hy are whooping cough cases on the rise, even in countries with widespread vaccination? Common explanations don’t hold up under scrutiny, scientists argue in a provocative paper published today.
The paper, published Tuesday in Proceedings of the Royal Society B, takes a global look at cases of whooping cough, a very contagious respiratory infection caused by the bacterium Bordetella pertussis. Whooping cough, also called pertussis, starts with cold-like symptoms and develops into serious coughing attacks that may produce a “whoop” between coughs.
Though vaccines became available 70 years ago, pertussis has re-emerged in recent years in developed countries. Cases in the United States — just a couple thousand a year in the 1970s — hit 32,000 in 2014. A similar story is playing out in other countries. The World Health Organization estimated that 195,000 children died from pertussis in 2008.
There have been various theories for why this might be the case. Some have said the vaccines wear off too quickly; others have speculated that it might be due to increased awareness of the disease.
But these explanations collectively fall short, said Pejman Rohani, a professor at the University of Georgia, and collaborators from the University of Michigan in the new review paper.
Here’s a look at three assertions they took issue with:
1. Whooping cough cases are rising globally.
Prevailing wisdom is that pertussis is re-emerging everywhere, the authors wrote. Looking at WHO data from 63 countries from 1980 to 2012, they found that about half of the countries did see increases in pertussis cases in that time period. But the other half “did not have any significant period of increase” during that time, they wrote. And those countries weren’t concentrated in one area — they were spread out around the world.
“The general observation that there’s a global pertussis resurgence — that’s not borne out with the data,” Rohani said.
Aaron Wendelboe, an associate professor in epidemiology at the University of Oklahoma Health Sciences Center whose pertussis research was included in the review, cast doubt over the finding. He said the authors should be more cautious in comparing pertussis rates across countries and across time, as the standards for different countries’ data can vary.
2. Whooping cough vaccine blocks the disease, but not transmission.
Some scientists have argued that pertussis vaccines “protect against disease but not against transmission,” Rohani said. That would mean that the population wouldn’t have so-called herd immunity, where unvaccinated individuals are protected against the disease.
As a counterpoint, Rohani et al. turned to Sweden, which took a 17-year hiatus from pertussis immunization, then reintroduced the vaccine. After infants started getting immunized, the number of adult cases went down, which “suggests that the vaccination of babies curbed transmission to adults,” he said. Pertussis rates also dropped among babies who are too young to be vaccinated, which also indicates some level of herd immunity, Rohani argued. The authors found similar evidence in Italy and Japan.
Dr. Sarah Long, chief of infectious diseases at St. Christopher’s Hospital for Children, said the authors are oversimplifying the prevailing wisdom in this area.
“I think they’re trying to debunk some things that they’ve stated black-and-white,” she said. Specifically, she points to the newer version of pertussis vaccines, called acellular vaccines. She said most scientists don’t back the wholesale claim that vaccines fail to block transmission: Instead they have found that acellular vaccines reduce transmission to a lesser degree than older vaccines did.
3. Immunity is short-lived.
A review Wendelboe conducted in 2005 estimated how long immunity against pertussis lasts: four to 20 years after a natural infection, and four to 12 years after vaccination. That finding is used to support the theory that perhaps the resurgence of whooping cough is a case of vaccinated people becoming susceptible again.
Rohani et al. argue that there’s not enough evidence to support that claim. His team conducted a mathematical analysis and found that “the prediction of what we expect to see at the population level” based on Wendelboe’s estimate didn’t match the actual prevalence of pertussis. They concluded that immunity may not be as short-lived as scientists think.
But the researchers concede that there’s a contradiction afoot: If immunity does last a long time, why do doctors see whooping cough show up in patients who were previously infected or vaccinated? Rohani offered one possible way of reconciling contradictory claims: Perhaps different people react differently to the vaccine. Because doctors are seeing only the patients who get sick, not those who are enjoying long-lasting immunity, he offered, they may see immunity as more short-lived than it really is.
A puzzling disease
So if the prevailing wisdom isn’t correctly capturing pertussis, then what’s causing the rise in cases?
Rohani et al. offer several hypotheses for further explanation. One is that the bacterium, Bordetella pertussis, is evolving in response to vaccines. Another is that humans are getting misdiagnosed with pertussis when they’ve really been infected by other bacterial species from the same genus Bordetella. Those infections may produce the same symptoms as whooping cough, but can’t be avoided by pertussis vaccines.
Long said those hypotheses are not new.
Rohani acknowledged that explanations are lacking. But “part of what we’re trying to point out is there are a reasonable amount of people walking their own puppy: An explanation might work for one set of data in one population, but ultimately what we really need is an explanation that is consistent for all data.”
So, there’s one thing the researchers can agree on: More study is needed.