All things being equal, you would rather not have pertussis in Philadelphia, where there is evidence of a vaccine-resistant strain of pertussis springing up. The CDC is downplaying the discovery a bit, because the resistance discovered affects just one component of the acellular whooping cough vaccine: “pertussis vaccines remain effective…,” the agency says.

Or, not ineffective, as research has already indicated that the “new” accellular vaccine, put into wide use in the 1990s, wanes in effectiveness earlier than the previous whole-cell vaccine, down to 70-percent effectiveness by the time children are in middle-school.

That’s obviously much, much better than nothing, but it’s been enough for whooping cough to regain a foothold. It achieved epidemic status in Washington in 2012, and last December, a King County baby died of pertussis. Nationally, it killed 18.

Oregon, by the way, has been experimenting with a whole-cell and acellular combination, since full immunization requires five shots anyway. Initial results seem to indicate that “priming” the process with a whole-cell dose does increase and extend effectiveness.

The recent letter to the New England Journal of Medicine reveals that, in an analysis of the pertussis that infected twelve children hospitalized in Philadelphia, “11 of the 12 B. pertussis isolates were negative for pertactin.” The strain has previously been detected in Japan, Finland, and France, but this is the first official report of it making a U.S. landing (the usual U.S. tests to confirm pertussis don’t distinguish between variants).

At this point, you are likely wondering what pertactin is. It turns out it’s a component in the acellular pertussis vaccine, “a protein that helps the bacteria attach to the lining of the airways,” explains the CDC.

“Pertactin-negative” means that the new pertussis strains have evolved ways to go about their business without employing the protein, which is associated with virulence. That’s supported by French research, which finds that although vaccinated children can still get the mutant pertussis, it’s less severe. AP says that according to the Pasteur Institute’s Nicola Guiso, the new pertactin-negative pertussis “accounts for 14 percent of cases there.” That said, the letter to the NEJM notes, “[p]ertactin mutants retain lethality in mouse models of infection,” and any future vaccine will need to account for it.