In light of recent outbreaks of pertussis, or whooping cough, in California and other states, health officials have renewed their emphasis on vaccinations against the illness for both children and adults whose immunity may have waned over time.
But a new study suggests widespread adult vaccinations may not bring about the reduction in pertussis cases some had hoped. Researchers projected that, given the patterns of social mingling seen in Europe, even if 75 percent of adults received pertussis boosters an unrealistic target, experts say it would reduce cases by only 15 percent.
"The presumed role for adults acting as a reservoir...we don't find empirical evidence for that," said Pejman Rohani, the study's lead author, a professor of ecology and evolutionary biology at the University of Michigan.
The research used data from Sweden and a model of disease transmission based on social interaction among different age groups.
The researchers chose to look at Sweden because of the availability of solid data on cases of pertussis, and a series of events that took place there that made it useful for the study.
In 1979, Sweden halted use of an older form of the vaccine over concerns about side effects. In 1996, a newer version came into use. The reintroduction of a vaccine allowed the researchers to look at different age groups over time some cohorts had received infant immunization and others had not.
They found that teenagers, who had not been vaccinated as infants, got pertussis at a higher rate than younger children. Children younger than teens, who had also not been vaccinated, contracted the disease at lower rates, because of the vaccine's use in infants younger than them.
The implication was that unvaccinated teenagers could be simply spreading disease among themselves.
"For teenagers, because they mix so much less with infants, there wasn't that concomitant reduction in teenagers as there was in the younger age groups," Rohani said.
A second finding of the study, Rohani said, was that the 17-year absence of the vaccine had a definite, noticeable effect, as rates of pertussis spiked during that time.
"The data from Sweden provide pretty strong evidence for the protective role that infant immunization programs can provide," he said. "It's an issue for which there is reasonably good empirical evidence. When you stop vaccinating, pertussis comes back."
Rohani said it is not clear if the patterns of pertussis transmission in Sweden would carry over to the United States.
However, he said, the evidence suggests that ensuring adults get boosters may not have as much of an impact as some suggest.
"Some of the current worries about adults playing a really big role in the circulation of pertussis, those worries may not be well-founded, Rohani said.
It remains to be seen what, if any, impact the study has on public health strategies regarding pertussis.
"I find it very provocative, and I think it will engender a fair amount of discussion among epidemiologists and public health people," said Dr. William Schaffner, chairman of the department of preventive medicine at the Vanderbilt University School of Medicine.
He said many in public health would have expected the impact of higher pertussis vaccination rates in adults to exceed the 15 percent shown by the model.
Pertussis presents a challenge to epidemiologists, because its symptoms range from mild to fatal, and so it isn't always clear who has it.
Schaffner said the recent outbreak in California is thought to be linked to vaccine-immunity wearing off, and children missing a booster shot they should receive around age 11. The outbreak has caused more than 6,400 cases and 10 infants have died so far this year, according to the Centers for Disease Control and Prevention.
One consequence of using the newer vaccine instead of the older one is that booster shots are needed to help maintain immunity.
But the model's findings do not necessarily indicate the futility of adult vaccination. Parents' vaccinations may not impact public health, but it helps protect their baby.
"The strategy may be much more effective on an individual basis than we can expect on a population basis," Schaffner said.
The findings will be published tomorrow (Nov. 12) in the journal Science.