In a new study, researchers found that COVID-19’s rapid spread throughout the world has been fueled in part by the virus’s ability to be transmitted by people who are not showing symptoms of infection.
They found that this silent phase of transmission can be a successful evolutionary strategy for pathogens such as viruses like the one that causes COVID-19.
The research was conducted by a team at Princeton University.
The study examined the pros and cons of silent transmission on the pathogen’s long-term survival.
Does transmission without symptoms enable the pathogen to infect greater numbers of people? Or does the lack of symptoms eventually lessen transmission and reduce the pathogen’s long-term survival?
The answer could inform how public health experts plan control measures such as quarantines, testing, and contact tracing.
The team says an asymptomatic stage for various reasons could provide certain benefits to the pathogen.
With the COVID-19 crisis, the importance of this asymptomatic phase has become extremely relevant.
Like more complex organisms, viruses can evolve by natural selection. New variants are generated by mutation and if these changes benefit pathogen transmission, then that strain of the virus will spread.
Species with strategies that contribute to their success will survive, while species with strategies that don’t promote transmission—such as killing the host before the virus can transmit to new susceptible individuals—will eventually die out.
The team says viral evolution involves a tradeoff between increasing the rate of transmission and maintaining the host as a base of transmission.
Species that navigate this tradeoff more effectively than others will come to displace those others in the population.
As the COVID-19 pandemic shows, a silent infection has certain short-term advantages. It makes control strategies—such as identification, quarantine, and contact tracing—difficult to implement.
Infectious people who lack symptoms tend to go about their lives, coming in contact with many susceptible people. In contrast, a person who develops a fever and cough may be more likely to self-isolate by, for example, staying home from work.
However there are also drawbacks: asymptomatic people may generate fewer infectious particles and thus fewer will escape from the infected person, for example in a violent sneeze or forceful cough. The overall transmission could be reduced over time.
The researchers used disease modeling to explore the tradeoffs between these scenarios.
They found that successful strategies emerged when the first stage of infection was completely symptomless, fully symptomatic, and somewhere in between.
They also found that the range of being symptomatic, from no symptoms to maximum symptoms, could be altered by small changes in disease control strategies.
The implication of this last part of the analysis is that disease control strategies could, over long time periods, an influence which strategy a pathogen deploys, and thus have impacts on the course of an epidemic.
The lead author of the study is Bryan Grenfell, Princeton’s Kathryn Briger and Sarah Fenton Professor of Ecology and Evolutionary Biology and Public Affairs, Woodrow Wilson School.
The study is published in the Proceedings of the National Academy of Sciences.
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