Farhana Khanam

Background: Understanding the extent of protection against typhoid fever conferred by naturally occurring typhoid is essential for transmission modeling and vaccine policy development. However, data on the level of protection from natural typhoid against recurrent typhoid episodes are sparse. Methods: We analyzed data from a cluster-randomized trial of a typhoid conjugate vaccine conducted in Dhaka, Bangladesh. A cohort of blood-culture confirmed typhoid patients (index cases) and matched controls (1:4 ratio) from the community who had not received the typhoid vaccine were followed for up to 6 years to identify recurrent typhoid episodes. A recurrent typhoid episode was defined as one occurring more than 30 days after the onset of the initial episode and/or involving a strain with a different antimicrobial resistance profile. Febrile patients who tested negative for Salmonella Typhi through passive surveillance were also evaluated as facility-based controls. Stratified Cox proportional hazards models were used to estimate hazard ratios (HRs) for recurrent typhoid episodes. Results: We identified 13 recurrent episodes among 940 index cases and 12 episodes among 3760 community-based controls, yielding incidence rates of 741 and 158 per 100 000 person-years, respectively, with an adjusted hazard ratio (aHR) of 3.7; 95% CI: 1.8, 7.9; P < .001. However, when using facility-based controls to prevent healthcare utilization bias, there was no statistically significant difference between groups (aHR: 1.5, 95% CI: 0.8, 2.8; P = .160). Conclusions: In this typhoid-endemic population comprising both children and adults, we found no evidence that natural typhoid fever confers protection against recurrent typhoid episodes.

Introduction: A cluster randomized trial (CRT) in Bangladesh found that Vi-tetanus toxoid (Vi-TT) vaccine conferred 85% protection to vaccinees at 18 months of follow-up; however, it failed to confer significant herd protection to non-vaccinees. Methods: In the CRT, children aged 9 months to <16 years from 150 clusters received one dose of either Vi-TT or Japanese encephalitis vaccine. To evaluate whether herd protection was evident, we analyzed two years of follow-up data by quartiles of clusters with ascending levels of Vi-TT coverage. Results: The average vaccine coverage across all clusters was 69% among targeted children and 20% in the entire population. Vi-TT provided 83% (95%CI: 74%,89%) total protection among vaccinees, 12% (95%CI: -17%,34%) indirect protection among non-vaccinees, and 53% (95%CI: 40%,63%) overall protection among entire population, irrespective of vaccine coverage. Significant herd protection (47%; 95%CI: 3%,71%) was observed only in the highest quartile of vaccine coverage in the entire population (range 21.5%-26%). No herd protection was evident among targeted children (<16 years), including the highest coverage quartile (72.1%-78.9%), where protection was - 8% (95%CI: -108%,44%). Conclusion: Vaccine herd protection, essential for typhoid conjugate vaccines to achieve high overall protection, was evident only in clusters with higher, albeit still modest, coverage in the entire population. Since even high coverage among children <16 years did not generate significant herd protection, higher vaccine coverage across all age groups, including vaccination of adults, may be necessary for TCVs to confer the combined direct and herd protection required for effective typhoid control. Summary box: What is already known on this topic?: Vi-tetanus toxoid (Vi-TT) vaccine provides high-level protection against typhoid fever in vaccinated children. However, overall analysis of a cluster-randomized trial of Vi-TT given to children under 16 years of age found no evidence of indirect (herd) protection by Vi-TT to unvaccinated individuals. What this study adds: In further analysis of the cluster randomized trial of Vi-TT, significant indirect protection (47%; 95%CI:3%,71%) was observed, but only in clusters with the highest levels of vaccine coverage of the entire population. In contrast, we found no evidence of vaccine herd protection in clusters with the highest levels of vaccine coverage of children, who were targeted by vaccination. How this study might affect research, practice, or policy: Achieving herd protection from typhoid conjugate vaccines (TCVs) may require higher vaccine coverage across the entire population, not only among children. Vaccination strategies that include adults may be necessary to maximize the public health impact of TCVs. These findings have important implications for typhoid control policies in endemic settings.

