It could be more serious; the factor would relate to 'herd immunity', broad well spread immunity due to years of everyone being exposed to the same pathogens. Slight mutations over a bit of time would still result in some disease in the population but infectious disease cases would be relatively mild in general.
Foreign migrants, especially children, could carry disease pathogens quite a bit different from the 'local' mutated variety; such infections could potentially be more serious than those caused by generally 'local' viral pathogens.
No evidence for that being the case but food for thought.
Nnnasty Foreigners have brought their sicknesses into our Country. I would not be surprised if your theory is right