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Measles transmission has been largely controlled in most Pacific Island Countries and Areas, but periodic large outbreaks have occurred in a number of places due to various factors, including vaccine coverage, susceptible population size and connections between countries. This research aims to estimate susceptibility build-up rates and forecast outbreak frequency in the Pacific using epidemiological and mathematical modelling with the goal of influencing and informing vaccine policy and practice for the Pacific Island Countries.
Camperdown - School of Public Health
Measles transmission has been achieved in most Pacific Island Countries and Areas (PICs). Yet large cyclical outbreaks have been observed in the Pacific for eg the Federated States of Micronesia in 2014, Samoa in 2019, Fiji in 2019, American Samoa in 2019 following incursions. Scale, size, and time between outbreaks have varied, likely related to vaccine coverage, the size of the susceptible population and connection within and between countries.
Measles is extremely infectious. In a fully susceptible population, each case will transmit the disease to an average 12-18 people. However, infection or vaccination leads to life-long immunity. Consequently, measles tends to spread in explosive epidemics which start and end rapidly as the disease ‘burns through’ the susceptible population leaving most people immune. However, the birth of children, If unvaccinated, will add to the population of people susceptible to measles, which may be sufficient to ‘fuel’ a new epidemic if the disease is reintroduced from another area. When the proportion of the population immune to measles (either due to vaccination or past infection) is above a threshold level of about 95%, reintroduction becomes very unlikely, and if reintroduced the disease is unable to spread through the population.
PICs are small, geographically dispersed remote islands and measles incursions are not as frequent. Even though the vaccination coverage for measles tends to be high in most Pacific Island Countries and Areas, there can be sizeable susceptible populations as a portion of the children are unvaccinated or fail to develop immunity following vaccination. In some places changing vaccine policy can impact or compromise the efficacy of the vaccine. In other places, immigration of unvaccinated persons adds to the susceptible population.
However, in small insular populations where the proportion of the population immune to measles is well below 95% there may not be an outbreak of measles for many years leading to critical questions:
- Why did Samoa only see the outbreak in 2019 when coverage had been staggering at 40-50% for many years?
- Tonga had an outbreak in a highly vaccinated population – 50% 2 dose; 30% one dose.
Hypothesis: In small geographically remote island countries, such as found in the Pacific, incursions of measles are infrequent; outbreaks can only occur when there is a build-up of susceptible populations and thresholds exceed.
Aim: To estimate the rate of build-up of susceptible populations and quantify and forecast the frequency of outbreaks in the Pacific Island Countries and Areas
Methods: Using epidemiological and mathematical modelling, this PhD will look at vaccination, surveillance, travel and other open—source data to predict the rate of build-up of susceptible populations under different vaccination coverage rates, and campaigns.
Outcome: This project will influence and inform measles vaccine policy and practice for the Pacific Island Countries
The opportunity ID for this research opportunity is 3340