I know the feeling... The CDC already recommends getting an MMR shot at 6 months for babies that will be traveling abroad. Unfortunately the measles situation in the U.S. is now that of a third world country, so IMO it makes sense to go ahead and get the shot at six months, especially if you're in an area where there have been cases reported.

Unfortunately the effectiveness of a single dose of the vaccine at six months is only something like 50%, but it's better than nothing.