Modelling the force of infection for hepatitis A in an urban population-based survey: a comparison of transmission patterns in Brazilian macro-regions

Abstract

Background: This study aimed to identify the transmission pattern of hepatitis A (HA) infection based on a primary dataset from the Brazilian National Hepatitis Survey in a pre-vaccination context. The national survey conducted in urban areas disclosed two epidemiological scenarios with low and intermediate HA endemicity.

Methods: A catalytic model of HA transmission was built based on a national seroprevalence survey (2005 to 2009). The seroprevalence data from 7,062 individuals aged 5-69 years from all the Brazilian macro-regions were included. We built up three models: fully homogeneous mixing model, with constant contact pattern; the highly assortative model and the highly assortative model with the additional component accounting for contacts with infected food/water. Curves of prevalence, force of infection (FOI) and the number of new infections with 99% confidence intervals (CIs) were compared between the intermediate (North, Northeast, Midwest and Federal District) and low (South and Southeast) endemicity areas. A contour plot was also constructed.

Results: The anti- HAV IgG seroprevalence was 68.8% (95% CI, 64.8%-72.5%) and 33.7% (95% CI, 32.4%-35.1%) for the intermediate and low endemicity areas, respectively, according to the field data analysis. The models showed that a higher force of infection was identified in the 10- to 19-year-old age cohort (∼9,000 infected individuals per year per 100,000 susceptible persons) in the intermediate endemicity area, whereas a higher force of infection occurred in the 15- to 29-year-old age cohort (∼6,000 infected individuals per year per 100,000 susceptible persons) for the other macro-regions.

Conclusion: Our findings support the shift of Brazil toward intermediate and low endemicity levels with the shift of the risk of infection to older age groups. These estimates of HA force of infection stratified by age and endemicity levels are useful information to characterize the pre-vaccination scenario in Brazil.

Figure 1. The Brazilian and National Hepatitis Survey populations and the Brazilian macro-regions according to their HA endemicity levels.

Figure 2. Seroprevalence fitted curves of HA infection (solid lines) for the intermediate (χ² = 6.7, p  = 0.349) and low endemicity (χ² = 85.5, p <0.001) areas (A), the corresponding 99% CIs (red dotted and blue dashed lines, respectively), and 95% confidence intervals for the observed data (error bars); (B) Seroprevalence ratio of HA infection between the intermediate and low endemicity areas by age (solid line) and its corresponding 99% CI (dashed lines).

A value of 1 denotes an equal prevalence in both areas, an integer >1 indicates a higher prevalence in the intermediate endemicity area, and a value <1 indicates a higher prevalence in the low endemicity area. The fitting parameters were: year⁻² and year⁻¹ for the intermediate endemicity area and year⁻² and year⁻¹ for the low endemicity area.

Figure 3. Force of HA infection by age (A), as estimated by the catalytic approach, for the intermediate and low endemicity areas (solid lines) and the corresponding 99% CIs for the curves (red dotted lines for the intermediate endemicity area and blue dashed lines for the low endemicity area).

(B) Hepatitis A force of infection curves by age and the ratio between the intermediate and low endemicity areas for the North region (solid line) and its corresponding 99% CI (dashed lines). A value of 1 denotes an equal number of new infected individuals per year in both areas, an integer >1 indicates a higher number of new infected individuals per year in the intermediate endemicity area, and a value <1 indicates a higher number of infected individuals in the low endemicity area. (C) Proportion of new hepatitis A infections for the intermediate and low endemicity regions (red and blue solid lines, respectively) and the corresponding 99% CIs (dotted and dashed lines, respectively). (D) HA incidence ratio (solid line) between the intermediate and low endemicity areas by age and its 99% CI (dashed lines).

Figure 4. The contact function and the respective contour plot for the intermediate (A, assortative; C, assortative with food and water transmission) and low (B, assortative; D, assortative with food and water transmission) endemicity areas.

The fitted parameters were: (A) year⁻², year⁻¹, year and ; (B) year⁻², year⁻¹, year and ; (C) year⁻², year⁻¹, year, and year⁻¹; (D) year⁻², year⁻¹, year, and year⁻¹; for the fully homogeneous model (not shown), year⁻¹ (intermediate) and year⁻¹ (low).