Seroprevalence of pertussis antitoxin (anti-PT) in Sweden before and 10 years after the introduction of a universal childhood pertussis vaccination program

Abstract

The prevalence of IgG ELISA antibodies against pertussis toxin (anti-PT) was studied in two Swedish seroepidemiological studies. One was performed in 1997 when the new pertussis vaccination program was 1 year old (n = 3420). In 2007, when Pa vaccines had been used countrywide for 10 years in the universal child vaccination program, this study was repeated to analyze the effect of vaccination on anti-PT prevalence (n = 2379). Before the statistical analysis of seroprevalence, children vaccinated within the last 2 years before the serosurveys were excluded. The results indicate a reduced exposure to Bordetella pertussis in the population. The proportion of sera without measurable anti-PT antibodies increased significantly, aggregated over all comparable age groups, from 3.8% in people sampled in 1997 to 16.3% in people sampled in 2007. For cord blood, 1% was without measurable anti-PT antibodies in 1997 compared to a significantly higher level, 12%, in 2007. With anti-PT concentrations of > or =50 and > or =100 EU/ml as cutoff points for 'recent infection' the proportion above the cutoff points for younger children was significantly higher in 1997 than in 2007 at both cutoff points. For all adults, 20 years of age and older, the difference in proportions above the lower cutoff point was close to statistically significant, comparing 1997 with 2007. This was not the case at 100 EU/ml. In the 1997 samples of children, there was a significant downward trend of 'recent infections' at both cutoff points for three sampled age groups between 5 and 15 years of age from 21% at 5.0-5.5 years of age to 7% at 14.7-15.7 years for the lowest cutoff. In the 2007 samples of children, on the contrary, there was a significant continuous upward trend of 'recent infections', at both cutoff points, for four sampled age groups between 4 and 18 years of age - from 4% at 4-5 years of age to 16% at 17-18 years at the lowest cutoff. The continuous increase, with age of children with high anti-PT concentrations, supports the recent change in the general Swedish childhood vaccination program to include a pre-school booster at 5-6 years and a school-leaving booster at 14-16 years of age.

Fig. 1

(A, B) Seroimmunity 1997 and 2007. Reverse cumulative distribution curves of ELISA IgG anti-PT antibodies in ‘not recently vaccinated’ children and adults in comparable age groups, all ages together.

Fig. 2

(A, B) Sero-survey 1997 and 2007. Reverse cumulative distribution curves of ELISA IgG anti-PT antibodies by comparable age groups in children after exclusion of ‘recently’ vaccinated children.

Fig. 3

(A, B) Sero-survey 1997 and 2007. Reverse cumulative distribution curves of ELISA IgG anti-PT antibodies in adults by comparable age groups. No cohort was vaccinated after 1979.

Fig. 4

Sero-survey 1997 and 2007. The proportion of sera with concentrations below MLD (minimum level of detection) distributed by age groups after exclusion of ‘recently’ vaccinated children and non-comparable age groups.

Fig. 5

(A, B) Sero-survey 1997 and 2007. The proportion of sera with concentrations ≥50 and ≥100 EU/ml, respectively, distributed by age groups after exclusion of ‘recently’ vaccinated children and non-comparable age groups.