The future of the novel coronavirus severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), responsible for the coronavirus disease 2019 (COVID-19) pandemic, is uncertain both in terms of prevalence and virulence. Some newly emerging pathogens become non-virulent as populations reach herd immunity. However, not all viruses behave similarly. But the fact that seasonal coronaviruses are less pathogenic and virulent could provide a degree of hope for humanity in the long term.
Using mathematical models to predict the future of SARS-CoV-2
SARS-CoV-2 could become just a seasonal nuisance in the next decade, causing just coughs and sniffles like a common cold, suggests new research. Scientists at the University of Utah, USA, tried to predict the possible future of the novel coronavirus using mathematical models that use learnings from the current pandemic on changes in human immunity over time. The research has been published in the journal Viruses.
The researchers developed a mathematical model to predict when SARS-CoV-2 may become avirulent based on the interplay between 3 factors: correlation of severity in consecutive infections, age-based susceptibility of a population, and decreased disease severity due to partial immunity.
Infections can be "high-" or "low-shedding" in the mathematical model, with low-shedding infections coinciding with a mild disease similar to the ones caused by seasonal coronaviruses. The researchers explored how these 3 mechanisms could change COVID-19 severity and how they can work together to rapidly promote non-virulence.
The combination of the 3 elements can rapidly reduce the frequency of severe disease
The study's findings suggest that rather than changes in the virus, the immunologic adaptation in human responses to the virus can drive changes in the disease. Each of the 3 elements can limit severe, infectious disease over time given the right circumstances, but when combined, they can rapidly bring down the frequency of more severe manifestation of disease over a broad range of conditions.
The team state:
We've shown that mild infections will win, as long as they train our immune systems to fight against severe infections."
Newer, more infectious variants with immune-escape capabilities may bypass the process proposed by this model
According to the authors, vaccines may complicate this evaluation as they might mimic natural infection with respect to the type and duration of immunity. They added that existing genetic variation in SARS-CoV-2 and further mutation might also change the evolutionary path of the virus in ways that this model cannot predict. More importantly, new and more infectious variants that can escape partial immunity may bypass the process proposed by this model.
In the long-term, beyond the duration considered by this model, SARS-CoV-2 and the human immune system may evolve together in the context of a genetic arms race rendering the virus more virulent and humans more resistant.
More studies needed to investigate if individuals with prior SARS-CoV-2 infection exhibit milder symptoms upon reinfection
As emerging virulent pathogens transition to a larger ecosystem, they often tend to become non-virulent. The role played by the human immune system in this transition is not clear. The COVID-19 pandemic offers an unprecedented opportunity to better understand how the immune system might facilitate disease evolution.
If it can help mitigate disease severity in a manner described by this model, then the study results suggest that mild or asymptomatic SARS-CoV-2 infections will become typical in the future. Although viral evolution and interaction of viruses with vaccines complicate the whole picture, there is hope that SARS-CoV-2 will eventually become a seasonal coronavirus.
As we capture more reinfections over the next few years, these mathematical models will help test if COVID-19 severity is beginning to decrease as per the predictions of this model and predict the future of the disease. Further studies are needed to determine if individuals with a history of prior SARS-CoV-2 infection exhibit milder symptoms upon getting reinfected. These investigations should account for whether people are vaccinated, because that could also be associated with mild disease.
The team concludes:
Because we have evidence for all three mechanisms for SARS-COV-2, it is quite possible that the initial outbreak composed of many severe, high-shedding infections can be followed by an endemic state characterized by mild, low-shedding infections much like the seasonal coronaviruses."