The effects of correlation between the first and second scheduled doses of measles vaccine on the maintainability of measles elimination
Despite 50 years of measles management, the disease continues to kill many thousands of children annually. In order to prevent this mortality, managers are working to achieve measles eradication – that is, the reduction of global measles prevalence to zero. This eradication process comprises two parts: achieving local elimination (i.e. reducing local prevalence to zero) and maintaining elimination in the face of new incursions. In order to maintain elimination, most countries use a two routine dose vaccine strategy, where individuals receive doses of the measles vaccine at two specified target ages. The second dose is administered with two aims: to catch individuals for whom the first dose was ineffective, and to provide a chance for individuals who missed the first dose to receive one. However, if every individual who receives a second dose has also received a first dose (i.e. the second dose is truly the second dose each individual receives, not just a second chance to receive at least one dose), then the second aim will not be addressed, impacting the success of the vaccination strategy. To explore this impact, we analyze the steady states of a discrete-time age-structured model for immunity levels within a population, for various age structures.
As correlation between doses – that is, the proportion of the second dose administered non-independently to people who had the first dose - increases, the proportional population immunity resulting from a two-dose vaccine strategy decreases, even when controlling for first and second dose coverage. This effect is similar in magnitude to that arising from different age structures. For example, when vaccinating with 90% first and second dose coverage, population immunity is maintainable at a level exceeding the accepted 95% herd immunity threshold in an idealized developed age structure with independent doses. However, if doses are administered in a totally correlated way, or the population has a developing age structure, only a 90% population immunity threshold can be reached. If the doses are totally correlated and vaccines are administered to a developing age structure, then not even a 90% population immunity threshold can be achieved and the risk of an outbreak is greatly increased. Our results suggest that, while increasing coverage of each dose will always be good, it is possible that we can adapt our strategies to “work smarter, not harder” and de-correlate the two doses.