Background: The dynamics of humoral immune responses following primary and booster COVID-19 vaccinations are crucial to understand in order to optimize vaccination strategies. This study evaluates the magnitude and durability of SARS-CoV-2-specific IgG antibody responses across different vaccines in a large cohort of Bangladeshi adults. Methods: A total of 6300 adults from nine hospitals across eight divisions of Bangladesh were enrolled. Participants received two primary doses of either ChAdOx1 nCoV-19 (Covishield, Serum Institute of India, n = 2855), mRNA-1273 (Moderna, n = 578), BNT162b2 (Pfizer-BioNTech, n = 121), or Vero-cell-inactivated (Sinopharm, n = 2746) vaccines. Booster doses were administered at one-year intervals post-primary vaccination. SARS-CoV-2 spike receptor-binding domain (RBD)-specific IgG antibody responses were measured by ELISA using serum from vaccinees at multiple time points after two primary and two booster doses. Results: A total of 3745 individuals received booster 1 (third dose), with 59% receiving heterologous boosters (a different vaccine regimen than the primary doses). Only 5.5% (n = 347) of participants received a second booster one year after the first booster (among them, 99% received BNT162b2). Our results suggest that heterologous boosters with the mRNA vaccine induced higher IgG levels than homologous boosters for individuals who received primary vaccination with adenovirus vector-based ChAdOx1 nCoV-19 or a Vero-cell-inactivated vaccine. However, in those who initially received the mRNA-based vaccine, both homologous and heterologous boosters produced comparable IgG responses. Among all vaccine types, booster immunization with the Vero-cell-inactivated vaccine induced the lowest antibody responses. Longitudinal analysis demonstrated significantly high IgG levels over the 12 months following the first booster (p < 0.0001); however, IgG levels declined significantly after the second booster dose (fourth dose). Conclusions: Heterologous boosting strategies, particularly those involving mRNA vaccines, elicit stronger and more sustained IgG responses compared to a homologous booster. However, antibody waning after the second booster highlights the need for continued monitoring and potential additional vaccine strategies.

The diagnosis of enteric fever has become difficult due to the nonspecific and overlapping clinical syndrome of typhoid and paratyphoid infections with other febrile illnesses. Moreover, the rapid emergence of fluoroquinolone-resistant typhoidal Salmonella and the lack of robust diagnostic methods highlight the urgent need for highly sensitive molecular techniques. Here, we evaluated the performance of a rapid, reliable, and cost-effective molecular diagnostic approach for detecting Salmonella Typhi, including the globally dominant haplotype H58 lineage (H58), and Salmonella Paratyphi A. An in-house-built conventional polymerase chain reaction (PCR) was performed on a collection of blood-culture-positive strains, and the sensitivity and specificity were compared with those of the standard blood culture results. H58 and non-H58 Typhi lineages with distinct resistance patterns were confirmed from the previously reported sequencing data. Our PCR result showed that target genes SSPA2308, STY2513, and STY0307 demonstrated 100% sensitivity and specificity for Salmonella Paratyphi A, Salmonella Typhi, and H58 Salmonella Typhi, respectively. The PCR assay reliably detected bacterial DNA at 5.2 × 10⁴ colony-forming units (CFUs), with consistent amplification observed up to 10⁻¹ dilution. This single-nucleotide polymorphism (SNP)-based diagnostic approach has added a new dimension to designing unique markers for multidrug-resistant (MDR)-associated H58 lineage detection and has the potential to inform local treatment algorithms.

Background: Shigella vaccine development is a critical priority due to high burden and long-term outcomes among children. Choosing a clinical efficacy endpoint for Shigella vaccine trials requires comparing existing diarrhea severity definitions among children with Shigella. Methods: Six- to 35-month-old children presenting with diarrhea were enrolled at 7 EFGH study sites. Among children with Shigella, 6 diarrhea severity definitions (Global Enteric Multicenter Study [GEMS] moderate-to-severe diarrhea [MSD], modified Vesikari score [MVS], MVS +/− dysentery, Clark score, MAL-ED score, and Shigella mortality score) were examined against 2 outcomes: change in length/height-for-age z-score (ΔLAZ/HAZ) over 3 months and death or hospitalization within 14 days. We compared the performance of each severity score to predict outcomes using linear regression and measures of diagnostic accuracy. Results: Moderate or severe diarrhea was negatively associated with ΔLAZ/HAZ by MVS (−0.07 z-score, 95% CI −0.10 to −0.03), MVS +/− dysentery (−0.05, 95% CI −0.09 to −0.02), and MAL-ED score (−0.04, 95% CI −0.07 to 0.00). GEMS MSD (93.6%), MAL-ED (87.2%), MVS +/− dysentery (85.1%), and MVS (80.9%) had high sensitivity in predicting death or hospitalization. Conclusions: MVS, MVS +/− dysentery, and MAL-ED score identified adverse outcomes following Shigella diarrhea and are viable options for a vaccine trial case definition.

