Interregional spread of influenza through United
States described by virus type, size of population and commuting
rates and distance
Researchers at the National Institutes of Health (NIH) conclude
that the regional spread of annual influenza epidemics throughout
the United States is more closely connected with rates of movement
of people to and from work than with geographical distance or air
travels. They also found that epidemics spread faster between more
populous locations.
"This valuable study highlights new approaches to using historical
data sources and statistical analysis to create epidemiological
models. These models not only help us understand the transmission
of influenza but also could guide policy for its control" said
Dr. Elias A. Zerhouni, NIH Director.
The research results, published this week in Science Express,
is based on 30 years of weekly data from the National Center for
Health Statistics on influenza-related mortality in different States
since 1972.
"This study quantifies the spread of influenza based on three
decades of data. We can correlate interstate spread with population
size, commuting, and virus type. The key point about our paper
is that we synthesize long-term data about disease incidence and
human movements using models," said Dr. Mark Miller, Associate
Director for Research at the Fogarty International Center (FIC),
part of NIH.
The investigators reached their conclusions by building a mathematical
model of influenza spread in the United States based on the historical
pattern of epidemics. The results suggest that when disease is
imported into the United States in a well-connected state, one
with many inflows and outflows of workers, for example, California,
disease spreads much faster than if disease is imported in a less-connected
state, for example, Wyoming. In observed epidemics, the initial
focus of infection varies from year to year, but epidemics tend
to emerge more often from California than other less populated
states.
The model can simulate influenza pandemic spread, a situation
where the greatest majority of the population is susceptible to
a new virus. In this case, transcontinental spread could occur
more quickly. Infection could reach all states within 2-4 weeks,
instead of the 5-7 weeks for annual epidemics, if seeded in a highly
connected state.
Past research highlighted the role of children in the local spread
of influenza, in particular, in schools and households. This study,
by contrast, suggests that adults are responsible for the regional
spread of influenza because they travel farther and more frequently.
Although the current modeling approach allowed researchers to
reproduce the spread of annual influenza epidemics in the United
States, there are caveats to extending the model to predict pandemic
influenza. While it is impossible to predict how an entirely new
strain of influenza would behave, comparison of the spread of influenza
between pandemic and epidemic seasons based on historical data
might shed some light on their differences. As the speed of influenza
spread between states is affected by work-related population movements
and transmissibility, interventions which limit inter-regional
routine travel might slow epidemic spread.
The researchers did not study the effect of specific interventions,
for example, school closure or travel restrictions to mitigate
or delay the spread of influenza. Describing the mechanisms of
local spread of influenza within cities or states is a key area
for future work and would provide insight into which control strategies
might be effective to control epidemic and pandemic influenza — in
particular, social distancing measures.
The study, supported by FIC, results from a collaboration among
researchers at FIC, the Center for Infectious Diseases Dynamics
(CIDD) at Pennsylvania State University and the National Institute
of Allergy and Infectious Diseases (NIAID, part of NIH). Study
authors are Cécile Viboud, PhD (FIC); Ottar N Bjørnstad,
PhD (CIDD, FIC); David J Smith, PhD, (FIC); Mark Miller, MD, (FIC);
Lone Simonsen, PhD, (NIAID); and Bryan T Grenfell, PhD (CIDD, FIC).
The John E. Fogarty International Center for Advanced Study
in the Health Sciences (FIC) is the international component of
the NIH. The center addresses global health challenges through
innovative and collaborative research and training programs and
supports and advances the NIH mission through international partnerships.
For more information about FIC and its programs, visit www.fic.nih.gov.
The National Institutes of Health (NIH) — The Nation's
Medical Research Agency — includes 27 Institutes and
Centers and is a component of the U.S. Department of Health and
Human Services. It is the primary federal agency for conducting
and supporting basic, clinical and translational medical research,
and it investigates the causes, treatments, and cures for both
common and rare diseases. For more information about NIH and
its programs, visit http://www.nih.gov. |