The current vaccine for whooping cough, or pertussis, may keep you or your baby healthy, but it may not stop either of you from spreading the disease, a new animal study suggests. Baboons can harbor and spread the disease even after receiving the vaccine, researchers have found. The study adds to growing evidence that the acellular pertussis vaccines, in which only parts of the pertussis bacterium are injected into the bloodstream to elicit a protective immune response, are not as good at controlling the disease as older, whole-cell vaccines were. However, a vaccine manufacturer argues that it's too early to conclude that a similar effect occurs in humans.
Pertussis starts out like a normal cough but causes severe coughing fits and can be lethal to infants. By the time of diagnosis, it is often untreatable with antibiotics. Historically associated with the slums of pre-World War II Europe and America, the disease has made a powerful resurgence in recent years. The United States alone experienced about 50,000 cases of pertussis last year, with 18 deaths, according to the Centers for Disease Control and Prevention. The increase could be due in part to more sensitive tools to diagnose pertussis that were widely introduced in 2010, or to pockets of children whose parents oppose vaccination. However, previous research also indicated that immunity in people vaccinated with the acellular vaccines, introduced in the 1990s, is less long-lasting than in users of the older, whole-cell vaccine.
The current study goes a step further and suggests that people who get the newer vaccine may still become infected and spread the germ. Tod Merkel, a microbiologist, and colleagues at the Food and Drug Administration in Bethesda, Maryland, examined response to the acellular vaccine in infant baboons, an animal that responds to the bacterium responsible for pertussis similarly to people. The researchers infected four groups of baboons , each group containing three or four babies, by anesthetizing the animals and dripping a pertussis-containing solution into their noses. One group had already received the standard three doses of the acellular vaccine; a second received the whole-cell vaccine. Members of the third group had previously had whooping cough. Those in the fourth group had not had the disease and received no vaccine before being exposed.
As expected, the unvaccinated baboons developed severe whooping cough, while the baboons that had been sick previously remained well, the research team reports today in the Proceedings of the National Academy of Sciences. Both groups of vaccinated animals also remained healthy. However, the germ persisted an average of 35 days in the throats of baboons vaccinated with the acellular shot, though it grew less thickly than it did in the throats of the sick, unvaccinated animals. Baboons vaccinated with the whole-cell shot harbored the germ for 18 days, and it did not grow at all in animals that previously had recovered from pertussis.
In another experiment, two baboons that had received acellular vaccines were exposed to whooping cough germs and then each was put in a cage 2 days later with previously unexposed baboons. In both cases, the vaccinated animals transmitted the germ to their cage mates, who developed pertussis. Follow-up studies showed that animals vaccinated with the acellular shots did not generate sufficient numbers of a particular variety of white blood cell to fight the pertussis infection as well as those receiving the older vaccine.
The researchers conclude that a new vaccine may be needed to provide so-called herd immunity, the ability of a community to stop an infection from spreading, and protect vulnerable babies from pertussis. “There's a difference between protecting individuals from illness and bringing down the incidence of pertussis in the population,” Merkel says. “To do both we may need a different vaccine.”
Sanofi Pasteur of Swiftwater, Pennsylvania, which makes one of the two acellular pertussis vaccines used in the United States, issued a statement cautioning that the study was not designed to evaluate the extent to which vaccination reduced transmission. “It cannot be said with certainty that these findings are directly applicable to humans,” the company said it its statement.
But other scientists applauded the work. “This is a very strong paper, even though it is a small sample,” says James Cherry, a vaccinologist at the University of California, Los Angeles, who was not involved in the study. Cherry argues that the efficacy of the acellular vaccines in trials held in Europe and Africa in the 1990s appeared high because case definitions did not count people with mild infections. The acellular vaccine was introduced because of public concerns and lawsuits arising from the whole-cell vaccine, which sometimes caused high fever and even seizures.
As for the claim that the new result may not be applicable to people, Merkel notes that, for ethical reasons, it may be difficult to duplicate the study in humans, as that would require purposefully exposing experimental subjects to a 3-month bout of pertussis.