In the TyVAC-Bangladesh cluster-randomized trial, a single dose of typhoid conjugate vaccine (TCV) in children aged 9 months to Salmonella Typhi and whether TCV introduction in endemic settings will induce herd effects. We hypothesize that the low indirect protection observed in the TyVAC-Bangladesh study could be explained by some combination of transmission across cluster boundaries (contamination), limited direct vaccine protection against infection, and insufficient population-wide vaccine coverage. To evaluate these hypotheses, we simulated the trial using two transmission dynamic models: a primary model fit exclusively to clinical surveillance data from the Bangladesh study site, and an alternative model simultaneously fit to clinical and serological data. Vaccinating 64% of eligible children (the coverage level achieved in the TyVAC-Bangladesh trial) induced 0% to 67% indirect protection in the primary model, and 0% to 54% indirect protection in the alternative model, depending on the transmissibility of chronic carriers and TCV efficacy against S. Typhi infection. The results of the TyVAC-Bangladesh trial were consistent with a TCV efficacy against infection of 15% to 85% in the primary model, and 60% to 90% in the alternative model. If adults had been vaccinated in the trial at the same coverage level as children, we estimate that indirect protection would have increased from 19% to between 40%-57%. These findings suggest that TCV confers protection against transmission from acute S. Typhi infections, and that the low indirect protection observed in the TyVAC-Bangladesh trial can be explained by transmission from unvaccinated adults.

Background: Enterotoxigenic Escherichia coli (ETEC) is a major cause of diarrheal illness, and population-based data on the incidence of clinically significant ETEC diarrhea in developing countries are limited. We provide insight into ETEC epidemiology; we followed a population-based cohort in a vaccine trial. Methods: We analyzed data from a cluster-randomized controlled trial of an oral cholera vaccine conducted in an urban slum in Dhaka, Bangladesh. The study covers 90 geographical clusters with an average population of 2988 households/cluster (baseline population 268 896). Two cohort analyses were conducted, 1 as dynamic cohort that included all subjects at vaccination, in-migrants, and births over 4 years and a closed cohort, which included only individuals present at baseline. We evaluated individuals placed under treatment center–based diarrheal surveillance between 2011 and 2015. Results: In the dynamic cohort, the ETEC incidence was 150/100 000 person-years (PY; 95% CI, 141–159), with seasonal peaks during warmer months, and in the closed cohort, the incidence was 153/100 000 PY (95% CI, 140, 166). The highest rate was seen in children aged 45 years (219/100 000 PY; 95% CI, 177–267). The rate of severe ETEC was ≤35/100 000 PY for persons aged ≤45 years (95% CI, 27–44), but rose to 82 for adults aged >45 years (95% CI, 58–113). Conclusions: ETEC diarrhea is a major health problem in young children and older adults, prevention through vaccination and improved water, sanitation, and hygiene should target both age groups.

Serologic surveillance of at-risk populations can be used to directly estimate the incidence of typhoidal Salmonella infection across a variety of settings, including those without access to facility-based blood-culture surveillance. We collected paired blood samples approximately three months apart from an age-stratified random sample of healthy children and adults in Bangladesh, Malawi, and Nepal as part of the Strategic Typhoid Alliance Across Asia and Africa (STRATAA) study. We used a multiplex bead assay to measure the concentration of IgG antibodies against seven Salmonella typhi/paratyphi antigens (CdtB, FliC, HlyE, LPSO2, LPSO9, Vi, and YncE) in each sample and identified recently infected participants by fitting a regression mixture model to the change in IgG concentration between participants’ samples. We estimated the seroincidence of infection in a Bayesian framework for each study site, age group, and antigen target. Finally, we compared the seroincidence estimates with crude and adjusted estimates of clinical incidence based on blood-culture surveillance. Seroincidence estimates were significantly higher than enteric fever incidence across all study sites, age groups, and antigen targets, even after adjusting for underreporting (median ratio: 25.4, interquartile range: 20.2-50.7). Seroincidence consistently peaked in the 0-4-year age group and declined moderately between children and adults (34% to 56% decline in HlyE seroincidence between the 5-9 and 30+ year old age groups), while enteric fever incidence peaked in older children and fell sharply in adults (71% to 95% decline in adjusted clinical incidence). Seroincidence estimates based on the HlyE and YncE antigens individually had the strongest correlation with observed enteric fever incidence across age groups and study sites (r = 0.63 and 0.71, respectively). These findings suggest that in endemic settings, both children and adults are frequently infected by typhoidal Salmonella serotypes, although only a fraction of these infections present as clinically identifiable enteric fever cases.

