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 [W] 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. Copyright (C) 2022 The Author(s). Published by Elsevier Ltd.