University of Utah Health
Summary The future prevalence and virulence of SARS-CoV-2 is uncertain. Some emerging pathogens become avirulent as populations approach herd immunity. Although not all viruses follow this path, the fact that seasonal coronaviruses are benign gives some hope. We developed a general mathematical model to predict when the interaction between three factors, the correlation of severity in consecutive infections, population heterogeneity in susceptibility due to age, and reduced severity due to partial immunity, will promote avirulence as SARS-CoV-2 becomes endemic. Each of these components has the potential to limit severe, high-dissemination cases over time in the right circumstances, but in combination they can rapidly reduce the frequency of more severe and infectious manifestations of the disease in a wide range of conditions. As more reinfections are captured in the data over the next few years, these models will help test whether the severity of COVID-19 is beginning to ease in the way our model predicts and predicts the disease. |
Comments
In the next decade, the new coronavirus responsible for COVID-19 could become little more than a nuisance, causing only common coughs and colds. That possible future is predicted through mathematical models that incorporate lessons learned from the current pandemic about how our body’s immunity changes over time. Scientists at the University of Utah carried out the research, now published in the journal Viruses .
"This shows a possible future that has not yet been fully addressed," says Fred Adler, PhD, professor of mathematics and biological sciences at the U. "Over the next decade, the severity of COVID-19 may decrease as the populations develop immunity collectively."
The findings suggest that changes in the disease could be driven by adaptations of our immune response rather than changes in the virus itself. Adler was lead author on the publication with Alexander Beams, first author and graduate student in the Department of Mathematics and Division of Health Epidemiology at the University of Utah, and undergraduate co-author Rebecca Bateman.
Although SARS-CoV-2 (the sometimes deadly coronavirus that causes COVID-19) is the best-known member of that family of viruses, other seasonal coronaviruses circulate in the human population and are much more benign. Some evidence indicates that one of these cold-causing relatives may have once been severe, leading to the "Russian flu" pandemic in the late 19th century. The parallels led U of U scientists to wonder if the severity of SARS-CoV-2 could similarly decrease over time.
To test the idea, they built mathematical models that incorporate evidence about the body’s immune response to SARS-CoV-2 based on the following data from the current pandemic.
It is likely that there is a dose response between exposure to the virus and the severity of the disease.
- A person exposed to a small dose of virus will be more likely to get a mild case of COVID-19 and shed small amounts of virus.
- In contrast, adults exposed to a large dose of virus are more likely to have severe illness and shed more virus.
- Masking and social distancing decrease the viral dose.
- Children are unlikely to develop serious illness.
- Adults who have had COVID-19 or been vaccinated are protected against serious illness.
Running several versions of these scenarios showed that the three mechanisms in combination create a situation in which an increasing proportion of the population will be predisposed to mild illnesses in the long term. Scientists felt the transformation was significant enough to need a new term. In this scenario, SARS-CoV-2 would become "Just Another Seasonal Coronavirus" , or JASC for short.
"At the beginning of the pandemic, no one had seen the virus before," explains Adler. "Our immune system was not prepared." Models show that as more adults become partially immune, either through prior infection or vaccination, serious infections will all but disappear over the next decade. Eventually, the only people who will be exposed to the virus for the first time will be children, and they are naturally less likely to suffer serious illness.
"The novel approach here is to recognize the competition taking place between mild and severe COVID-19 infections and ask which type will persist in the long term," Beams says. "We have shown that mild infections will win, as long as they get into our immune system to fight serious infections."
The models do not take into account all possible influences on the disease trajectory. For example, if new virus variants overcome partial immunity, COVID-19 could worsen. Furthermore, the predictions are based on the key assumptions of the model being held.
"Our next step is to compare our model predictions with the most current disease data to assess which direction the pandemic is going as it occurs," Adler says. "Do things seem to be going in a good or bad direction? Is the proportion of mild cases increasing? Knowing that could affect the decisions we make as a society."
