Gateway virus
Via NPR:
Epidemiologists have noticed for a long time that vaccinating for measles
alone seems to protect against other diseases as well. The initial assumption was that getting vaxed for measles was just an indication of better health care in general, but that wasn't the whole story.
Instead, it appears that the measles virus is especially nasty. It weakens the immune system in general, and can delete previously acquired immunities. Lots of data from Euro countries with good health care systems bears this out. When kids get several viral diseases in a short time interval, measles tends to come first.
Conclusion would be obvious: In a poor health care system like USA STRONG, where budgets and time are limited, where you may not get a chance to do a long series of shots, do measles first.