Modeling Immunity to Malaria with an Age-Structured PDE Framework

TL;DR

This paper introduces a new age-structured PDE model for malaria that integrates vector-host dynamics with immunity development, providing insights into disease spread, immunity acquisition, and vaccination impacts.

Contribution

The paper presents a novel PDE framework coupling immunity dynamics with transmission, including a probabilistic interpretation of $\

Findings

Model captures age-specific immunity and infection heterogeneity.

Endemic equilibrium exists with a forward bifurcation at $\

Vaccination reduces severe disease in children but may increase it in older children due to immunity effects.

Abstract

Malaria is one of the deadliest infectious diseases globally, causing hundreds of thousands of deaths each year. It disproportionately affects young children, with two-thirds of fatalities occurring in under-fives. Individuals acquire protection from disease through repeated exposure, and this immunity plays a crucial role in the dynamics of malaria spread. We develop a novel age-structured PDE malaria model, which couples vector-host epidemiological dynamics with immunity dynamics. Our model tracks the acquisition and loss of anti-disease immunity during transmission and its corresponding nonlinear feedback onto the transmission parameters. We derive the basic reproduction number ($\mathcal{R}_0$) as the threshold condition for the stability of disease-free equilibrium; we also interpret $\mathcal{R}_0$ probabilistically as a weighted sum of cases generated by infected individuals at different infectious stages and different ages. We parametrize our model using demographic and immunological data from sub-Saharan regions. Numerical bifurcation analysis demonstrates the existence of an endemic equilibrium, and we observe a forward bifurcation in $\mathcal{R}_0$. Our numerical simulations reproduce the heterogeneity in the age distributions of immunity profiles and infection status created by frequent exposure. Motivated by the recently approved RTS,S vaccine, we also study the impact of vaccination; our results show a reduction in severe disease among young children but a small increase in severe malaria among older children due to lower acquired immunity from delayed exposure.