Dr. Natalie Exner Dean, a postdoctoral associate in the department of biostatistics at the University of Florida College of Public Health and Health Professions and the College of Medicine, and Dr. Ira Longini, a professor in the department and co-director of UF’s Center for Statistics and Quantitative Infectious Diseases, collaborated with researchers from Northeastern University and the University of Washington on a computer model that indicates new cases of Zika in the U.S. probably will not be widespread.
It is estimated that about 80 percent of Zika infections are asymptomatic or have symptoms so mild that the disease is not detected. This means the number of cases reported by disease surveillance systems in the U.S. and across the world might be only a small fraction of the actual number of infections. In fact, it’s likely we are underestimating imported cases in the U.S. and even likely some locally spread cases.
In this situation, mathematical and computational models that account for mosquito populations, human mobility, infrastructure and other factors that influence the spread of Zika are valuable because they can generate estimates of the full extent of the epidemic.
This is what our research group, made up of physicists, biostatisticians and computer scientists, has done for Zika. The Global Epidemic and Mobility Model (GLEAM) can model the spread of Zika through countries and geographical regions.
Our model suggests that while more cases of Zika can be expected in the continental U.S., outbreaks will probably be small and are not projected to spread. By contrast, some countries, like Brazil, have already seen widespread outbreaks.
How does the model work?
Zika is primarily transmitted by Aedes mosquitoes. For a mosquito to transmit Zika to a human, it must first have bitten a human infected with the virus. If enough people infected with Zika travel to a new area with these mosquitoes, the virus could spread in a new geographic region.
That means models for Zika transmission need to take factors like mosquito population, human mobility and temperature, among others, into account.
So we begin by dividing the population of the Americas into geographical cells of similar size, and grouping these cells into subpopulations centered around major transportation hubs. Our model also incorporates data on the density of the mosquitoes that transmit Zika, Aedes aegypti and Aedes albopictus, within those subpopulations. Mosquitoes need warm weather to thrive, so we include a daily estimated temperature for each subpopulation. That allows us to factor seasonal temperature changes into our simulations.
To breed, mosquitoes need standing water, and to spread Zika, they need people to feed on. Areas with standing water, fewer window screens and less air conditioning, which are often lower-income areas, are at greater risk. The model uses detailed data about socioeconomics for each subpopulation, as well as data on the relationship between socioeconomic status and risk of exposure to mosquito-borne disease.
Once all of these factors are incorporated into the model, we simulate a Zika outbreak. These simulations are meant to project what will happen next with Zika, so they need to include information about what has already happened. The simulations were calibrated to match data from countries that experienced the epidemic first, like Brazil and Colombia.