The critical vaccine efficacy. The group-to-group reproduction number under vaccination in a population of day care cohorts of equal size, for different values of vaccine efficacy against acquisition. For each line the size of the day care cohort is given as the number of day care attendees (50, 30, and 20). The entire cohort including family members were used in the simulations. In each simulation roughly half of the day care attendees had a sibling attending the same day care centre, i.e., one third of the families had two children in the day care centre and two thirds of the families had one child in the day care centre.