Objectives: Rates of pertussis immunisation among children in Estonia are high (∼95%), but pertussis is still the most common vaccine preventable childhood disease. Adults are suspected to be sources of pertussis in children. We aimed to measure pertussis toxin (PT) IgG in adults to estimate pertussis infection activity and compare estimated and reported pertussis incidences.
Methods: In a cross-sectional serosurvey, consecutive leftover blood sera (n=3327) from subjects aged 20-99 years old were collected at Quattromed HTI laboratories between the 7th January and 27th February 2013. Anti-PT IgG concentration was measured by ELISA (Euroimmun, Lübeck, Germany). Estimated annual pertussis incidence was calculated for 10-year age classes using de Melker et al. (2006. J Infect. 53(2):106-13) formula.
Results: The mean number of samples in each 10-year age class was 466 (SD 20.5), except for 90-99 year olds which contained 65 samples. More than half of all subjects (58.1%) had anti-PT IgG <5.0IU/mL, 2.7% had 62.5 to <125IU/mL and 0.6% ≥125IU/mL; no differences occurred between 10-year age classes. Estimated incidence of pertussis infection was 5.8% (95% CI 4.8-7.0) in 2012, with peaks observed in 20-29 year olds (11.0%; 95% CI 7.4-15.6) and 90-99 year olds (10.8%; 95% CI 3.0-26.2). Estimated pertussis incidence rate was 915 times higher than reported. Of 80 subjects with anti-PT IgG ≥62.5IU/mL, 25 (31.3%) had complained of coughing to their GP during the previous six months.
Conclusion: The frequency of pertussis infection was similar for all ages, suggesting similar Bordetella pertussis activity in adults and children. The wide gap between reported and estimated incidence indicates poor recognition of pertussis, likely owing to it being an asymptomatic or mild disease.
Keywords: Anti-PT IgG; Immunisation; Pertussis incidence; Seroepidemiology; Whooping cough.
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