A resurgence of mumps cases in countries with high rates of vaccination has raised concerns regarding the effectiveness of the vaccines currently being utilized. In countries such as the United States, immunization has been very successful in decreasing the number of mumps cases, and reduced the incidence of mumps by 99.9%, making the virus a prospect for eradication until these resurgences. A high percentage of these mumps cases have been in individuals who have followed vaccination recommendation guidelines. Two proposed reasons for this epidemiological trend are 1) a decrease in immunity from vaccination over time (waning immunity hypothesis) and 2) the ability of current circulating strains to evade vaccine-derived immunity (leaky vaccine hypothesis).
CEID’s Deven Gokhale, Tobias Brett, and Pejman Rohani collaborated with a research team to create transmission models to determine which of these mechanisms is responsible for the resurgence of mumps cases in the United States. These models used vaccine uptake and demographic data to test these two hypotheses.
The team created two transmission models that capture age and genotype for each vaccine failure mechanism. Age is an important factor to consider for the “waning immunity hypothesis”, since as individuals age, some may lose their mumps immunity they had previously acquired from an earlier vaccination. Genotypes (genetic makeup) are also essential to examine for the “leaky” hypothesis regarding virus genotype G. Vaccines are usually derived from the virus A genotype, but recent mumps epidemics in the United States, Canada, and the United Kingdom were caused by genotype G viruses.
It is important to understand where this vaccine failure is occurring to better understand transmission and prevent disease spread. If waning immunity is the issue, booster vaccinations could be needed periodically; however, for a “leaky” vaccine, boosters developed from a different genotype would not be effective and current vaccines must be altered.
Results indicated the exponential waning model was the most consistent with data observed from trends in rates of mumps cases and age groups affected at both the population and individual level. This means that the most likely cause for mumps reemergence in the United States is a decrease in immunity from vaccines. The model found that about 50% of individuals maintain their immunity throughout their lifetime, but 11.77% of vaccinated individuals lose immunity by the time they reach 18 years of age. This may explain why a majority of mumps cases occur in individuals that have been vaccinated.
Given that waning immunity is the primary cause of mumps resurgence, the current vaccine and immunization schedule is not adequate. In order to attain herd immunity, regular booster doses should be administered to increase immunity at the population level. Future research should examine efficient and cost-effective vaccination programs to control the spread of mumps in the United States in the hopes of eventually eliminating the disease.
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By: Brenna Daly