The Virulence-Transmission Trade-off
By Nicole Blattman
The threat of COVID-19 seems to have diminished over time.
The “Virulence-Transmission Trade-off” is a theory of disease evolution that says the transmission of a pathogen can be limited by high virulence.
The more sick you are, the more likely you are to stay home and isolate– inhibiting the spread of disease. Viruses may become less severe in order to encourage transmission.
Since 2020 the world has become aware of the speed and degree to which a virus can be transmitted. For months we stayed home, but eventually the world opened back up. Even with variant after variant of COVID-19 emerging, we did not shut back down. At some point, the threat lost its intensity. Why? Part of the reason can be attributed to vaccines and medical advances, but another part is the “Virulence-Transmission Trade-off.”
The Goal of Disease
The “Virulence-Transmission Trade-off” is a theory of disease evolution that says the transmission of a pathogen can be limited by high virulence. The virulence of a disease is its severity, its deadliness. The trade-off suggests that a disease of great severity will not be transmitted as much as one with lower severity.
This matters because, at the end of the day, the main goal of a disease is to spread to as many hosts as possible. Variants of a virus that can be transmitted more often than others will outcompete those less transmitted variants. Thus, the trade-off indicates that since a less deadly virus is more transmissible, less deadly variants will begin to emerge.
But, why? Why is a less virulent virus more transmissible? Think about it. When you’re really sick, like so sick you can’t get out of bed, what do you do? You stay home. You tell your friends and family that you don’t want them to catch what you have. You don’t go to school or work, you don’t go grocery shopping, you don’t go out to eat or to the movies or anywhere. This takes away almost all of the disease’s opportunities to spread to others. Isolating yourself prohibits the main goal of disease– transmission.
If a less virulent variant emerged, things would be different. Maybe your nose is running, you have a bit of a cough, but you don’t halt your day-to-day routine. You still go out, you still see people. And this is how the virus spreads. This trade-off makes diminishing virulence seem attractive to a virus, leading to the arrival of less severe viruses.