I’m saying this without any actual proof, but I strongly suspect the Mennonite outbreak is among a group of Old Colony Mennonites. Due to pressure from various governments, Old Colony Mennonites migrated from Ukraine to the Russian Empire, then to Canada (where some stayed), and then to Mexico; this left them with a cultural distrust of governments and government agencies (such as public health agencies). They settled in rural Mexico and avoided the government and public contact as much as possible, meaning that (among other things), they managed to avoid the great vaccine push of the 1950s and 1960s, whereas more mainstream communities were educated on the benefits of vaccines and got to see those benefits in their everyday lives. When Mexico’s economy deteriorated in the 1980s and 1990s, some of those Old Colony Mennonites moved to Texas and others back to Canada, where they remain in under-vaccinated pockets.
I haven’t heard anything about which specific branch of Mennonites are involved in the outbreak but, as I said, I suspect they’re mostly Old Colony Mennonites.
I know that Canada and the US used to often do things very similarly (sorry about gestures broadly). I’m not sure how similar we were in measles vaccination. In the US, the vaccination recommendations are:
If you were born in or before 1957, you’re presumed to be immune.
If you were born after 1957 and were never vaccinated, get vaccinated.
If you were vaccinated before 1967, they strongly recommend you get a booster shot. (The original vaccine wasn’t as effective as the later vaccine.)
If you were vaccinated between 1968 and 1989, they’d like you to get a booster shot. (They originally thought that vaccine provided lifetime immunity, but that eventually wore down. A booster brings you back up to snuff.)
If you were vaccinated after 1989, you should have gotten two shots and should be fine. If you’re concerned, you can tested for immunity.