundation.

Projecting the cost of introducing typhoid conjugate vaccine (TCV) in the national immunization program in Malawi using a standardized costing framework

Background

There is a substantial typhoid burden in sub-Saharan Africa, and TCV has been introduced in two African countries to date. Decision-makers in Malawi decided to introduce TCV and applied for financial support from Gavi, the Vaccine Alliance in 2020. The current plan is to introduce TCV as part of the national immunization program in late 2022. The introduction will include a nationwide campaign targeting all children aged 9 months to 15 years. Following the campaign, TCV will be provided through routine immunization at 9 months. This study aims to estimate the cost of TCV introduction and recurrent delivery as part of the national immunization program.

Methods

This costing analysis is conducted from the government's perspective and focuses on projecting the incremental cost of TCV introduction and delivery for Malawi’s existing immunization program before vaccine introduction. The study uses a costing tool developed by Levin & Morgan through a partnership between the International Vaccine Institute and the World Health Organization and leverages primary and secondary data collected through key informant interviews with representatives of the Malawi Expanded Programme on Immunization team at various levels.

Results

The total financial and economic costs of TCV introduction over three years in Malawi are projected to be US$8.5 million and US$29.8 million, respectively. More than two-thirds of the total cost is made up of recurrent costs. Major cost drivers include the procurement of vaccines and injection supplies and service delivery costs. Without vaccine cost, we estimate the cost per child immunized to be substantially lower than US$1.

Discussion

Findings from this analysis may be used to assess the economic implications of introducing TCV in Malawi. Major cost drivers highlighted by the analysis may also inform decision-makers in the region as they assess the value and feasibility of TCV introduction in their national immunization program.

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.).

Typhoid conjugate vaccine in Africa and Asia: Status of clinical evaluation and vaccine introduction

The disease burden of typhoid fever remains high in endemic areas in Asia and Africa, especially in children. Recent clinical trials conducted by the Typhoid Vaccine Acceleration Consortium show typhoid conjugate vaccine (TCV) to be safe, immunogenic, and efficacious at preventing blood culture-confirmed typhoid fever in African and Asian children. Pakistan, Liberia, and Zimbabwe recently introduced TCV through campaigns and routine childhood immunizations, providing protection for this vulnerable population. It is essential to continue this momentum while simultaneously filling data gaps - including typhoid complications - to inform decision-making on TCV introduction. A multidisciplinary approach including surveillance, water, sanitation, and hygiene investments, and large-scale TCV introduction is needed to decrease the burden and mortality of typhoid fever.

Prospects of future typhoid and paratyphoid vaccines in endemic countries

Low- and middle-income countries face a high burden of typhoid and paratyphoid fever due to poor water quality and inadequate sanitation. The World Health Organization (WHO) recommends the use of typhoid conjugate vaccines (TCV) in endemic settings and Gavi, the Vaccine Alliance, supports TCV introduction. There are currently two WHO-prequalified TCVs with Typbar TCV® introduced in Pakistan, Liberia, and Zimbabwe. Countries should assess disease burden and consider introduction of TCV for programmatic use. Several paratyphoid vaccine candidates are in early stages of development. An effective bivalent vaccine would be the most efficient way to control typhoid and paratyphoid fever.

The Surveillance for Enteric Fever in Asia Project (SEAP), Severe Typhoid Fever Surveillance in Africa (SETA), Surveillance of Enteric Fever in India (SEFI), and Strategic Typhoid Alliance Across Africa and Asia (STRATAA) Population-based Enteric Fever Studies: A Review of Methodological Similarities and Differences

Building on previous multicountry surveillance studies of typhoid and others salmonelloses such as the Diseases of the Most Impoverished program and the Typhoid Surveillance in Africa Project, several ongoing blood culture surveillance studies are generating important data about incidence, severity, transmission, and clinical features of invasive Salmonella infections in sub-Saharan Africa and South Asia. These studies are also characterizing drug resistance patterns in their respective study sites. Each study answers a different set of research questions and employs slightly different methodologies, and the geographies under surveillance differ in size, population density, physician practices, access to healthcare facilities, and access to microbiologically safe water and improved sanitation. These differences in part reflect the heterogeneity of the epidemiology of invasive salmonellosis globally, and thus enable generation of data that are useful to policymakers in decision-making for the introduction of typhoid conjugate vaccines (TCVs). Moreover, each study is evaluating the large-scale deployment of TCVs, and may ultimately be used to assess post-introduction vaccine impact. The data generated by these studies will also be used to refine global disease burden estimates. It is important to ensure that lessons learned from these studies not only inform vaccination policy, but also are incorporated into sustainable, low-cost, integrated vaccine-preventable disease surveillance systems.

