Background and aims: 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.
Keywords: Anti-PT IgG; Immunisation; Pertussis incidence; Seroepidemiology; Whooping cough.
Copyright © 2014 Elsevier Ltd. All rights reserved.