Children largely spread whooping cough among themselves, so blanket vaccination campaigns targeting teens and adults may be a waste of time, according to a study that looks at how social patterns affect disease transmission.
The findings, published on Thursday in the journal Science, contradict the notion that infected adults are behind outbreaks in California and elsewhere of whooping cough, a contagious disease caused by the bacterium Bordetella pertussis.
A U.S. advisory panel last month recommended that adults over 65 be given a booster of the "Tdap" vaccine for tetanus, diphtheria and pertussis, or whooping cough, to protect infants under a year old, who are too young to be vaccinated.
But older people may not be the main culprit, Pejman Rohani of the University of Michigan and colleagues say.
Whooping cough, which causes uncontrollable, violent coughing, infects 30-50 million people a year globally and kills about 300,000, mostly children in developing countries.
There are regular outbreaks in developed countries, including one in California that has affected more than 6,400 people and killed at least 10 infants, the U.S. Centers for Disease Control and Prevention said.
To study the effects of social interactions on spreading whooping cough, the research team used a situation in Sweden.
That country halted its whooping cough vaccination program in 1979 because of vaccine safety concerns, and did not resume routine vaccination for 17 years. But health authorities continued to track cases of whooping cough by age group.
"We took advantage of an unplanned natural experiment," Rohani said in a telephone interview.
The team compared this to a 2008 study of more than 7,000 people from eight European countries that tracked social contacts by age. They plugged this into a computer model to see how social contacts affected the spread of whooping cough.
The team found that when Sweden resumed vaccinating young children, there was a big drop in the number of cases of whooping cough in all age categories except in teenagers.
With the patterns of social mixing, children mostly interact with other children, and are unlikely to be infected by adults, they found.
"Infant immunization produces a protective effect for other children, who are likely to be mixing with other infants," the team wrote. Infected adults did not play a major role in spreading whooping cough to children.
Rohani said other studies would be needed to explain how social networks affect regional disease outbreaks.
But he said looking at social networks is another way to better understanding how infectious diseases are spread -- and may save health officials the time and expense of mass vaccination campaigns that may not work.