People who have visited one of those places and think they may have been exposed can get a measles vaccine. It takes about 2 weeks for IgM levels to increase, and the incubation period of measles is sometimes longer (range: 7 to 21 days). So it can be possible to stave off the disease by getting vaccinated right away.
Of course, people who have already been vaccinated do not need to worry. The vaccine is over 99% effective.
A very small child who is too young to be vaccinated is at the most risk of getting the disease.
There are many others with illnesses who cannot be vaccinated. So, we hold our breath around kids or wear masks.
The MMR is a live vaccine, so immune compromised people cannot get vaccinated.