A new generation of typhoid conjugate vaccines (TCVs) have been shown to be safe and provide long-lasting immunity in children. In India, Typbar TCV Vi-TT (Vi capsular polysaccharide conjugated with tetanus toxoid protein) is manufactured by Bharat Biotech International Limited (BBIL). Immunogenicity studies by BBIL showed \>90% detectable antibodies to Typbar TCV vaccine at 42 days and \>70% at 720 days in persons 2-45 years old. A single dose of vaccine was effective in children 6-23 months old with 98% seroconversion at 42 days and persistent high antibody titers at 720 days. Another randomized controlled trial study by University of Oxford tested Vi-TT with a human challenge model of typhoid fever in which participants ingested Salmonella Typhi after vaccination. Typhoid fever was diagnosed by clinical criteria and/or positive blood culture at 2 weeks in 77% of controls (n=31) as compared to 35% in the Vi-TT group (n=37) to give vaccine efficacy of 54% (95% confidence interval of 26-71). Typbar TCV has been licensed in India for children \>6 months old since 2013 and is currently used in the private sector. In January 2018, the World Health Organization (WHO) recommended the use of typhoid conjugate vaccines especially in those countries with the highest burden of disease and the highest burden of antimicrobial resistance. However, there are no field effectiveness studies yet for Typbar TCV.
In this study, a hybrid facility- and community-based typhoid surveillance strategy will be used to evaluate the impact of a government-led typhoid conjugate vaccine introduction program on typhoid disease burden in Navi Mumbai, India. Navi Mumbai is a metropolitan area extending from Mumbai with approximately 1.4 million people and wide demographic and socioeconomic strata, including slums. Navi Mumbai is comprised of 13 area nodes, and 8 of these nodes are under the jurisdiction of the local government body, Navi Mumbai Municipal Corporation (NMMC). Navi Mumbai has high enteric fever burden: a 2008 study found 98 blood culture-confirmed pediatric cases over 16 months in 2 hospitals with a majority of isolates exhibiting multi-drug resistance. Data from one private laboratory in 2015 showed over 225 S. Typhi cases including 3 cases with ceftriaxone resistance.
NMMC has decided to introduce TCV into public sector routine immunization services and will vaccinate all children 9 months to \<15 years old within its jurisdiction - estimated 390,000 children - with TCV over 2 years. NMMC government officials are collaborating with researchers from WHO India, Institute of Cholera and Enteric Diseases (NICED), Centers for Disease Control and Prevention (CDC), and Stanford University to implement the TCV introduction in a two-phase cluster-randomized program in order to allow rigorous evaluation of population impact. NMMC has 22 urban health posts (UHPs) that give routine childhood immunizations. UHPs were first grouped into 3 strata based on % of population living in slums: low, medium, or high. Half of the UHPs within each stratum were then randomly selected to receive TCV in 2018 (Phase 1) and the remaining UHPs to receive TCV one year later in 2019 (Phase 2).
Evaluation studies of NMMC's TCV introduction program include: impact on blood-culture confirmed typhoid fever; impact on clinical syndromes of febrile illness; vaccine safety; vaccine efficacy; vaccination campaign coverage and acceptability surveys; environmental water contamination with Salmonella Typhi; seroconversion associated with typhoid infection; and cost-effectiveness of typhoid conjugate vaccination campaign.
