you have proof the DNA of the disease is the same then as now?
That's a possibility. Virulence factors can change over time. It's been hypothesized that the near complete absence of Rheumatic Fever might be due to less virulent streptococcal A bacteria. Or it might be due to increased treatment, better nutrition, sunspots or something else. It's hard to prove.
FTFA
As much pertussis as we're seeing now, we're still seeing in most places pretty good control in the very young," who are at the highest risk of dying form pertussis, Halperin says. "We're seeing lapsed immunity in school-age kids and we have to solve that. But those kids aren't dying."
Can be interpreted as either the bacterium is the same and the host defenses are better in younger children due to a less effective (but still reasonably useful) vaccine or it could be that current Pertussis strains are not as virulent, especially in infants, but still contagious. Should be possible to determine this by looking at older frozen samples of Pertussis, which I assume are available (but don't really know).