Early Insights From Clinical Trials of Typhoid Conjugate Vaccine

Clinical trials of typhoid conjugate vaccine (TCV) are ongoing in 4 countries. Early data confirm safety, tolerability, and immunogenicity of typhoid conjugate vaccine, and early efficacy results are promising. These data support World Health Organization recommendations and planned country introductions. Forthcoming trial data will continue to inform programmatic use of typhoid conjugate vaccine.

Vaccines against antimicrobial resistance: a promising escape route for multidrug resistance

Antibiotic resistance has become a global health problem requiring urgent intervention. The pace of development and frequency of transmission of antimicrobial resistance have tremendously surpassed the number of antibiotics developed in the past few decades. Emergence and transmission of multidrug-resistant genes, for example, mcr-1 and mcr-5.3, against the last resort of antibiotics has challenged the treatment options. Vaccination is a promising approach with no instance of antimicrobial resistance generation or transmission reported so far. The time required for developing a vaccine, extensive pre- and post-licensure studies and the financial constraints for the R&D has hampered vaccine development over the past few decades. Vaccine can prove to be an effective future strategy for combating antimicrobial resistance.

Typhoid fever infection - Antibiotic resistance and vaccination strategies: A narrative review

Typhoid fever is a bacterial infection caused by the Gram-negative bacterium Salmonella enterica subspecies enterica serovar Typhi (S. Typhi), prevalent in many low- and middle-income countries. In high-income territories, typhoid fever is predominantly travel-related, consequent to travel in typhoid-endemic regions; however, data show that the level of typhoid vaccination in travellers is low. Successful management of typhoid fever using antibiotics is becoming increasingly difficult due to drug resistance; emerging resistance has spread geographically due to factors such as increasing travel connectivity, affecting those in endemic regions and travellers alike. This review provides an overview of: the epidemiology and diagnosis of typhoid fever; the emergence of drug-resistant typhoid strains in the endemic setting; drug resistance observed in travellers; vaccines currently available to prevent typhoid fever; vaccine recommendations for people living in typhoid-endemic regions; strategies for the introduction of typhoid vaccines and stakeholders in vaccination programmes; and travel recommendations for a selection of destinations with a medium or high incidence of typhoid fever.

Safety and immunogenicity of co-administration of meningococcal type A and measles-rubella vaccines with typhoid conjugate vaccine in children aged 15-23 months in Burkina Faso

OBJECTIVES

The World Health Organization pre-qualified single-dose typhoid conjugate vaccine (TCV) and requested data on co-administration with routine vaccines. The co-administration of Typbar TCV (Bharat Biotech International) with routine group A meningococcal conjugate vaccine (MCV-A) and measles-rubella (MR) vaccine was tested.

METHODS

This was a double-blind, randomized controlled trial performed in Ouagadougou, Burkina Faso. Children were recruited at the 15-month vaccination visit and were assigned randomly (1:1:1) to three groups. Group 1 children received TCV plus control vaccine (inactivated polio vaccine) and MCV-A 28 days later; group 2 children received TCV and MCV-A; group 3 children received MCV-A and control vaccine. Routine MR vaccine was administered to all participants. Safety was assessed at 0, 3, and 7 days after immunization, and unsolicited adverse events and serious adverse events were assessed for 28 days and 6 months after immunization, respectively.

RESULTS

A total of 150 children were recruited and vaccinated. Solicited symptoms were infrequent and similar for TCV and control recipients, as were adverse events (group 1, 61.2%; group 2, 64.0%; group 3, 68.6%) and serious adverse events (group 1, 2.0%; group 2, 8.0%; group 3, 5.9%). TCV generated robust immunity without interference with MCV-A vaccine.

CONCLUSIONS

TCV can be safely co-administered at 15 months with MCV-A without interference. This novel study on the co-administration of TCV with MCV-A provides data to support large-scale uptake in sub-Saharan Africa.

