University of Michigan. US: A key to victory in battle, according to Chinese general and military
strategist Sun Tzu, is to know your enemy. In the current fight against
whooping cough resurgence, perhaps the biggest obstacle is an
incomplete understanding of its underlying causes, according to a
University of Michigan population ecologist.
Though the root causes of the resurgence remain "elusive and
contentious," public health officials in many countries—including the
United States—have introduced whooping cough booster shots into
childhood immunization schedules, Pejman Rohani and a co-author state in
a paper scheduled for online publication Jan. 19 in Proceeding of the
National Academy of Sciences.
If those booster schedules are based on a misdiagnosis of the
problem, the shots are likely to be "epidemiologically ineffective and
economically costly," they conclude. Rohani is a professor of ecology
and evolutionary biology, a professor of complex systems, and a
professor of epidemiology. The first author of the PNAS paper is former
U-M doctoral student Maria Riolo, who is now a postdoctoral researcher
at the University of Chicago.
Rohani said he strongly supports childhood vaccinations and a
tightening of personal-belief exemptions that allow some parents to opt
out of vaccinating their children. In the U.S., the Centers for Disease
Control and Prevention recommends a series of five shots for whooping
cough, also known as pertussis, for children under 7. Additional shots
are recommended for older children and some adults.
Riolo and Rohani used a mathematical model to identify pertussis
booster schedules that result in the greatest reduction in disease for
the lowest economic cost. They looked at several potential causes of the
pertussis resurgence separately, though in the real world it's likely
that more than one of the mechanisms are simultaneously at play.
Their mathematical model uses a "genetic algorithm" that mimics how
evolution by natural selection would operate on a population of diverse
booster schedules.
They found that the most effective booster schedules varied depending
on the resurgence mechanisms. They looked at four possible mechanisms:
insufficient vaccine coverage, low vaccine efficacy, waning immunity and
a leaky vaccine.
Insufficient vaccine coverage can occur when large-scale vaccination
campaigns fail to reach enough children or when parents opt out of a
vaccination program.
Low efficacy means the vaccine sometimes fails to provide protection.
Waning vaccine immunity means the vaccine is initially protective, but
protection wears off over time. For the waning immunity scenario in
their study, Riolo and Rohani investigated booster strategies in cases
where a pertussis vaccine protects for an average of five, 15, 25, 35 or
45 years.
A "leaky" vaccine reduces the risk that a previously vaccinated
person becomes infected but does not eliminate it entirely. That can
occur when the microbial strains used in the vaccine don't include all
the strains circulating in the general population, as happened with this
year's influenza vaccine.
Riolo and Rohani found that when a simulated pertussis resurgence was
primarily due to low coverage or low vaccine efficacy, the most
effective booster strategy was a single booster shot before children
enter kindergarten.
For waning immunity, the most successful schedules called for a
series of boosters between ages 5 and 45 to maintain population
immunity. The frequency of the boosters depends on the duration of
immunity afforded by the vaccine.
If leaky immunity was the primary cause, the team's genetic algorithm
found no booster schedules that further reduced the incidence of
pertussis.
"If a vaccine is too leaky, the pathogen can continue to circulate in
a fully vaccinated population, and you won't be able to get elimination
using that vaccine alone," Riolo said. "You can still get a large
reduction from pre-vaccine levels of disease, but the leakiness limits
how far you can get."
Thanks to widespread childhood vaccination, 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.
The U-M study was supported by the U.S. Department of Homeland
Security, the Fogarty International Center, and the U.S. National
Institutes of Health.