If you can give the second dose of MMR as early as 28 days after the first dose, why do we routinely wait until kindergarten entry to give the second dose?

The second dose of MMR may be given as early as 4 weeks after the first dose, and be counted as a valid dose, if both doses were given after the first birthday. The second dose is not a booster, but rather it is intended to produce immunity in the small number of people who fail to respond to the first dose. The risk of measles is higher in school-age children than those of preschool age, so it is important to receive the second dose by school entry. It is also convenient to give the second dose at this age, since the child will have an immunization visit for other school entry vaccines.