The trial was approved by the Malawi National Health Sciences Research Committee; the Malawi Pharmacy, Medicines, and Regulatory Authority; the institutional review board at the University of Maryland, Baltimore; and the research ethics committee at the University of Liverpool. Bharat Biotech International supplied Vi-TCV free of charge. The sponsors had no role in the design of the trial; the collection, analysis, or interpretation of the data; or the writing of the manuscript.

This single-center, phase 3, double-blind, individually randomized, active-controlled trial was conducted in two urban townships (Ndirande and Zingwangwa) in Blantyre, Malawi. Detailed methods have been published previously.23,24 In brief, we planned to enroll approximately 28,000 healthy children 9 months to 12 years of age who were residing in the trial areas. Eligible children had no previous typhoid vaccination and no acute illness or history of allergy or hypersensitivity. Parents or guardians provided written informed consent, and assent was obtained from children who were 8 years of age or older. Human immunodeficiency virus (HIV) status was solicited verbally; positive status was confirmed by the participant’s health passport, where possible. Participants were recruited through government health centers and primary schools. Safety data (adverse events and serious adverse events) were recorded prospectively.

Participants were randomly assigned in a 1:1 ratio to receive a single dose of Vi-TCV or a control vaccine (meningococcal capsular group A conjugate [MenA] vaccine), with the use of block randomization, with block sizes varying from 6 to 12. The randomization sequence was generated with the use of the blockrand package (version 1.3) in R Software, version 3.4.1 (R Foundation for Statistical Computing), and was concealed before randomization (which occurred immediately before vaccination). Parents, guardians, participants, and trial staff involved in screening, eligibility assessment, and follow-up were unaware of the trial-group assignments. Nurses who were aware of the trial-group assignments prepared and administered vaccines in a private area and had no further role in the trial.

The trial vaccine was Typbar-TCV (Bharat Biotech International), a WHO-prequalified tetanus-toxoid conjugated Vi-TCV containing 25 μg of Vi polysaccharide per 0.5-ml dose. MenA (MenAfriVac, Serum Institute of India) was used as the control vaccine and was administered at a dose of 10 μg per 0.5 ml to children 1 year of age or older and at a dose of 5 μg per 0.5 ml to children younger than 1 year of age. Vaccines were administered intramuscularly in the left thigh (in children <1 year of age) or in the left arm (in children ≥1 year of age). Both Vi-TCV and the MenA vaccine were administered with routine measles–rubella vaccine (in the right thigh) in children 9 to 11 months of age (according to the Malawi Expanded Program on Immunization guidelines).

All participants were monitored for 30 minutes after vaccination for immediate adverse events. Enhanced passive surveillance for fever and serious adverse events was conducted at four primary health centers (in Ndirande and Zingwangwa and at Gateway Clinic and Nancholi Youth Organization Clinic) and at Queen Elizabeth Central Hospital, a government referral hospital, where parents and guardians were instructed to bring unwell children at any time. Usual provision of health service was enhanced by telephone calls and community messaging. If children presented with febrile illness (subjective fever for ≥72 hours, an axillary temperature of ≥38°C, or hospitalization with a history of fever of any duration), a blood culture was obtained (5 ml [in children <5 years of age] or 10 ml [in children ≥5 years of age]) and a rapid diagnostic test for malaria was performed. Antimicrobial resistance of S. Typhi isolates was tested by means of disk diffusion.25 Isolates that showed pefloxacin resistance underwent confirmatory testing for ciprofloxacin resistance with the use of Etest (bioMérieux), with a minimum inhibitory concentration of more than 0.06 mg per liter indicating resistance. Hospital admission and antimicrobial treatment were at the discretion of the facility clinician. Participants with blood culture–confirmed S. Typhi were contacted every 2 weeks until they were asymptomatic to monitor treatment response and outcomes.

The primary outcome was blood culture–confirmed typhoid fever occurring at any time after vaccination. Secondary outcomes were the safety profiles of Vi-TCV and the MenA vaccine (assessed according to the number of adverse events detected in the first 30 minutes after vaccination), the number of serious adverse events within 28 days after vaccination, and the number of adverse events within 6 months after vaccination. For the primary evaluation of vaccine efficacy, all children were under enhanced passive surveillance for at least 18 months (from February 21, 2018, to April 3, 2020).

Details regarding the sample-size and power calculations have been reported previously.23 In brief, assuming a vaccine efficacy of 75%, we calculated that a minimum of 30 cases of typhoid fever would be needed to test the null hypothesis of no protective efficacy (i.e., vaccine efficacy ≤0%). The primary analysis was performed in the intention-to-treat population, which included all children who underwent randomization and received a dose of a vaccine. The first episode of blood culture–confirmed typhoid fever occurring after vaccination was used for the primary analysis. In the intention-to-treat analysis, the vaccine group was defined according to the vaccine that was assigned, not the vaccine that was received. The per-protocol analysis of vaccine efficacy included children who completed the trial without any protocol deviations, received the vaccine to which they were assigned, and had the first episode of blood culture–confirmed typhoid fever at least 14 days after vaccination.

Because of the interruption of surveillance (starting on April 3, 2020) as a result of coronavirus disease 2019, the protocol was amended to allow the primary efficacy analysis to be conducted as of April 3, 2020, encompassing 18 to 24 months of follow-up per participant. The data and safety monitoring board approved the amendment because the trial had reached the prespecified number of typhoid cases, and a prolonged disruption in surveillance would affect evaluations of incidence, cases prevented, and the number needed to vaccinate. Surveillance of the full cohort, conducted in a blinded manner, is planned to continue until September 30, 2021 (a minimum of 36 months of follow-up) for secondary longer-term efficacy and subgroup analyses.

The incidence rate was calculated as the number of first episodes of blood culture–confirmed typhoid fever divided by the total follow-up time. Individual follow-up time was the smallest of the following: time to the first episode of typhoid fever; time to withdrawal from the trial, loss to follow-up, death, or relocation out of the trial area; or the time to the end of the analysis period. The incidence rate ratio was calculated as the ratio of the incidence rate in the Vi-TCV group to that in the MenA group. The vaccine efficacy was calculated as (1−incidence rate ratio)×100%. Subgroup analyses were conducted to evaluate vaccine efficacy according to sex, trial site (Ndirande or Zingwangwa), and age at the time of vaccination (<5 years or ≥5 years). Poisson regression with the interaction term between each prespecified subgroup of interest and the vaccine group was used to compare vaccine efficacy across subgroups.

The absolute risk reduction was calculated as the risk of blood culture–confirmed typhoid fever in the MenA group minus that in the Vi-TCV group. The number needed to vaccinate was calculated as 1÷absolute risk reduction, representing the number of children who would need to be vaccinated to prevent one case of blood culture–confirmed typhoid fever. The cumulative incidence of typhoid fever for each vaccine group was calculated with the use of the Kaplan–Meier method, and vaccine efficacy was estimated at 12, 18, and 24 months after vaccination with the use of the life-table method. All analyses were performed according to the prespecified statistical analysis plan with the use of Stata software, version 16 (StataCorp). Full details of the trial design and conduct are provided in the protocol, available with the full text of this article at NEJM.org.