But it all comes down to a single factor -- kids tend to play with kids.
That's the conclusion of a study that used data from a "unique natural experiment" in Sweden, according to Pejman Rohani, PhD, of the University of Michigan in Ann Arbor, and colleagues.
Combined with a European study of interpersonal contacts, the data show that age-specific contacts alone can explain changes in who gets whooping cough, Rohani and colleagues reported in the Nov. 12 issue of Science.
The finding may have important policy implications, the researchers noted. Specifically, proposals for pertussis booster vaccinations in teens and adults are likely to have little effect, they argued.
Despite the availability of effective vaccines, whooping cough continues to take a substantial toll in developed countries -- more than 295,000 infant and childhood deaths per year, according to one estimate.
Although vaccine use has been relatively high in recent years, many countries have seen a resurgence in the disease, Rohani and colleagues noted. One possible explanation has been the idea that adults may form a reservoir, since the disease is usually without symptoms in grown-ups. But genetic changes, waning immunity, better disease reporting, and other explanations have also all been touted.
To clarify the issue, the researchers examined the pattern of social interaction revealed in the 2008 POLYMOD study, in which for one day more than 7,000 people from eight European countries recorded their contacts with other people. A central result was that children interacted much less frequently with adults than with other kids, the researchers noted.
Rohani and colleagues reported that they created a mathematical model that incorporated the contact data but ignored "the myriad complexities that have been proposed as explanations for recent pertussis epidemiology."
They compared the model with age-stratified incidence data from Sweden. In that country, a vaccine was introduced in 1952, but withdrawn in 1979 as a result of concerns over safety and efficacy. An acellular vaccine was reintroduced in 1996.
Despite the simplicity of the model, Rohani and colleagues reported, it accurately reproduced the observed incidence data in Sweden from 1985 through 2005 -- from a period in which no vaccine was given to the modern vaccine era.
Overall incidence was variable from year to year, but remained higher than 100 cases per 100,000 population until the vaccine was introduced in 1996. From then on, incidence fell sharply, approaching zero in 2005.
The model showed the same pattern, the researchers reported.
The only major discrepancy, they reported, was that in children ages 6 through 10 the model predicted the vaccine would have little effect on incidence. In fact, observation showed a sharp decline in new cases starting when the vaccine was introduced.
After the vaccine introduction, the model predicted a relatively steady number of cases among those ages 11 through 19, as well as a sharp decline in cases among those ages 20 through 75. Both effects were also seen in the real-world data, the researchers said.
In essence, the data show that social interactions alone can explain what has been seen in countries like Sweden where the vaccine was withdrawn and then reintroduced, according to co-author Aaron King, PhD, also of the University of Michigan.
There is a characteristic pattern, he said -- a sharp drop in cases in children, combined with an uptick among teens, which has led to the suggestion that waning immunity might be the cause.
But that idea, he told MedPage Today, is not needed to explain the data. "The most important thing you have to pay attention to is the nature of contacts" among people, he said. Equally unneeded, he said, are the ideas that adults serve as a reservoir or that the disease has evolved to beat the vaccine.
He added that the Swedish picture may not apply precisely to the U.S., because of differences in vaccination rates and the greater diversity of the population. But, he said, "we are emboldened by the match of the prediction to the data (in Sweden) to suggest that the general idea would work in the U.S."
What's needed, King said, is a solid analysis of how people in the U.S. interact, similar to the European study.
The researchers reported support from the Department of Homeland Security, the National Institutes of Health, and the Bill & Melinda Gates Foundation. They did not report any potential conflicts.
Zalman S. Agus, MD Emeritus Professor
University of Pennsylvania School of Medicine and Dorothy Caputo, MA, RN, BC-ADM, CDE, Nurse Planner