Use of Typhoid Vi-Polysaccharide Vaccine as a Vaccine Probe to Delineate Clinical Criteria for Typhoid Fever

Blood cultures (BCs) detect an estimated 50% of typhoid fever cases. There is need for validated clinical criteria to define cases that are BC negative, both to help direct empiric antibiotic treatment and to better evaluate the magnitude of protection conferred by typhoid vaccines. To derive and validate a clinical rule for defining BC-negative typhoid fever, we assessed, in a cluster-randomized effectiveness trial of Vi-polysaccharide (ViPS) typhoid vaccine in Kolkata, India, 14,797 episodes of fever lasting at least 3 days during 4 years of comprehensive, BC-based surveillance of 70,865 persons. A recursive partitioning algorithm was used to develop a decision rule to predict BC-proven typhoid cases with a diagnostic specificity of 97–98%. To validate this rule as a definition for BC-negative typhoid fever, we assessed whether the rule defined culture-negative syndromes prevented by ViPS vaccine. In a training subset of individuals, we identified the following two rules: rule 1: patients aged < 15 years with prolonged fever accompanied by a measured body temperature ≥ 100°F, headache, and nausea; rule 2: patients aged ≥ 15 years with prolonged fever accompanied by nausea and palpable liver but without constipation. The adjusted protective efficacy of ViPS against clinical typhoid defined by these rules in persons aged ≥ 2 years in a separate validation subset was 33% (95% CI: 4–53%). We have defined and validated a clinical rule for predicting BC-negative typhoid fever using a novel vaccine probe approach. If validated in other settings, this rule may be useful to guide clinical care and to enhance typhoid vaccine evaluations.

Reviving the "Moore Swab": a Classic Environmental Surveillance Tool Involving Filtration of Flowing Surface Water and Sewage Water To Recover Typhoidal Salmonella Bacteria

The “Moore swab” is a classic environmental surveillance tool whereby a gauze pad tied with string is suspended in flowing water or wastewater contaminated with human feces and harboring enteric pathogens that pose a human health threat. In contrast to single volume “grab” samples, Moore swabs act as continuous filters to “trap” microorganisms, which are subsequently isolated and confirmed using appropriate laboratory methods. Continuous filtration is valuable for the isolation of transiently present pathogens such as human-restricted Salmonella enterica serovars Typhi and Paratyphi A and B.

The “Moore swab” is a classic environmental surveillance tool whereby a gauze pad tied with string is suspended in flowing water or wastewater contaminated with human feces and harboring enteric pathogens that pose a human health threat. In contrast to single volume “grab” samples, Moore swabs act as continuous filters to “trap” microorganisms, which are subsequently isolated and confirmed using appropriate laboratory methods. Continuous filtration is valuable for the isolation of transiently present pathogens such as human-restricted Salmonella enterica serovars Typhi and Paratyphi A and B. The technique was first proposed (1948) to trace Salmonella Paratyphi B systematically through sewers to pinpoint the residence of a chronic carrier responsible for sporadic outbreaks of paratyphoid fever. From 1948 to 1986, Moore swabs proved instrumental to identify long-term human reservoirs (chronic carriers) and long-cycle environmental transmission pathways of S. Typhi and Paratyphi, for example, to decipher endemic transmission in Santiago, Chile, during the 1980s. Despite limitations such as intermittent shedding of typhoidal Salmonella by humans and the effects of dilution, S. Typhi and S. Paratyphi have been recovered from sewers, surface waters, irrigation canals, storm drains, flush toilets, and septic tanks by using Moore swabs. Driven by the emergence of multiple antibiotic-resistant S. Typhi and S. Paratyphi A strains that limit treatment options, several countries are embarking on accelerated typhoid control programs using vaccines and environmental interventions. Moore swabs, which are regaining appreciation as important components of the public health/environmental microbiology toolbox, can enhance environmental surveillance for typhoidal Salmonella, thereby contributing to the control of typhoid fever.

