Princeton scientists say COVID-19’s silent spread is what makes it so powerful

As nations like Germany and the US struggle to determine the best ways to reopen their economies while keeping everyone safe, the global confirmed COVID-19 case count has exceeded four million. Frustratingly, and despite the world’s best efforts, the coronavirus is continuing to spread. 

One of the biggest factors allowing COVID-19 to continuously infiltrate populations and communities is its ability to spread silently via asymptomatic carriers. As devastating as this virus has been, as far as modern science can tell, most people who come into contact with COVID-19 won’t end up developing symptoms. However, those asymptomatic carriers are still quite contagious.

One would assume that COVID-19’s inability to bring about symptoms in many carriers represents a deficiency or weakness in the virus. A team of researchers at Princeton University disagrees. According to their new study, the asymptomatic transmission is a logical evolutionary step for pathogens.

As humanity’s knowledge of disease and medical capabilities have improved over the years, it’s become harder and harder for pathogens and viruses to thrive like they did hundreds of years ago. It makes all the evolutionary sense in the world that new viral strains would eventually develop the ability to spread secretly via asymptomatic carriers. It’s a chilling concept, but an important realization: we’ve gotten smarter, and now pathogens have as well. 

These findings certainly aren’t limited to just COVID-19. It’s very possible, perhaps even predictable, that more pathogens and viruses with similar tendencies will appear in the future.

For their research, the team at Princeton examined all of the pros and cons associated with asymptomatic transmission for pathogens. Will no symptoms help a virus spread in silence? Absolutely, but there’s also no denying that it’s easier for a virus to spread when it causes symptoms (sneezes, coughs). 

After a complex analysis of how pathogens can spread through populations, they found that asymptomatic transmission can be a successful “strategy” for viral survival. Moreover, the mathematical model used by researchers also suggests that, over long periods, containment measures taken by societies can even influence how a pathogen manifests itself in carriers (asymptomatic, very symptomatic, and anywhere in between).

“An asymptomatic stage for various reasons could provide certain benefits to the pathogen,” says Bryan Grenfell, Princeton’s Kathryn Briger and Sarah Fenton Professor of Ecology and Evolutionary Biology and Public Affairs, Woodrow Wilson School, in a university release. “With the COVID-19 crisis, the importance of this asymptomatic phase has become extremely relevant.”

We don’t usually place viruses and pathogens on the same level as animals or ourselves, but just like more complex organisms, viruses are subject to natural selection. Viral traits that help strains survive and spread persist, while characteristics that don’t aid survival die out eventually. For example, if a virus were so powerful that it immediately killed its host before that organism had a chance to infect anyone else, that virus isn’t going to last very long.

Insidiously, the fact that COVID-19 is quite mild in many patients has helped it spread all over the world and harm so many others. It’s impossible to keep track of all asymptomatic COVID-19 carriers, meaning there are almost certainly countless people venturing outside today and spreading the coronavirus to many other people.

“Viral evolution involves a tradeoff between increasing the rate of transmission and maintaining the host as a base of transmission,” says Simon Levin, Princeton’s James S. McDonnell Distinguished University Professor in Ecology and Evolutionary Biology. “Species that navigate this tradeoff more effectively than others will come to displace those others in the population.”

In many ways, the relationship between any virus and its host is parasitic. The virus needs its host to stay alive, at least for a certain period.

“These are host-parasite interactions,” Levin explains, “and thinking about them from an evolutionary perspective is something we, along with many other scientists, have been interested in for a long time.”

Research for this study started long before COVID-19 emerged; initially, the study’s authors were interested in examining why some influenza strains are asymptomatic. 

“I wondered why asymptomatic flu would arise in evolution,” graduate student Chadi Saad-Roy notes, “and so as a team, we formulated a simple model to try to understand why evolution would favor such behavior.”

These findings have a wide range of implications. Besides just helping us better understand COVID-19’s nature, they also serve as a warning that future mutations of the coronavirus may behave quite differently than the current strains we’re dealing with. The virus will evolve and mutate depending on what gives it the best chances of survival.

“Based on our model,” Saad-Roy concludes, “it’s a natural evolutionary endpoint for certain diseases.”

The full study can be found here, published in Proceedings of the National Academy of Sciences.

John Anderer is a frequent contributor to Ladders News.