The Vi Conjugate Typhoid Vaccine Is Safe, Elicits Protective Levels of IgG Anti-Vi, and Is Compatible with Routine Infant Vaccines

Typhoid fever remains a serious problem in developing countries. Current vaccines are licensed for individuals who are 5 years old or older. A conjugate of the capsular polysaccharide (CP) of Salmonella enterica serovar Typhi (Vi) bound to recombinant exoprotein A of Pseudomonas aeruginosa (Vi-rEPA) enhanced Vi immunogenicity and protected 2- to 5-year-olds in Vietnam. In this study, Vi-rEPA was evaluated for use in infants. A total of 301 full-term Vietnamese infants received Expanded Program on Immunization (EPI) vaccines alone or with Vi-rEPA or Haemophilus influenzae type b-tetanus toxoid conjugate (Hib-TT) at 2, 4, and 6 months and Vi-rEPA or Hib-TT alone at 12 months. Infants were visited 6, 24, and 48 h after each injection to monitor adverse reactions. Maternal, cord, and infant sera were assayed for IgG anti-Vi and for IgG antibodies to Hib CP and the diphtheria, tetanus, and pertussis toxins at 7, 12, and 13 months. No vaccine-related serious adverse reactions occurred. In the Vi-rEPA group, the IgG anti-Vi geometric mean (GM) increased from the cord level of 0.66 to 17.4 enzyme-linked immunosorbent assay units (EU) at 7 months, declined to 4.76 EU at 12 months, and increased to 50.1 EU 1 month after the 4th dose (95% of infants had levels of ≥ 3.5 EU, the estimated protective level). Controls had no increase of the IgG anti-Vi GM. Infants with cord anti-Vi levels of <3.5 EU responded with significantly higher IgG anti-Vi levels than those with levels of ≥ 3.5 EU. Anti-diphtheria, -tetanus, and -pertussis toxin levels were similar in all groups. Vi-rEPA was safe, induced protective anti-Vi levels, and was compatible with EPI vaccines, and it can be used in infants. High cord IgG anti-Vi levels partially suppressed infant responses to Vi-rEPA.