These findings suggest that a lower rate of hospital admission is associated with adherence to the routinely recommended schedule. Healthy vaccinee bias, defined as when children who are more susceptible to illness are vaccinated later or not at all, resulting in an overestimation of the beneficial effect of the next vaccination, may have been present and partly explains the lower rate of infections observed for receipt of an additional vaccine, rather than being an effect of specifically receiving MMR.
The findings of this large scale observational study on non-specific effects emphasise the extreme difficulty in interpreting such results given the likely presence of healthy vaccinee bias.