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

Summary: 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. [Copyright &y& Elsevier]

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