In itself, the simulation model showed that an early delivery of MMR2 has the potential to reduce the risk of large measles outbreaks. Improving coverage remains the best way to fully mitigate outbreak risk.

Across scenarios and sensitivity analyses, bringing MMR2 forward led to a short- and medium-term reduction in case numbers in the simulations. This decrease was impacted by coverage: if MMR2 coverage decreased by 3-5% in the new schedule, only 4.8% of cases were averted.

Vaccine-induced immunity did not wane in the reference scenario. When a slow waning was included (from our previous study www.thelancet.com/journals/lan...), the impact of early MMR2 delivery decreased slightly (5.28% reduction). If waning started at a younger age, the reduction dropped to 3.79%.

The different vaccination strategies changed the age distribution of the cases: an earlier delivery of MMR2 led to a lower proportion of measles cases in children aged between 2 and 4. A 0.5% increase of MMR1 uptake decreased the number of measles cases across all age groups.

We used a compartmental model stratified by age and region, and simulated outbreaks with different vaccination schedules and coverage. Without changes to coverage, moving MMR2 to 2 years reduced the number of cases by 11.86%, similar to increasing MMR1 coverage by 0.5% across all regions and years.

Changes in the vaccination schedule affect transmission: it may impact vaccine coverage, and close an immunity gap by quickly protecting children who did not react to the first dose, but it also requires an extra vaccination appointment. This may be difficult for parents, and lead to lower coverage.