For viruses, symptomless patients show strengths and weaknesses of evolving germs

PRINCETON, N.J. — The coronavirus continues to be a global threat, especially because many patients don’t even know they have the illness. To that end, A new study out of Princeton University reveals the “silent phases” of viruses play a major role in how a pathogen evolves. Researchers say how these viruses mutate could mean the difference between a short-term illness or a global pandemic.

Researchers analyze the good and the bad that come along with asymptomatic transmission, examining its role in long-term survival. Chadi Saad-Roy and his team modified a standard mathematical model of how diseases are spread through populations. Using the influenza virus for their model, the team compartmentalized the research by representing susceptible, infected, and recovered individuals.

The “infected” compartment is also broken into two parts. During the first part, researchers change the levels of symptoms so some participants wouldn’t have any at all, some would have mild symptoms, and others would develop significant symptoms. The second part has fully symptomatic individuals. The study then looks at the effects the symptoms have on individuals during part one of the study.

Is silently spreading a virus better or worse?

Researchers find that those who have no symptoms, some symptoms, and significant symptoms all could have improved their health by making small changes in controlling the disease.

“An asymptomatic stage for various reasons could provide certain benefits to the pathogen,” Prof. Bryan Grenfell says in a university release. “With the COVID-19 crisis, the importance of this asymptomatic phase has become extremely relevant.”

There’s not a definite way of knowing who’s immune system will be affected by viruses, seeing as much of it has to do with natural selection. Variants of pathogens work by mutating and if the pathogen’s transmission benefits from these changes, the virus can spread.

“Viral evolution involves a trade off between increasing the rate of transmission and maintaining the host as a base of transmission,” co-researcher Simon Levin adds. “Species that navigate this tradeoff more effectively than others will come to displace those others in the population.”

Visible symptoms also change behavior

Someone showing symptoms of COVID-19 is likely to quarantine and isolate themselves from the general public and at-risk family members. Asymptomatic people however, will likely go about their day and continue on to work. Asymptomatic carriers have the disadvantage of not being able to get the germs and infectious particles out of their system, unlike a person who shows systems like sneezes and coughs and are now able to reduce the effects quicker.

It’s important to recognize this study began before the coronavirus pandemic. As mentioned, it initially focused on the spread of influenza in 2019.

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

Saad-Roy also notes the research may helps explain a broader aspect of COVID-19 and epidemic nature, in general. It is not to his surprise that the coronavirus appears symptomless in some cases.

The study is published in the journal Proceedings of the National Academy of Sciences.

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