A Phase II, Randomized, Double-blind, Controlled Safety and Immunogenicity Trial of Typhoid Conjugate Vaccine in Children Under 2 Years of Age in Ouagadougou, Burkina Faso: A Methods Paper

The recent Typhoid Fever Surveillance in Africa Program demonstrated an overall adjusted incidence of typhoid fever 2–3 times higher than previous estimates in Africa. Recently, a single-dose typhoid conjugate vaccine that allows infants as young as 6 months old to be vaccinated was prequalified by the World Health Organization (WHO). This Vi-based conjugate vaccine demonstrated robust immunogenicity after 1 dose in infants and children 6 through 23 months of age in India with no safety signal, and is currently being tested for the first time on the African continent in Malawi. The WHO Strategic Advisory Group of Experts recommends studies to evaluate co-administering Vi-typhoid conjugate vaccine (Vi-TCV) with routine childhood vaccines in typhoid-endemic countries. The Burkina Faso immunization schedule includes yellow fever vaccine (YFV) at 9 months and meningococcal A conjugate vaccine (MCV-A) at 15 months, in addition to measles-rubella vaccine at both 9 and 15 months. Co-administration testing of Vi-TCV with these routine vaccinations will provide the data needed to support large-scale uptake of Vi-TCV in sub-Saharan Africa. A randomized, controlled, Phase II trial of Vi-TCV co-administration with the vaccinations routinely given at 9 and 15 months of age is planned in Burkina Faso. The overall aim is to assess the safety and immunogenicity of Vi-TCV when co-administered with YFV at 9 months of age and with MCV-A at 15 months of age. A total of 250 participants (100 infants aged 9–11 months and 150 children aged 15–23 months) will be enrolled.

Clinical Trials Registration. NCT03614533.

Progress in the overall understanding of typhoid fever: implications for vaccine development

INTRODUCTION

Typhoid fever continues to have a substantial impact on human health, especially in Asia and sub-Saharan Africa. Access to safe water, and adequate sanitation and hygiene remain the cornerstone of prevention, but these are not widely available in many impoverished settings. The emergence of antibiotic resistance affects typhoid treatment and adds urgency to typhoid control efforts. Vaccines provide opportunities to prevent and control typhoid fever in endemic settings.

AREAS COVERED

Literature search was performed looking for evidence concerning the global burden of typhoid and strategies for the prevention and treatment of typhoid fever. Cost of illness, available typhoid and paratyphoid vaccines and cost-effectiveness were also reviewed. The objective was to provide a critical overview of typhoid fever, in order to assess the current understanding and potential future directions for typhoid treatment and control.

EXPERT COMMENTARY

Our understanding of typhoid burden and methods of prevention has grown over recent years. However, typhoid fever still has a significant impact on health in low and middle-income countries. Introduction of typhoid conjugate vaccines to the immunization schedule is expected to make a major contribution to control of typhoid fever in endemic countries, although vaccination alone is unlikely to eliminate the disease.

Distinct climate influences on the risk of typhoid compared to invasive non-typhoid Salmonella disease in Blantyre, Malawi

Invasive Salmonella diseases, both typhoid and invasive non-typhoidal Salmonella (iNTS), are seasonal bloodstream infections causing important morbidity and mortality globally in Africa. The reservoirs and transmission of both are not fully understood. We hypothesised that differences in the time-lagged relationships of rainfall or temperature with typhoid and iNTS incidence might infer differences in epidemiology. We assessed the dynamics of invasive Salmonella incidence over a 16-year period of surveillance, quantifying incidence peaks, seasonal variations, and nonlinear effects of rainfall and temperature exposures on the relative risks of typhoid and iNTS, using monthly lags. An increased relative risk of iNTS incidence was short-lasting but immediate after the onset of the rains, whereas that of typhoid was long-lasting but with a two months delayed start, implying a possible difference in transmission. The relative-risk function of temperature for typhoid was bimodal, with higher risk at both lower (with a 1 month lag) and higher (with a ≥4 months lag) temperatures, possibly reflecting the known patterns of short and long cycle typhoid transmission. In contrast, the relative-risk of iNTS was only increased at lower temperatures, suggesting distinct transmission mechanisms. Environmental and sanitation control strategies may be different for iNTS compared to typhoid disease.

Typhoidal Salmonella human challenge studies: ethical and practical challenges and considerations for low-resource settings

Typhoidal Salmonella is a major global problem affecting more than 12 million people annually. Controlled human infection models (CHIMs) in high-resource settings have had an important role in accelerating the development of conjugate vaccines against Salmonella Typhi.

The typhoidal Salmonella model has an established safety profile in over 2000 volunteers in high-income settings, and trial protocols, with modification, could be readily transferred to new study sites. To date, a typhoidal Salmonella CHIM has not been conducted in a low-resource setting, although it is being considered.

Our article describes the challenges posed by a typhoidal Salmonella CHIM in the high-resource setting of Oxford and explores considerations for an endemic setting.

