Using these data, VIDD Staff Scientist Dr. Dennis Chao and VIDD Member Betz Halloran and collaborators developed a mathematical model to investigate the association between an individual’s risk of typhoid outcome and the vaccination status and age of people living nearby (see figure). Using a Cox proportional hazards model, they found that young children and those who were not vaccinated had higher rates of typhoid illness during the study. They hypothesized that living near more high-risk people (e.g., unvaccinated people or young children) would be associated with higher rates of typhoid illness. To test this hypothesis, they defined a novel covariate called potential exposure to be the sum of the relative risks (defined by vaccination status and age) of all people living within 100 meters of each individual. Potential exposure was significantly associated with typhoid illness. These data provide insight into how herd immunity contributes to infectious disease transmission within populations. It is the hope that learning more about how individuals affect the infection risk of others will help inform the development of future preventative and therapeutic treatments.
Chao DL, Park JK, Marks F, Ochiai RL, Longini IM, Halloran ME. The contribution of neighbours to an individual's risk of typhoid outcome. Epidemiol Infect. 2015 May 4:1-8.