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Children, Not Adults, Spread Whooping Cough

November 12, 2010

A new study on whooping cough outbreaks and how social patterns affect transmission suggests that blanket vaccination campaigns that target teens and adults could be a waste of time, as children largely spread the disease among themselves.

The findings contradict the common idea that infected adults are behind outbreaks of whooping cough, a contagious disease caused by the bacterium Bordetella pertussis.

A US advisory panel last month recommended that adults over 65 get a booster vaccine for tetanus, diphtheria and whooping cough to protect infants that are too young to be vaccinated.

But according to research by Pejman Rohani and colleagues at the University of Michigan, older people may not be the culprit.

Whooping cough infects 30 to 50 million people a year around the world and kills around 300,000, mostly children in developing countries.

Developed countries have regular outbreaks, including one in California that has affected more than 6,000 people and killed at least 10 infants, said the US Centers for Disease Control and Prevention (CDC).

The researchers used a situation that occurred in Sweden to study the effects of social interactions around the spread of whooping cough.

Sweden stopped its whooping cough vaccination program in 1979 because of safety concerns, and did not resume routine vaccinations for 17 years. But during that time, health officials continued to track cases of whooping cough by age group.

“We took advantage of an unplanned natural experiment,” Rohani told Reuters 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 used a computer model to see how social contacts affected the spread of the disease.

The team discovered that once Sweden resumed vaccinating young children, there was a big drop in the number of whooping cough cases in all age groups except teenagers. They found that with social mixing patterns, children mostly interact with other children and are unlikely to be infected by adults.

“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 big role in spreading the disease to children.

The UM study shows that, overall, “the role of adults in transmission is minimal,” and that blanket booster-vaccinations of adults would unlikely be an efficient strategy for controlling whooping cough, said Aaron King, an assistant professor of ecology and evolutionary biology with a joint appointment in mathematics, who was part of the study.

The researchers stressed that because the study used incidence data from Sweden, results could vary with any incidences in the United States, where infant vaccination compliance rates are lower and the population is much more diverse. “We need similar analyses for the United States,” said Rohani.

The study makes two strong conclusions, King said. “The first point is that we see strong evidence for the efficacy of vaccination directed at children when compliance is high. The second is that better knowledge of actual contact patterns among age groups is crucial for the design of more effective and economical vaccination strategies.”

Rohani said more studies would need to be conducted to explain how social networks affect regional disease outbreaks. But looking at social networks is another way to better understand how infectious diseases are spread and could save time and the expense of mass vaccinations that may not work.

The findings from the study were published Thursday in the journal Science.

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