Demographic changes and behavioural responses shape vulnerability to infectious disease outbreaks

Demographic shifts are reshaping population age structures worldwide, with implications for infectious disease dynamics. Since contact patterns, susceptibility and infectiousness often vary by age, the risk that pathogen introductions initiate a substantial outbreak depends on the populations age distribution and associated behavioural characteristics. We develop an age-structured mathematical model to estimate the risk that a single pathogen introduction leads to sustained transmission (the probability of a major outbreak) under long-term demographic transitions, incorporating changes in age-specific contact patterns and behavioural adaptation. Using the Republic of Korea (projected to become the worlds oldest population by 2050) as a case study, we show that population ageing generally reduces the probability of a major outbreak due to older individuals lower contact rates. However, this effect is attenuated for pathogens with increasing susceptibility or infectiousness with age, and if future older cohorts have higher contact levels than at present (e.g. through extended workforce participation in an ageing society). These findings demonstrate that, while outbreak risks are affected by demographic changes, they are further modified by associated behavioural responses, highlighting the importance of accounting for demographic and socio-behavioural context when assessing future infectious disease outbreak risks. Author SummaryIn the early stages of an infectious disease outbreak, the risk that initial cases lead to a substantial outbreak is shaped by a range of factors including the characteristics of the host population. Demographic changes, such as population ageing, are transforming societies worldwide, yet their implications for infectious disease emergence remain unclear. Here, we show that ageing populations reduce the likelihood that imported infections trigger major infectious disease outbreaks due to lower contact rates between individuals of older ages. However, this effect depends on how susceptibility, infectiousness and host behaviour vary with age. For example, increased social and economic activity among future older adults (due to a higher retirement age) could offset the decrease in the outbreak risk. These findings underscore the need to account for demographic and socio-behavioural factors, in addition to biological factors, when assessing future outbreak risks and designing robust public health strategies, particularly in societies undergoing rapid demographic change.