Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.
Temporal trends in strain frequencies for the fim2 (A), fim3 (B), ptxA (C), ptxP (D), and prn (E) alleles. Four periods were defined to reflect the worldwide changes in pertussis vaccination, the early WCV period (earlier than 1960), the period in which mainly WCVs were used (WCV period, 1960 to 1995), the period in which both WCVs and ACVs were used (WCV/ACV period, 1996 to 2000), and a period in which mainly ACVs were used (ACV period, later than 2000).
