Lack of herd immunity against measles in individuals aged <35 years could explain re-emergence of measles in Catalonia (Spain)

OBJECTIVES: The objective of this study was to assess the association between lack of herd immunity in some population groups and the re-emergence of measles in Catalonia in 2006. METHODS: Herd immunity was considered established in the different population groups when the prevalence of positive serological results to measles was higher than the herd immunity threshold of 90%. The Chi-square test and the odds ratio were used to assess the association between lack of herd immunity and measles cases in the outbreak of 2006-2007. RESULTS: Herd immunity was not established against measles in individuals aged 0-14 years and 25-34 years, as the prevalence of positive serological results was <90% in these groups. In the measles outbreak of 2006-2007, 91% of cases occurred in age groups without herd immunity, and only 9% of cases occurred in age groups with herd immunity (p<0.001). The odds ratio for the association between lack of herd immunity and measles cases in the outbreak of 2006-2007 was 104 (p<0.001). CONCLUSION: Lack of herd immunity in individuals aged<35 years could be one of the factors underlying the re-emergence of measles in Catalonia.