In Chapter 2, we described the measles outbreak of 2013/14 that occurred in The Netherlands. The outbreak took mainly place within the orthodox Protestant community. In total, 2700 measles cases, 181 hospitalisations and 1 death were reported. In Chapter 3, we assessed the actual burden of the 2013/14 measles outbreak by assessing the completeness of reporting. Given that 9% of cases were reported, the estimated total size of the 2013/14 outbreak approximated 31,400 measles cases. In Chapter 4, we assessed the severity and contagiousness of 2539 unvaccinated (94%), 121 once vaccinated (5%) and 16 (1%) at least twice-vaccinated measles cases. Compared with unvaccinated and once-vaccinated cases, twice-vaccinated cases were less severe and less infectious. Our findings support the recommendation of the WHO of a two-dose MMR vaccination schedule. In Chapters 5, 6, and 7, we discussed the uptake, effectiveness, and tolerability of an early MMR vaccination offered to infants between 6 and 14 months of age. The national vaccination register is individual-based and is therefore suitable for tailored interventions such as the administration of an early MMR to infants between 6-14 months of age who live in municipalities with low vaccination coverage (<90%). The vaccine uptake of invited infants was 57% (5800 out of 10,097 infants). The crude estimate of the early MMR vaccine effectiveness was estimated to be 94% (95%CI 79%-98%). However, when we, in contrary to previous studies, took into account of the different levels in exposure between the vaccinated and unvaccinated infants, the effectiveness decreased to 71% (95%CI -72%-95%). Administering the vaccine to infants of six months old was considered well-tolerated. Adverse events were similar or fewer than observed among 14 month-olds infants. In Chapter 8, we assessed the correlate of protection against measles and subclinical (asymptomatic) measles among once vaccinated children. The correlate of protection against measles was estimated to be below 0.345 IU/ml, ...