Introduction: Millions of Euvichol-Plus doses have been deployed from the global oral cholera vaccine stockpile in over 20 cholera-affected countries. However, information on Euvichol-Plus’s effectiveness is limited. Using this vaccine in a cholera epidemic in Dhaka, Bangladesh, provided the opportunity to evaluate the vaccine effectiveness (VE) using a test-negative design.Methods: A two-dose regimen of Euvichol-Plus was administered to individuals aged >1 year in a population of ca. 900 000 in two campaign rounds between June and August 2022, with prospective registration of all persons who received at least one dose. We conducted systematic surveillance in two key facilities, enrolling patients with acute watery diarrhoea who were eligible for vaccination from the campaign’s start and who presented for care between 21 August 2022 and 20 August 2023. Faecal culture-positive cholera cases were matched to up to four faecal culture-negative controls by age, presentation date and facility. Vaccination status was documented without knowledge of culture results. Conditional logistic regression models estimated the OR for the vaccination-cholera association, and the VE of the two-dose regimen was calculated as [(1−OR) × 100].Results: The analysis included 226 cases and 552 matched controls. The adjusted VE of two doses of the Euvichol-Plus vaccine against medically attended cholera was 66% (99.5% CI: 30 to 83) for all recipients. Limited protection (12%; 95% CI: −95 to 60) was observed for children aged 1–4 years; whereas, protection was 79% (95% CI: 60 to 89) for those aged ≥5 years. VE against cholera with moderate to severe dehydration was 69% (95% CI: 44 to 83) overall but 6% (95% CI: −206 to 71) for children aged 1–4 years.Conclusion: Euvichol-Plus provided significant protection against medically attended cholera of any severity as well as cholera with moderate to severe dehydration. However, significant levels of protection were only observed for those aged ≥5 years.

Background: When delivered through vaccination Vi-polysaccharide antigen of Salmonella enterica serotype Typhi protects against typhoid by inducing IgG anti-Vi antibodies. We aimed to determine whether the presence of antibodies following natural infection is associated with a lower incidence of typhoid fever in endemic regions. Methods: We analyzed data from a cohort study of typhoid fever conducted in Dhaka, Bangladesh. Plasma IgG anti-Vi antibodies were measured using a standard enzyme-linked immunosorbent assay in random serosurveys of a population that had not previously received typhoid vaccination. Participants were followed for up to 20 months for culture-confirmed typhoid fever. The receiver operating characteristic (ROC) curve and Cox proportional hazard models were used to evaluate the associations between antibody levels and typhoid risk. Results: The ROC analysis revealed that IgG anti-Vi antibody titers were predictive of typhoid risk among the 8,261 serosurvey participants (area under the curve: 0·63; 95% confidence interval (CI): 0·58─0·67). Detection of any antibodies was associated with a lower risk of typhoid in crude analyses (hazard ratio (HR): 0·13; 95% CI: 0·03─0·52), though this association declined after adjustment (HR: 0·32; 95% CI: 0·07─1·40). A positive correlation was observed between IgG anti-Vi titers and age (correlation coefficient 0·35; p < 0·001), with a stepwise increase in detectable antibodies by age, ranging from 12·0% (280/2,333) in children under 5 years to 54·2% (463/854) in individuals 50 years and older (p < 0·001). Conclusions: In typhoid-endemic populations, IgG anti-Vi antibodies may indicate natural immunity to typhoid. The increasing seroprevalence with age suggests that these antibodies could serve as markers of cumulative past typhoid infections. Future research should explore these possibilities. Clinical trial number: Not applicable.