Longevity of serologic responses following a single dose of typhoid conjugate vaccine among children living with HIV in Pakistan: A prospective cohort study

Background: This study aimed to assess the longevity of serologic response and seroconversion rates at several time points following TCV vaccination among children living with HIV aged 6 months to 15 years in Pakistan. Methods: From November 20, 2020, to January 2, 2021; 336 children were enrolled and followed up prospectively for 12 months. Blood samples were collected before the administration of TCV and at 4–6 weeks, 6 months, and 1 year after administration of a single dose (0.5 ml) of intramuscular Typbar TCV®. Samples were analyzed for anti-Vi-IgG antibodies using ELISA. Geometric mean titers (GMTs), seroconversion rates (fourfold rise in anti-Vi-IgG from baseline), and factors associated with sustained seroconversion at 1 year were evaluated using generalized linear mixed models. Findings: The seroconversion rates were significantly lower in children aged 6 months to 5 years compared to children > 5 years; (127/216 (58·8%)) versus (81/111 (73·0%)) at 6 months and (110/217 (50·7%)) versus (78/109 (71·6%)) at 1 year, only two-third; 188/326 (57·7%) remained seroconverted at 1 year. The GMTs (95 % CI) were significantly lower in children aged 6 months to 5 years compared to children > 5 years, 9·6 (7·6, 12·0) versus 28·9 (20·2, 41·4) at 6 months, and 6·6 (5·4, 8·0) versus 23·1 (16·4, 32·5) at 1 year time point. The odds of sustained seroconversion significantly decreased with time (adjusted odds ratio (aOR): 0·232; 95 % CI: 0·136,0·396). The odds of sustained seroconversion following 1 year of TCV vaccination were significantly higher among children with non-severe HIV clinical disease (aOR: 10·6; 95 % CI: 1·5, 73·9) and children in elder age group (aOR: 7·45; 95 % CI: 11·2, 47·0). Conclusions: There was a decline in seroconversion after one year of TCV in children living with HIV. The decline was significant in children with severe or advanced HIV disease. A significant decrease in seroconversion rates was observed among children living with HIV following one year of TCV administration.