Because pertussis vaccine targets a specific, purified protein from the target microbe (that is, a fairly non-conserved element, that is fairly small and provid

Because pertussis vaccine targets a specific, purified protein from the target microbe (that is, a fairly non-conserved element, that is fairly small and provides fewer targets for variation in Ab targets). Most vaccines don't do this, and none of the new vaccines under development do this (that I am aware of, admittedly.) It's historical inertia at this point. But going back at least as far as "since when I was a kid" it's a fundamental point of vaccine design that you aim for things that the microbe has a difficult time evolving away from (highly conserved molecules), and that offers multiple antigen targets. I can't say that's never deviated from - not letting the perfect be the enemy of the good and what-have-you - but it's the norm.