If you have a high enough vaccination rate from the people who are here in this country’s then any foreigners coming in with a disease like that won’t cause an outbreak.
It pays to have a high vaccination rate here!
It also pays to have some people not vaccinated so we can do these studies. If at least 5% of the population is not vaccinated there is a control group to study the affects of the vaccination. Given that the vaccination changes every few years (companies change, process changes, etc.), and the illness morphs, there are always good reasons to see if the vaccine is working or is faulty.