title: Seroprevalence of IgG antibodies to pertussis toxin in children and adolescents in Estonia.
abstract: OBJECTIVE
  Despite high immunisation coverage and frequent booster doses, the national notification rates of pertussis in Estonia have been increasing. The peak of 97/100,000 was reached in 2010 which is the highest incidence rate since 1962 (210/100,000). We aimed to measure the prevalence of pertussis toxin (PT) IgG type antibodies in subjects of <18 years and to estimate the pertussis infection activity in a recently non-immunised cohort.
  
  
  METHODS
  In a cross-sectional serosurvey, all consecutive leftover sera were collected in the Tartu University Hospital during April-August 2012. Anti-PT IgG concentration was measured by commercial ELISA and analysed in yearly cohorts. The antibody concentrations ≥62.5 IU/mL was considered suggestive to pertussis in the last year among 9- to 14-year-olds.
  
  
  RESULTS
  The GMC of the anti-PT-IgG was 7.4 IU/mL (95% CI 6.9-8.0). In the total of 1053 serum samples, the highest proportion of sera with high antibody titres ≥125 IU/mL and ≥62.5 IU/mL were at the ages when pertussis vaccine boosters were given: 7 years 10.9% (95% CI 4.1-22.3) and 2 years 36.9% (95% CI 25.3-49.8), respectively. Approximately half of all sera had undetectable anti-PT IgG levels. The estimated incidence of Bordetella pertussis infection among 9- to 14-year-olds in the year before serum sampling was 6.3% (95% CI 3.3-10.8), which is at least 60 times higher than the officially reported incidence of pertussis disease in respective years.
  
  
  CONCLUSIONS
  The serologic method is not suitable for diagnosing pertussis in instances when the last pertussis immunisation was less than one year ago. The relatively high proportion of subjects with undetectable anti-PT IgG levels and the relatively low rate of officially reported pertussis cases suggest that low antibody levels do not necessarily indicate the absence of protection. The estimated incidence rate of pertussis is much higher than officially reported figures, which suggests that asymptomatic/mild B. pertussis infection remains unrecognised and unreported.
authors.affiliation: The Institute of Microbiology, University of Tartu, Ravila 19, 50411 Tartu, Estonia; Children's Clinic of Tartu University Hospital, Lunini 6, 51014 Tartu, Estonia. Electronic address: Piia.Jogi@kliinikum.ee.
authors.forename: Piia
authors.initials: P
authors.lastname: Jõgi
authors.affiliation: Department of Family Medicine, University of Tartu, Puusepa 1a, 50406 Tartu, Estonia.
authors.forename: Marje
authors.initials: M
authors.lastname: Oona
authors.affiliation: Department of Public Health, University of Tartu, Ravila 19, 50411 Tartu, Estonia.
authors.forename: Karolin
authors.initials: K
authors.lastname: Toompere
authors.affiliation: Department of the Children's Clinic, United Laboratories, Tartu University Hospital, Lunini 6, 51014 Tartu, Estonia.
authors.forename: Sirje
authors.initials: S
authors.lastname: Leedo
authors.affiliation: Department of Communicable Disease Surveillance and Control, Health Board, Paldiski mnt 81, 10617 Tallinn, Estonia.
authors.forename: Jevgenia
authors.initials: J
authors.lastname: Epstein
authors.affiliation: The Institute of Microbiology, University of Tartu, Ravila 19, 50411 Tartu, Estonia.
authors.forename: Irja
authors.initials: I
authors.lastname: Lutsar
mesh_terms: D000293:Adolescent; D000907:Antibodies, Bacterial / Q000097:blood*; D002648:Child; D003430:Cross-Sectional Studies; D004957:Estonia / Q000453:epidemiology; D006801:Humans; D007074:Immunoglobulin G / Q000097:blood*; D037342:Pertussis Toxin / Q000276:immunology*; D010567:Pertussis Vaccine / Q000627:therapeutic use*; D015995:Prevalence; D016036:Seroepidemiologic Studies; D014917:Whooping Cough / Q000453:epidemiology / Q000517:prevention & control
chemical_list: D000907:Antibodies, Bacterial; D007074:Immunoglobulin G; D010567:Pertussis Vaccine; D037342:Pertussis Toxin
keywords: Anti-PT IgG; Immunisation; Pertussis incidence; Seroepidemiology; Whooping cough