Changes in the Dutch Bordetella pertussis population in the first 20 years after the introduction of whole-cell vaccines.

Despite the introduction of mass vaccination in 1953 in The Netherlands, pertussis is currently an endemic disease with regular epidemic outbreaks. Changes in the Bordetella pertussis population in the first 20 years after the introduction of vaccination were studied by indexing IS1002 fingerprint types, fimbrial serotypes and 15 genes encoding surface proteins. Three periods were compared, the pre-vaccination period (1949-1952) and two subsequent periods, 1953-1958 and 1965-1972. Except for fimbrial serotypes, no changes were observed in the B. pertussis population between the first two periods. Mortality decreased fivefold and 543-fold in the periods 1953-1958 and 1965-1972, respectively, compared to the pre-vaccination period. The largest decrease in mortality coincided with significant changes in the B. pertussis population with respect to the frequencies of fimbrial serotypes, fingerprint types and ptxS1 alleles. A new fingerprint type (ft29), associated with the novel ptxS1 allele ptxS1A was observed in 50% of the isolates in the period 1965-1972. Of the 15 investigated genes, only ptxS1 showed a mismatch between the vaccine strains and clinical isolates, suggesting that it may have played a role in driving the observed changes. It is proposed that, within 10-20 years after the introduction of mass vaccination, an adaptive response occurred consisting of clonal expansion of strains, which expressed a pertussis toxin variant distinct from the vaccine variants. This adaptation had very little, if any, effect on mortality, however.