MMR vaccine effectiveness in an outbreak that involved day-care and primary schools

► We studied a large measles outbreak in day-care and pre-school centres. ► The immunization schedule included two doses of MMR vaccine at 15 months and 4 years. ► Attack rate in children aged 6-11 months was higher than 12-23 months (22% and 9%). ► Vaccine effectiveness after receiving at least 1 dose of MMR vaccine was high (>96%). ► Advancing the first dose to 12 months instead of 15 would have prevented 91% of cases.