For the 60 largest cities, the authors plot the number of weeks per year during
which no pertussis cases were reported against the respective population size.
The prevaccine (1944–1957) and vaccine (1957–1974) era are plotted
in dark and light filled circles, respectively. To illustrate the concept, the
incidence reports for three different cities, Birmingham, Sunderland and
Walsall, for both prevaccine (dark) and vaccine (light) eras have also been
presented. The dots in these insets indicate that no pertussis cases were
reported during that week.