Age-dependent acquisition of IgG antibodies to Shigella serotypes-a retrospective analysis of seroprevalence in Kenyan children with implications for infant vaccination

Abstract

Background: Shigellosis mainly affects children under 5 years of age living in low- and middle-income countries, who are the target population for vaccination. There are, however, limited data available to define the appropriate timing for vaccine administration in this age group. Information on antibody responses following natural infection, proxy for exposure, could help guide vaccination strategies.

Methods: We undertook a retrospective analysis of antibodies to five of the most prevalent Shigella serotypes among children aged <5 years in Kenya. Serum samples from a cross-sectional serosurvey in three Kenyan sites (Nairobi, Siaya, and Kilifi) were analyzed by standardized ELISA to measure IgG against Shigella sonnei and Shigella flexneri 1b, 2a, 3a, and 6. We identified factors associated with seropositivity to each Shigella serotype, including seropositivity to other Shigella serotypes.

Results: A total of 474 samples, one for each participant, were analyzed: Nairobi (n = 169), Siaya (n = 185), and Kilifi (n = 120). The median age of the participants was 13.4 months (IQR 7.0-35.6), and the male:female ratio was 1:1. Geometric mean concentrations (GMCs) for each serotype increased with age, mostly in the second year of life. The overall seroprevalence of IgG antibodies increased with age except for S. flexneri 6 which was high across all age subgroups. In the second year of life, there was a statistically significant increase of antibody GMCs against all five serotypes (p = 0.01-0.0001) and a significant increase of seroprevalence for S. flexneri 2a (p = 0.006), S. flexneri 3a (p = 0.006), and S. sonnei (p = 0.05) compared with the second part of the first year of life. Among all possible pairwise comparisons of antibody seropositivity, there was a significant association between S. flexneri 1b and 2a (OR = 6.75, 95% CI 3-14, p < 0.001) and between S. flexneri 1b and 3a (OR = 23.85, 95% CI 11-54, p < 0.001).

Conclusion: Children living in low- and middle-income settings such as Kenya are exposed to Shigella infection starting from the first year of life and acquire serotype-specific antibodies against multiple serotypes. The data from this study suggest that Shigella vaccination should be targeted to infants, ideally at 6 or at least 9 months of age, to ensure children are protected in the second year of life when exposure significantly increases.

Keywords: IgG antibodies; Kenya; Shigella flexneri; Shigella sonnei; sero-prevalence; serotype.

Figure 1

Prevalence of IgG antibody responses by age. IgG antibody responses to the OAg of (A) Shigella flexneri 1b, (B) S. flexneri 2a, (C) S. flexneri 3a, (D) S. flexneri 6, and (E) S. sonnei were measured by ELISA. Bar plot showing the proportion of positive individuals based on the participants with responses above the four-fold lower limit of quantification (LLoQ). Shown is the overall prevalence with 95% Agresti–Coull confidence intervals. Chi-square test—test for trend. Significance considered at p <0.05.

Figure 2

Seroprevalence heatmap by age and site. Serum antibody prevalence by age against the different serotypes was determined by ELISA. The heatmap represents all the three sites and the overall prevalence with proportion indicated in each individual box. The gradient of the heatmap from blue to red indicates the lowest to the highest proportions, respectively.

Figure 3

Overall magnitude of IgG antibodies by age. Violin plots of the anti-OAg IgG ELISA units (EU) against (A) Shigella flexneri 1b, (B) S. flexneri 2a, (C) S. flexneri 3a, (D) S. flexneri 6, and (E) S. sonnei. Indicated within each violin plot are boxplots with the median (dark solid line) and the lower and upper quartiles with the geometric mean concentration represented in blue dots. Each individual is represented by each individual black closed circle. The dotted line represents the seroresponse cutoff of the antibodies. The Kruskal–Wallis test with multiple comparison adjustment was used. A test for all pairwise comparisons between age groups and ELISA unit concentrations was conducted. Significance is denoted by an asterisk with significance considered at p <0.05, where *p < 0.05, **p < 0.01, ***p < 0.001, and ****p < 0.0001. ns, not significant.

Figure 4

Geometric mean concentration heatmap by age and site. Serum antibody geometric mean concentration by age against the different serotypes was determined by ELISA. The heatmap represents all the three sites and the overall with the geomean indicated in each individual box. The gradient of the map from blue to red represents the lowest to the highest concentrations, respectively.

Figure 5

Breadth of antibody responses by (A) age and (B) site. The breadth of antibody response was defined as the number (1–5) of serotypes to which individuals had antibody concentrations above the seroresponse cutoff. Each segment of the stacked bar represents the percentage of seropositive individuals with responses to the number (n) of serotypes.