An infant gets immunized in this undated historical photo. American doctors no longer use so-called "gluteal injection sites" for immunizations.
An infant gets immunized in this undated historical photo. American doctors no longer use so-called "gluteal injection sites" for immunizations. Photo made available through the CDC Public Health Image Library

This is not your parents’ pertussis. Over the last few years, this once-common childhood illness, also known as whooping cough, has evolved in response to its own vaccine, according to a new study from the U.S. Centers for Disease Control and Prevention.

The mutated pertussis is not more dangerous than the original, and vaccines against the illness are still effective. So the findings don’t change researchers’ and doctors’ understanding of whooping cough as a public health problem. Instead, they’re a glimpse into a fascinating natural phenomenon, and one that’s happening as we speak. (Though this isn’t the first time this has happened. Populations of pneumococcal illnesses changed after the introduction of its vaccine, prompting changes to the vaccine to deal with newly popular strains.) The whooping cough findings have kicked off research to determine whether this is related to the recent increase in whooping cough cases in the U.S.

Paul Offit, chief of the division of infectious diseases at the Children’s Hospital of Philadelphia, was not involved in the CDC study, but took part in a conference call about the study in which CDC researchers presented their results. He talked with Popular Science over the phone immediately after the call.

“We discussed the data and what the data allow us to say and what they don’t allow us to say,” he says. “The data don’t support the notion that the mutants are more virulent.” Babies who fall ill from un-mutated pertussis have the same symptoms as babies who fall ill from mutated pertussis, he says.

“The data don’t support the notion that the mutants are more virulent,” Offit says.

There’s also no evidence yet that pertussis vaccines are less effective against the new strains, say all of the researchers Popular Science talked with, both in and outside of the CDC. Everyone recommended Americans continue to get vaccinated right on schedule. “Currently, we know it’s the best form of protection we have,” says Lucia Pawloski, the CDC scientist who led the new research. Kids who don’t get vaccinated against pertussis are still eight times as likely as vaccinated kids to get the disease, she adds.

Nevertheless, it’s plausible whooping cough’s changes do affect the vaccine’s ability to protect. “Yes, theoretically, there could be an issue, but we don’t have any data to say that yet,” Pawloski says. The CDC is interested in answering this question and hopefully will have data to do so in six or eight months, Offit says.

Evolution photo

Micrograph of Pertussis Bacteria, 1979

So what exactly is different about pertussis nowadays? Since 2010, Pawloski and her colleagues have recorded a sharp rise in the U.S. in strains of the illness that don’t make a protein called pertactin. Before 2010, researchers saw only one U.S. pertussis infection, from 1994, in which the bacteria didn’t make pertactin. By 2012, the majority of pertussis infections researchers studied were caused by non-pertactin-making pertussis. Pawloski and her team examined 1,300 samples taken from whooping cough outbreaks between 1935 and 2012.

Pertussis bacteria make thousands of proteins, but pertactin happens to be one of the few included in many forms of the vaccine used in developed countries, including the U.S. “We’ve created this selective pressure,” Offit says, by widely using vaccines with pertactin in them.

When the proteins and other molecules in a vaccine enter your body through a shot, your immune system finds them, learns to recognize them, and develops defenses against them. Now, if you ever truly encounter that illness, your body will respond quickly to quash it. If the vaccine contains pertactin and the actual illness doesn’t, that means your body is learning to recognize something that’s useless to it when it encounters whooping cough.

All pertussis vaccines used in developed countries have one, two, or three other pertussis molecules besides pertactin, so there’s still stuff in there for your body to train on, even if pertactin is now useless. How much of a blow—if any at all—losing pertactin deals to vaccines that have other active molecules is still unclear. “I think at this point it’s hard to say exactly what it means for the vaccines’ effectiveness,” says Nicola Klein, co-director of the Kaiser Permanente Vaccine Study Center in California. Klein was not involved in the CDC study.

“We’ve created this selective pressure,” Offit says.

Meanwhile, pertussis vaccines often used in developing countries contain entire killed Bortedella pertussis bacteria, so they actually contain perhaps thousands of things the body is able to use to learn to recognize whooping cough. Those vaccines, which are cheaper to make, were discontinued in developed countries because they cause severe, if non-permanent, side effects, such as seizures in babies.

CDC researchers now are looking into whether losing pertactin is related to another phenomenon associated with the increasing popularity of the newer whooping cough vaccines. Recent studies have found that even those kids who get all their recommended pertussis shots—in the U.S. administered as the DTaP shot—are vulnerable again to whooping cough at about age 8 or 10. Researchers think the new vaccine is why the U.S. saw more cases of whooping cough in 2012 than it has in any year since 1955. The older shot, with the killed bacteria, protected people for longer. Scientists are unsure what exactly made the tougher stuff was longer-lasting, however.

Is pertussis’ evolution the answer? That’s not certain yet. “We couldn’t say yes or no at this point whether pertactin had a role in the waning of immunity,” Pawloski says.