[Measures of the effect of vaccination in an outbreak of measles]

Vaccine efficacy, the effectiveness of a vaccination programme and the fraction of a population that would be necessary to vaccinate in order to prevent a measles outbreak are studied in a population having a vaccination programme. A dynamic model for acute, directly transmitted diseases was applied, based on the rate of transmission, namely, the probability that the infectious agent is transmitted from an infected to a susceptible person within a short space of time. We carried out a population-based study where we observed the final attack rates among vaccinated and unvaccinated people for a measles outbreak. In this population, the vaccination programme is responsible for a reduction of: 82.41% of all cases in the non-vaccinated sector; 98.45% of the average risk of disease among the vaccinated; and 97.29% of all cases among the population as a whole. Under these conditions there is the possibility of disease outbreaks arising, outbreaks which could be prevented if the percentage of the vaccinated population exceeded 95%.