The potential for the avian influenza strain H5N1 to spread into the human population and kill tens of millions of people has got researchers studying the question of how influenza spreads. Some researchers at the US National Institues of Health found through mathematical modelling that long commuter trips play a large role in spreading influenza.
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.
These results make intuitive sense. Picture a married couple where the husband commutes in one direction and the wife commutes to a very different direction. Then, for example, the husband can get exposed to a virus at work and bring the virus home. Then the wife gets exposed at home and goes to work before feeling any symptoms. Then at work before she starts showing symptoms she passes the virus along to co-workers.
The role of workplaces as nexuses of virus spread most obviously argue for the importance of efforts to reduce the potential for pathogen spread in work environments. Less obviously, households with multiple members in the labor market are also nexuses for virus spread and rearrangements of living arrangements and jobs could reduce the rate of spread of a deadly pandemic virus. For example, in some cases a couple could get jobs at the same employer. Or during a pandemic members of a multi-worker household could try to make arrangements to telecommute. Or people who are in roommate situations could try to get roommates from companies where they work.
States that are well connected get sick before less connected states.
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.
While children in schools have long been viewed as the biggest spreaders of flu these latest results put workplaces as bigger flu spreaders.
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.
Kids might infect more people but kids do not infect so many people from distant places. Adults create longer chains of infection.
Reduction of inter-regional travel could slow the spead of a deadly pandemic influenza strain into different commuting zones.
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.
Picture the United States as broken up into a bunch of regions which each have major cities and surrounding suburbs that serve as commuting work destinations. If a flu strain gets into one of those regions then commuters will spread the strain around that region. Since there is far more travel within a region than between regions restrictions on travel between regions would cause less economic disruption than restrictions within regions.
In my post "Economic Collapse Avoidable During An Influenza Pandemic" I proposed the use of "workplace cocooning" - basically living in workplaces - as a way to reduce the spread of a deadly pandemic influenza strain.
Update: A team at Los Alamos National Laboratory developed a simulation for spread of a pandemic flu virus and found that vaccines that provided only partial immunity would have considerable value in slowing the rate of spread of the virus.
The large-scale, stochastic simulation model examines the nationwide spread of a pandemic influenza virus strain, such as an evolved avian H5N1 virus, should it become transmissible human-to-human. The simulation rolls out a city- and census-tract-level picture of the spread of infection through a synthetic population of 281 million people over the course of 180 days, and examines the impact of interventions, from antiviral therapy to school closures and travel restrictions, as the vaccine industry struggles to catch up with the evolving virus.
“Based on the present work ... we believe that a large stockpile of avian influenza-based vaccine containing potential pandemic influenza antigens, coupled with the capacity to rapidly make a better-matched vaccine based on human strains, would be the best strategy to mitigate pandemic influenza,” say the authors, Timothy Germann, Kai Kadau, Ira Longini and Catherine Macken.
“It's probably not going to be practical to contain a potential pandemic by merely trying to limit contact between people (such as by travel restrictions, quarantine or even closing schools), but we find that these measures are useful in buying time to produce and distribute sufficient quantities of vaccine and antiviral drugs,” said Germann of Los Alamos’ Applied Physics Division.
“Based on our results, combinations of mitigation strategies such as stockpiling vaccines or antiviral agents, along with social distancing measures could be particularly effective in slowing pandemic flu spread in the U.S.,” added Longini.
If a partial immunity vaccine is the best that can be developed in advance of a pandemic then that vaccine is still worth developing.
The results show that advance preparation of a modestly effective vaccine in large quantities appears to be preferable to waiting for the development of a well-matched vaccine that may not become available until a pandemic has already reached the United States.
“Because it is currently impossible to predict which of the diverging strains of avian H5N1 influenza virus is most likely to adapt to human transmission, studies of broadly cross-reactive avian-influenza based vaccines with even modest immunogenicity in humans are important,” said Macken, an influenza researcher in the Los Alamos Theoretical Division. Ideally, both vaccine strategies would be done in parallel: Stockpile a modestly effective vaccine to use while the better-matched one is being developed, the authors suggest.
I have no idea what the odds are for H5N1 to accumulate enough mutations to propagate easily between humans. But it if does cross over and achieve pandemic status then a vaccine that provides partial immunity would be extremely valuable.