Interacting contagious diseases like influenza and pneumonia follow the same complex spreading patterns as social trends. This new finding, published in Nature Physics, could lead to better tracking and intervention when multiple diseases spread through a population at the same time.
The information comes as the COVID-19 coronavirus is still making its slow rounds across the globe. While the World Health Organization last week stopped short of calling the virus a pandemic, they cautioned that a declining rate of spread may be temporary, and it's too soon to tell if the threat is truly subsiding.
When disease modelers map an epidemic like coronavirus, Ebola or the flu, they traditionally treat them as isolated pathogens. Under these so-called "simple" dynamics, it's generally accepted that the forecasted size of the epidemic will be proportional to the rate of transmission.
But according to Laurent Hébert-Dufresne, professor of computer science at University of Vermont, and co-lead of the research, the presence of even one more contagion in the population can dramatically shift the dynamics from simple to complex.
Once this shift occurs, microscopic changes in the transmission rate trigger macroscopic jumps in the expected epidemic size -- a spreading pattern that social scientists have observed in the adoption of innovative technologies, slang, and other contagious social behaviors.
WHAT'S THE IMPACT
The researchers first began to compare biological contagions and social contagions in 2015 at the Santa Fe Institute, a transdisciplinary research center where Hébert-Dufresne was modeling how social trends propagate through reinforcement. A classic example of social reinforcement: 10 friends telling you to see a new movie, which is significantly different than one friend saying the same thing 10 times.
Like multiple friends reinforcing a social behavior, the presence of multiple diseases makes an infection more contagious that it would be on its own. Biological diseases can reinforce each other through symptoms, as in the case of a sneezing virus that helps to spread a second infection like pneumonia. Or, one disease can weaken the host's immune system, making the population more susceptible to a second, third, or additional contagion.
When diseases reinforce each other, they rapidly accelerate through the population, then fizzle out as they run out of new hosts. According to the researchers' model, the same super-exponential pattern characterizes the spread of social trends, like viral videos, which are widely shared and then cease to be relevant after a critical mass of people have viewed them.
A second important finding is that the same complex patterns that arise for interacting diseases also arise when a biological contagion interacts with a social contagion, as in the example of a virus spreading in conjunction with an anti-vaccination campaign. The paper details a 2005 Dengue outbreak in Puerto Rico, and Hébert-Dufresne cites an additional example of a 2017 Dengue outbreak in Puerto Rico, where failure to accurately account for the interplay of Dengue strains reduced the effectiveness of a Dengue vaccine.
This in turn sparked an anti-vaccination movement -- a social epidemic -- that ultimately led to the resurgence of measles, a second biological epidemic. It's a classic example of real-world complexity, where unintended consequences emerge from many interacting phenomena.
THE LARGER TREND
As the coronavirus spreads abroad, Health and Human Services Secretary Alex Azar on Tuesday gave assurances that the immediate threat in the U.S. remains low, as other government health officials sent mixed messages about what to expect from the epidemic.
The Trump Administration said the situation is under control, while Dr. Nancy Messonnier, director of the CDC's National Center for Immunization and Respiratory Diseases said, "Ultimately, we expect we will see community spread in this country. It's not so much a question of if this will happen anymore but rather more a question of exactly when this will happen and how many people in this country will have severe illness."
Azar said the number of COVID-19 cases in this country remains steady, only jumping from 14, where it has stood since February 11, to 53 due to officials bringing home repatriated Americans infected from the Diamond Princess cruise line and Americans repatriated from Wuhan, China.
However, abroad, COVID-19 is spreading quite rapidly, Azar said, and the U.S. will likely see more cases.
Dr. Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases said he expected a vaccine to be in human trial in a month-and-a-half. Still, this means that it will take at least a year to scale up to vaccine production.