Rubella

So what does this mean exactly? Most likely the mumps portion of the MMR vaccine is less effective in the Marshallese population in particular due to some genetic difference in their immune response. It is also possible this virus is “drifting” by changing a protein here or there or an H or N molecule as similar to the influenza virus. So all in all, there are likely small changes in the natural mumps virus altering the landscape for everyone; however those with Marshallese background are naturally more susceptible.