5% of people do not respond to the 1st dose of MMR
, and rubella) vaccine.
A 2nd dose is thus given to catch this small group of non responders. Unlike other vaccines then, the second dose is not a booster--it is another chance for those 5 % who did not make antibodies the first time. If you were a non responder exposed to rubella after your first dose--you could catch the disease!