The current vaccine for pertussis, or whooping cough, failed to prevent the bacterial pathogen from colonizing an immunized baboon, and spreading to other baboons, researchers found. The surprising discovery might explain why the respiratory illness has recently become a public health issue, with the number of reported cases in the U.S. hitting a 50-year high of 42,000 last year.

To understand this bacterial resurgence, researchers vaccinated infant baboons when they were 2, 4, and 6 months old, with either the currently used acellular pertussis (aP) vaccine or the whole-cell (wP) vaccine, which hasn’t been used since the 1990s due to side-effect issues. The monkeys were then challenged with the bacterial pathogen, Bordetella pertussis, when they reached 7 months.

While the baboons that were given the modern aP vaccine were protected from any severe pertussis-like symptoms after exposure, their throats still became colonized and they were unable to clear the infection more efficiently than their unvaccinated counterparts. Importantly, these monkeys could easily transmit the pathogen onto unvaccinated monkeys that they made contact with.

In comparison, monkeys given the superseded wP vaccine benefited from a more rapid clearance of the pathogen than either the aP vaccinated monkeys or the untreated ones. Monkeys that were infected once before didn’t become colonized when exposed to the pathogen a second time around.

“When you’re newly vaccinated you are an asymptomatic carrier, which is good for you, but not for the population,” Tod J. Merkel, lead author of the study and researcher at the Office of Vaccines Research and Review at the U.S. Food and Drug Administration, told The New York Times.

The researchers also found something revealing when they looked at the specific immune response of each group of monkeys. “Although all vaccinated and previously infected animals had robust serum antibody responses, we found key differences in T-cell immunity,” the authors wrote in PNAS. Specifically, previously infected animals and wP-vaccinated animals both exhibited the same kind of boost in immune response while the aP vaccination elicited a response that was slightly different.

“The observation that aP, which induces an immune response mismatched to that induced by natural infection, fails to prevent colonization or transmission, provides a plausible explanation for the resurgence of pertussis and suggests that optimal control of pertussis will require the development of improved vaccines,” the authors concluded in their study.

The pertussis vaccine is normally administered along with vaccines for diphtheria and tetanus, known together as DTaP. The Centers for Disease Control and Prevention (CDC) recommends administering DTaP vaccines to infants as young as 2 months but also warns that the protection provided by a DTaP vaccination “fades over time.”

This loss in immunity seems particularly true for people who receive an aP vaccine. A study published this year found that during a pertussis outbreak in 2009 and 2010, teenagers who received DTaP in childhood were six times more likley of contracting pertussis compared to those who received DTwP. Even an aP booster shot wasn’t able to provide any protective advantage over receiving a DTwP vaccine in the first place.

Source: Warfel J, Zimmerman L, Merkel T, et al. Acellular pertussis vaccines protect against disease but fail to prevent infection and transmission in a nonhuman primate model. Proceeding of the National Academy of Sciences. 2013.