Vaccine and Infectious Disease Division

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Can your neighbors affect your risk of disease?

Despite the fact that for over a century we have known that vaccinating on a population level can reduce risk of infection on the individual level, vaccination to some infectious diseases remains a controversial issue; recently in certain parts of the United States. So why would vaccinating myself affect your child? This is referred to as herd immunity. Not unlike a herd of cattle, people congregate and interact with each other constantly: family, neighbors, strangers on the bus, children in school. Therefore, an individual’s risk of infection can be influenced by the susceptibility of others in the community.

To test the effect of vaccination on herd immunity to typhoid fever, a disease caused by the food- and water-borne pathogen Salmonella enterica, VIDD scientists used mathematical modeling to assess the risk of typhoid based on exposure to neighbors in a typhoid vaccine clinical trial carried out by outside investigators in 2009. The clinical trial assessed risk of typhoid fever in approximately 19,000 participants vaccinated with a typhoid vaccine and 19,000 vaccinated with a control.

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.

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