we had eradicated it
Clearly we hadn't.
We horded the vaccine domestically, patented efficient methods for manufacturer and distribution, and curtailed it's use in states we considered too poor or too evil.
Some of the highest rates of measles today can be found in Yemen, a country we've been bombing since the early '10s.
We didn't exterminate the disease. We incubated it. And now we're reaping what we've sown.