Development of CHIMs in endemic settings is scientifically justifiable as it remains unclear whether findings from challenge studies performed in high-resource non-endemic settings can be extrapolated to endemic settings, where the burden of invasive Salmonella is highest. Volunteers are likely to differ across a range of important variables such as previous Salmonella exposure, diet, intestinal microbiota, and genetic profile. CHIMs in endemic settings arguably are ethically justifiable as affected communities are more likely to gain benefit from the study. Local training and research capacity may be bolstered.

Safety was of primary importance in the Oxford model. Risk of harm to the individual was mitigated by careful inclusion and exclusion criteria; close monitoring with online diary and daily visits; 24/7 on-call staffing; and access to appropriate hospital facilities with capacity for in-patient admission. Risk of harm to the community was mitigated by exclusion of participants with contact with vulnerable persons; stringent hygiene and sanitation precautions; and demonstration of clearance of Salmonella infection from stool following antibiotic treatment.

Safety measures should be more stringent in settings where health systems, transport networks, and sanitation are less robust.

We compare the following issues between high- and low-resource settings: scientific justification, risk of harm to the individual and community, benefits to the individual and community, participant understanding, compensation, and regulatory requirements.

We conclude that, with careful consideration of country-specific ethical and practical issues, a typhoidal Salmonella CHIM in an endemic setting is possible.

Phase 3 Efficacy Analysis of a Typhoid Conjugate Vaccine Trial in Nepal

BACKGROUND

Salmonella Typhi is a major cause of fever in children in low- and middle-income countries. A typhoid conjugate vaccine (TCV) that was recently prequalified by the World Health Organization was shown to be efficacious in a human challenge model, but data from efficacy trials in areas where typhoid is endemic are lacking.

METHODS

In this phase 3, randomized, controlled trial in Lalitpur, Nepal, in which both the participants and observers were unaware of the trial-group assignments, we randomly assigned children who were between 9 months and 16 years of age, in a 1:1 ratio, to receive either a TCV or a capsular group A meningococcal conjugate vaccine (MenA) as a control. The primary outcome was typhoid fever confirmed by blood culture. We present the prespecified analysis of the primary and main secondary outcomes (including an immunogenicity subgroup); the 2-year trial follow-up is ongoing.

RESULTS

A total of 10,005 participants received the TCV and 10,014 received the MenA vaccine. Blood culture-confirmed typhoid fever occurred in 7 participants who received TCV (79 cases per 100,000 person-years) and in 38 who received MenA vaccine (428 cases per 100,000 person-years) (vaccine efficacy, 81.6%; 95% confidence interval, 58.8 to 91.8; P<0.001). A total of 132 serious adverse events (61 in the TCV group and 71 in the MenA vaccine group) occurred in the first 6 months, and 1 event (pyrexia) was identified as being vaccine-related; the participant remained unaware of the trial-group assignment. Similar rates of adverse events were noted in the two trial groups; fever developed in 5.0% of participants in the TCV group and 5.4% in the MenA vaccine group in the first week after vaccination. In the immunogenicity subgroup, seroconversion (a Vi IgG level that at least quadrupled 28 days after vaccination) was 99% in the TCV group (677 of 683 participants) and 2% in the MenA vaccine group (8 of 380 participants).

CONCLUSIONS

A single dose of TCV was immunogenic and effective in reducing S. Typhi bacteremia in children 9 months to 16 years of age. (Funded by the Bill and Melinda Gates Foundation; Current Controlled Trials number, ISRCTN43385161.).

Accelerating Typhoid Conjugate Vaccine Introduction: What Can Be Learned From Prior New Vaccine Introduction Initiatives?

The health consequences of typhoid, including increasing prevalence of drug-resistant strains, can stress healthcare systems. While vaccination is one of the most successful and cost-effective health interventions, vaccine introduction can take years and require considerable effort. The Typhoid Vaccine Acceleration Consortium (TyVAC) employs an integrated, proactive approach to accelerate the introduction of a new typhoid conjugate vaccine to reduce the burden of typhoid in countries eligible for support from Gavi, the Vaccine Alliance. TyVAC and its partners are executing a plan, informed by prior successful vaccine introductions, and tailored to the nuances of typhoid disease and the typhoid conjugate vaccine. The iterative process detailed herein summarizes the strategy and experience gained from the first 2 years of the project.