No abstract

Infectious disease control measures often require collective compliance of large numbers of individuals to benefit public health. This raises ethical questions regarding the value of the public health benefit created by individual and collective compliance. Answering these requires estimating the extent to which individual actions prevent infection of others. We develop mathematical techniques enabling quantification of the impacts of individuals or groups complying with three public health measures: border quarantine, isolation of infected individuals, and prevention via vaccination/prophylaxis. The results suggest that (i) these interventions exhibit synergy: They become more effective on a per-individual basis as compliance increases, and (ii) there is often substantial “overdetermination” of transmission. If a susceptible person contacts multiple infectious individuals, an intervention preventing one transmission may not change the ultimate outcome (thus, risk imposed by some individuals may erode the benefits of others’ compliance). These results have implications for public health policy during epidemics.

Compliance with infectious disease control measures can benefit public health but be burdensome for individuals. This raises ethical questions regarding the value of the public health benefit created by individual and collective compliance. Answering such questions requires estimating the total benefit from an individual’s compliance, and how much of that benefit is experienced by others. This is complicated by “overdetermination” in infectious disease transmission: each susceptible person may have contact with more than one infectious individual, such that preventing one transmission may have no net effect if the same susceptible person is infected later. This article explores mathematical techniques enabling quantification of the impacts of individuals and groups complying with three types of public health measures: quarantine of arrivals, isolation of infected individuals, and vaccination/prophylaxis. The models presented suggest that these interventions all exhibit synergy: each intervention becomes more effective on a per-individual basis as the number complying increases, because overdetermination of outcomes is reduced, Thus additional compliance reduces transmission to a greater degree.