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Date of Award
Master of Science (MS)
Department of Integrated Science and Technology
After an effective vaccine was widely available, measles incidence fell by 98%. Immunization programs and surveillance systems in the United States (US) are so successful there has been no endemic spread since 2000. The threat from measles lies in its high infectivity, an asymptomatic infectious state that lasts an average of four days and the decreasing popularity of vaccination. The lack of first-hand experience with the infection (due to immunization) has caused some to wonder if the vaccine is necessary. Herd immunity threshold is a measure of the fraction of immune individuals present in a population to keep disease reproduction rate below one. This threshold varies with population and disease characteristics. While below herd immunity threshold an index case could cause a small outbreak. In contrast, a gradual decrease in vaccination rates (or an increase in exemption rates) above herd immunity threshold coupled with an index case can lead to an epidemic. Endemic state is attained if the chain of infection persists for greater than one calendar year. Such a return to endemic state as has been seen in the United Kingdom. This study uses System Dynamics methodology to create a Measles Aging Chain Susceptible-Exposed-Infected-Recovered model (named MACSEIR) to analyze the vaccination conditions under which outbreaks and possibly endemic spread of measles could occur in the state of Virginia. The model utilizes a fictional population with demographic characteristics taken from US census data in addition to epidemiologic data from the Virginia Department of Health and Centers for Disease Prevention and Control (CDC). Outbreaks have been simulated under varying vaccination conditions. The study shows that outbreaks will occur in any vaccination rate conditions while greater than 4% of the population is susceptible to measles. While measles incidence is infrequent in the US, healthcare providers should still maintain a high level of suspicion in differential diagnosis because it is endemic in many countries that American families frequent. Though national vaccination rates are still high, some community rates are not; they should be the focus of prevention efforts.
Gowe, Ivan W., "System dynamics modeling study of the possibility of endemic measles in the state of Virginia" (2013). Masters Theses. 224.