[Effect of previous vaccination on the dynamics of measles outbreak]

The effectiveness of a vaccine should really be measured through controlled clinical testing. Ethical questions, however, prevent this from being possible. Orenstein suggests that one of the methods that could be used to estimate the effectiveness of a vaccine is the study of the epidemic outbreaks of the disease wherever possible. This paper examines the calculation of the effectiveness of vaccinations in the field and of the protective effects of a vaccination programme through the analysis of an epidemic outbreak of measles.

A retrospective cohort study. The covered population was a group of children born in the municipality of Aznalcollar (a town with 5,571 inhabitants falling under the Alfarafe-Seville Health Authority District) between two epidemic outbreaks of measles recorded there in 1986 and 1994. Those children who had not reached vaccination age at the start of the 1994 outbreak were excluded from the cohort as were those who had been previously exposed to the virus--the children born before and during the 1986 epidemic. The study included both children who had been vaccinated and children who had not. All the cases fulfilled the diagnostic criteria set. The vaccination history of all the children being monitored was reviewed. After calculating the infection rate in those who had been vaccinated and those who had not, the vaccination effectiveness and the indirect, total and average effects of the vaccination programme were calculated.

The vaccination was effective in 91.5% of the children vaccinated. The vaccination programme led to a 78.9% reduction in the number of cases among the unvaccinated children, 98% among the children who had been vaccinated and 92% in the overall cohort studied, whose vaccination coverage was 66.4%.

The effects of vaccination and vaccination effectiveness can and should be calculated by taking advantage of the epidemic outbreaks that take place wherever possible. It is recommended that vaccination coverage of the population be increased, thus enhancing the protective effects of the programme.