Data were obtained from a previously published phase 3 randomised controlled trial comparing Vi-polysaccharide tetanus-toxoid conjugate vaccine (Typbar-TCV; Bharat Biotech Intl Ltd, India): (Vi-TT) with Vi-polysaccharide (Typbar;Bharat Biotech Intl Ltd, India): (Vi-PS) in participants aged 2- 45 years. An additional open-label arm administered Vi-TT to children aged 6 months to 23 months. The proportion of participants with presumed clinical or subclinical infection (‘seroincidence’), was determined using mixture models and compared using relative risks.
Seroincidence was lower in those randomised to Vi-TT than Vi-PS in those aged 2-45 years; 21/155 (13.5%) vs 47/129 (36.4%); RR 0.372 (95%CI 0.235–0.588), p<0.0001 and in those aged 2-15 years RR 0.424 (95%CI 0.231– 0.778), p=0.0039. Vaccine seroefficacy was 85% (95%CI 80–88%).
This is the first field estimate of the seroefficacy of a Vi-TT vaccine and shows that Typbar TCV substantially reduces the number of serologically defined (sub) clinical infections in infants, children and adults.