Vaccination in a growing population: An age-structured model of trachoma

Simple models of the dynamics of infectious diseases, and the concept of the basic reproductive number, are often based on an assumption of constant populations. There is some foundational work on the role of the birth rate in defining the basic net replacement ratio, but it has rarely been applied, except in the context of HIV. Here we consider trachoma, which results from repeated infection of the conjuctiva with Chlamydia trachomatis and remains a leading infectious cause of blindness globally particularly affecting populations with high poverty and limited access to healthcare. It is estimated that 103 million individuals are at risk of blindness from trachoma, and that control by antibiotic mass drug administration (MDA) is reducing this risk in many populations but the persistence and resurgence of active (inflammatory) trachoma in endemic areas highlight the need for additional interventions. Birth rates in endemic countries are high, and on the decadal timeline of interventions may affect the dynamics of transmission. This study presents an age-structured deterministic mathematical model to evaluate the potential impact of future C. trachomatis vaccination in a growing population. The model captures critical vaccination thresholds, disease dynamics under different birth rates, and the effects of vaccination efficacy, susceptibility reduction, and waning immunity. Results reveal that increased population growth can reduce disease prevalence and critical vaccination thresholds, thereby enhancing the effectiveness of vaccination strategies. Conversely, declining birth rates in trachoma-endemic populations may increase the vaccination coverage required for elimination, highlighting the importance of accounting for demographic change when designing long-term vaccination strategies. Our findings underscore the potential role of a trachoma vaccine in not only achieving but sustaining elimination in endemic regions, especially when targeted to appropriate ages.