Two Forms of Epidemic Risk
“The problem is that young, healthy people interact with at-risk people, and they don’t have enough respect for that.” Dr. Eric Topol, Executive Vice President, Scripps Research
The quote above appeared in a recent article from Wired.com titled “There’s a Huge Covid Surge Right Now and Nobody Is Talking About It.” Dr. Topol is commenting on the danger that exists when people do not take precautions to avoid spreading infectious diseases to other people who face a dangerous medical condition. I wholeheartedly agree with the direct reading. However, implicit in his quote lies a more nuanced understanding of a type of risk that is commonly ignored when discussing Covid-19 and other infectious diseases.
Multiple types of risk exist in an infectious disease outbreak. The one commonly discussed, and the one directly referred to above, concerns risks of the medically vulnerable. This risk measures the likelihood that an infection results in hospitalization, death, a chronic condition like long-Covid, or another adverse heath outcome. I refer to this type of risk as a “health risk.” Typically it is larger for people with known underlying medical conditions but some amount of health risk exists for everyone. However, one never faces this “health risk” unless one is exposed and becomes infected.
We create a second type of risk in outbreaks through our actions which expose others to potential infections. In my forthcoming book, I term this type of risk “exposure risk.” Exposure risk is created by our interactions with others in society. It weaves through the tapestry of our day to day lives whenever we come in contact with other people. It doesn’t depend on a known medical vulnerability. Exposure risk is higher the more frequent and more intense our interactions with others. Someone like a city bus driver, who encounters hundreds of people every day for extended periods of time holds high exposure risk regardless of their health status. They are more likely to be infected, and if infected, they are more likely to pass the infection to others.
In this way, exposure risk is a shared risk. When the bus driver finishes a work shift, they share their exposure risk with everyone they contact in their home, at the social events they attend, and with everyone they interact with on their next work shift. Their children then share this exposure risk with their school classmates and teachers. Their friends share this exposure risk with their friends and loved ones. Their passengers on the bus share it with everyone they contact in their homes, schools, and places of work.
Exposure risk doesn’t have to be shared directly. We share it with our friends, the friends of our friends and their friends too, onward and onward to many people that we may never meet in person. Exposure risk propagates outwards in wave after wave of these inter-personal contacts like ripples from a stone tossed in a pond. Some people in each of these ripples of social connectedness will be medically vulnerable with high levels of health risk. This is the impact to which Dr. Topol refers but it is more broad than just our direct contacts with medically vulnerable “at risk” people.
This shared exposure risk provides one reason why “you do you” and “look out for yourself” will never work well in an infectious disease outbreak. If you only account for your own exposure and health risk and make decisions based solely on your own circumstances, you will never account for the extra risk that you create for others and the people that you will impact-some of whom you will never know or even meet. A firm that only considers its own interest will pollute a river and force others downstream to manage the mess that they create. Exposure risk works in the same way. Selfish people expose others and force those they contact to pay too. In our society many that pay the largest costs from epidemic outbreaks, in terms of health and finance, are the socially vulnerable (a term introduced to me by Anne Sosin of Dartmouth College). They are people who lack access to high quality healthcare, advanced medicines, nutritious foods, financial and housing security, and a host of other advantages. These people, with the most disadvantage, have paid the largest costs throughout the ongoing pandemic and in epidemic outbreaks long past.
When an epidemic outbreak strikes, our individual actions determine the safety of others in society through an aggregate exposure risk. We each determine the size of the pebble that we toss in the pond. If someone takes on large exposure risk in a “you do you” world because they do not feel personally “at risk” (in the sense of health risk) they create a bigger pebble. They create larger and longer lasting ripples in the pond that spread far and wide. They create an epidemic outbreak that lasts longer and affects more people–some of whom are medically or socially vulnerable.
On the other hand, we can make the pebble that we toss in the pond smaller by taking precautions. We can wear high quality masks in crowded public spaces, stay home where we feel unwell and test for an infection, and avoid exposure risk by shunning crowded gatherings that aren’t necessary among other protections. It is up to each of us to determine the size of the epidemic ripples that we each create and how far they propagate. It is up to each of us to continue looking out for each other and, to use Dr. Topol’s word, to have more “respect” for the exposure risk that we each create for ourselves, our loved ones, and everyone in society. It is through this respect that we can all help to protect each other and limit the suffering that many continue to face.
Troy Tassier is a professor at Fordham University and the author of the forthcoming book, The Rich Flee and the Poor Take the Bus: How Our Unequal Society Fails Us during Outbreaks.