ANN ARBOR, Mich.—Two U-M population ecologists have been awarded a $1.7 million grant from the National Institutes of Health for a five-year study that will try to explain the changing patterns of whooping cough outbreaks, using records from several countries spanning more than 70 years.
Thanks to widespread childhood vaccination, whooping cough (pertussis) once seemed to be under control. But the bacterial illness, which in infants causes violent, gasping coughing spells, has made a comeback in the United States and some other developed countries since the 1980s. In addition, there's been a shift in who's getting sick, with fewer cases seen in preschool children and more in teenagers.
Unlike a conventional epidemiological investigation of a disease outbreak, the new U-M study will rely heavily on the use of long-term incidence reports, mathematical models of pertussis transmission and statistical methods for extracting information from data. Records from recent and historical outbreaks in several countries – including England, Wales, Sweden, Denmark, Senegal and the United States – will be analyzed.
"I think pertussis research has, until now, been piecemeal. Researchers have used different methods on different data sets and arrived at diverse conclusions," said U-M population ecologist Pejman Rohani, co-leader of the new NIH-funded study.
"What we have proposed to do would, we believe, represent the most comprehensive stab at a systematic examination of pertussis," said Rohani, a professor of ecology and evolutionary biology, a professor of complex systems and a professor of epidemiology at the School of Public Health. The co-principal investigator on the pertussis study is Aaron King, an associate professor of ecology and evolutionary biology and an associate professor of mathematics.
Some researchers say the reason for the observed changes in pertussis outbreak patterns since the 1980s is clear: The childhood vaccine for whooping cough wears off quicker than experts had previously believed. But Rohani and King say the explanation may not be that simple.
"That's probably the easiest hypothesis to understand because it's fairly straightforward, but infectious disease systems are usually much more complicated," King said.
"We're trying to understand why we see these patterns of pertussis, both the resurgence in the total number of cases and the change in the age distribution of those cases," he said. "We don't really understand either of those things, and we see it happening across the United States."
The new U-M study will use archival data from outbreaks that occurred in the pre-vaccination era to pin down some of the fundamental biology of pertussis: the duration of infection-derived immunity and the epidemiological impact of repeat infections, as well as the respective contributions of population demography, age-specific contacts and seasonality. A second phase of the research will address the vaccine-specific aspects of the problem by focusing on countries with known vaccine usage and immunization schedules.
"The ultimate test of whether our understanding of pertussis has significantly advanced will be to explain an enigmatic body of data, namely the changing patterns of pertussis incidence in the United States over the past 30 years," Rohani said.
New insights provided by the study could, for example, lead to recommendations about revising the pertussis immunization schedule for children or the design of optimal adult/adolescent boosters, Rohani and King said.
Currently, the U.S. Centers for Disease Control and Prevention recommends that infants and children receive the diphtheria-tetanus-acellular pertussis (DTaP) vaccine at 2 months, 4 months, 6 months and 15 to 18 months of age. A booster of DTaP is given at 4 through 6 years of age and another dose of whooping cough vaccine, known as Tdap, is recommended for adolescents.
In 2010, 27,550 cases of pertussis were reported nationally with 27 deaths – 25 of which were infants. Worldwide, there are 30 million to 50 million cases of whooping cough each year, resulting in about 300,000 deaths, according to CDC.
In 2010, 9,143 cases of pertussis (including 10 infant deaths) were reported in California, the most cases reported in that state since 1947.
The state of Washington reported 965 cases in 2011, compared to 608 cases in 2010. Through April 7 of this year, 776 cases have been reported in Washington, according to the CDC.
- Pejman Rohani: http://www.lsa.umich.edu/eeb/directory/faculty/rohani/
- Aaron King: http://www.lsa.umich.edu/eeb/directory/faculty/kingaa/