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Temporal trends in the frequency of pertussis toxin S1 subunit gene (ptxA) variants in the United Kingdom B. pertussis population. The number of strains containing distinct ptxA variants was determined for each of the six decades shown. The proportion of strains containing ptxA(1) out of the total number of strains is shown for each period. Error bars show the 95% CI for these percentages. The United Kingdom WCV strains contain ptxA(1) and ptxA(2).
