title: Lack of herd immunity against measles in individuals aged <35 years could explain re-emergence of measles in Catalonia (Spain).
abstract: OBJECTIVE
  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).
  
  
  CONCLUSIONS
  Lack of herd immunity in individuals aged<35 years could be one of the factors underlying the re-emergence of measles in Catalonia.
authors.affiliation: Public Health Agency of Catalonia, Department of Health of Catalonia, Bac de Roda 83-85, 08005 Barcelona, Spain; CIBER - Epidemiology and Public Health (CIBERESP), Instituto de Salud Carlos III, Madrid, Spain. Electronic address: pedro.plans@gencat.cat.
authors.forename: Pedro
authors.initials: P
authors.lastname: Plans
authors.affiliation: Public Health Agency of Catalonia, Department of Health of Catalonia, Bac de Roda 83-85, 08005 Barcelona, Spain; CIBER - Epidemiology and Public Health (CIBERESP), Instituto de Salud Carlos III, Madrid, Spain.
authors.forename: Nuria
authors.initials: N
authors.lastname: Torner
authors.affiliation: Public Health Agency of Catalonia, Department of Health of Catalonia, Bac de Roda 83-85, 08005 Barcelona, Spain; CIBER - Epidemiology and Public Health (CIBERESP), Instituto de Salud Carlos III, Madrid, Spain.
authors.forename: Pere
authors.initials: P
authors.lastname: Godoy
authors.affiliation: Public Health Agency of Catalonia, Department of Health of Catalonia, Bac de Roda 83-85, 08005 Barcelona, Spain.
authors.forename: Mireia
authors.initials: M
authors.lastname: Jané
mesh_terms: D000293:Adolescent; D000328:Adult; D002648:Child; D002675:Child, Preschool; D004196:Disease Outbreaks; D006801:Humans; D032521:Immunity, Herd; D007223:Infant; D008457:Measles / Q000453:epidemiology* / Q000276:immunology / Q000517:prevention & control; D008458:Measles Vaccine / Q000627:therapeutic use; D015995:Prevalence; D013030:Spain / Q000453:epidemiology; D014611:Vaccination / Q000706:statistics & numerical data; D055815:Young Adult
chemical_list: D008458:Measles Vaccine
keywords: Herd immunity; Measles elimination; Measles outbreaks; Prevention; Seroepidemiology