Measles immunity and vaccination policy in Catalonia
Introduction
Measles is still a disease which causes substantial morbidity and mortality especially in the less developed countries, being one of the ten primary causes of death.[1]. Its reservoir is exclusively human, there are no asymptomatic carriers, and a highly effective vaccine exists. For these reasons, the European Region of the WHO has designated the elimination of the illness as one of the targets formulated in the Health for All Strategy[2]. The Catalan Health Plan also includes the elimination of indigenous measles as one of its targets for the year 2000[3].
The disease is highly infectious when compared with other infectious agents such as the chickenpox or mumps viruses[4]and this makes its total elimination very difficult. The existence of a certain number of susceptible subjects is sufficient to produce outbreaks; these have been reported even when the vaccine coverage is 98%5, 6, 7, 8. A highly elevated percentage of the population bearing anti-measles antibodies is needed before herd immunity becomes an effective barrier against the virus. Mathematic models of the disease's behaviour indicate that the immune population must be over 90% before this barrier is produced9, 10, 11, which, assuming that the vaccine's efficacy is 95%, means that vaccine coverage must be superior to 95%10, 12, 13.
Studies of the prevalence of anti-measles antibodies in a population are of interest not only as a way of objectively measuring the proportion of the population that is immune, but also as a tool for adjusting specific strategies for the elimination of measles and for the designation of resources. Seroepidemiological surveys of measles were carried out in Catalonia in 1983 and 198614, 15; the data obtained in this last study supported the introduction of a second dose of vaccine into the schedule of systematic vaccinations of the Generalitat of Catalonia. Thus, since 1989, two doses of measles–mumps–rubella vaccine have been administered in Catalonia; one at 15 months and the other at 11 years.
The objective of the present study is to find out the prevalence of anti-measles antibodies in a representative sample of the school population of Catalonia and to compare the results obtained with those of the earlier seroepidemiological studies, in order to evaluate the changes which have occurred.
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Subjects and methods
The study was carried out in 1996 using a representative sample of schoolchildren aged 6–7 years, 10–11 years, 13–14 years and 15–16 years in Catalonia, an autonomous community of 6 million inhabitants situated in the north–east of Spain. The samples were obtained by means of a random sample by clusters (schools) stratified by provinces. 40 schools were selected at random from a list containing all the schools in Catalonia: 25 schools were of the province of Barcelona, 5 of Tarragona, 5 of
Results
1568 subjects were approached. The total number of subjects who consented and were included in the study were 1231 schoolchildren aged from 6–16 years, being the participation rate of 79%. The distribution of the sample by the sociodemographic variables considered in the study was similar to that of the Catalan population17, 18. According to vaccination cards, 91.1±1.5% of the schoolchildren had been vaccinated for measles (one or two doses), this figure being highest in the 13–14 age group
Discussion
The most notable facts to emerge from the results of this study is that the prevalence of antibodies in the school population in Catalonia is high and that it has increased significantly in all age groups with respect to the prevalence found in the study carried out in 1986 using the same methodology. The overall prevalence found in the present study supposes a notable increase with respect to that found in 1983 and 198614, 15. These data are in full accordance with the downward evolution of
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2011, VaccineCitation Excerpt :In 1987, the age of MMR administration was changed from 12 to 15 months, and in 1988 a second dose of MMR at 11 years was added. To ensure that the proportion of vaccinated children aged less than 10 years reached 95% and to achieve the elimination of measles by 2000, from the last quarter of 1998, the second dose of MMR was advanced to 4 years [15]. Therefore, at the time of the outbreak studied here, routine vaccination consisted of two doses of MMR at 15 months and 4 years, using the Schwarz (Priorix®) and Enders (MSD®) strains.
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