One day, the pandemic will end. Scientists don't know how that finale will play out but a new model offers a teaser: The deadly SARS-CoV-2 may not totally disappear but instead become a commonly circulating cold virus that just causes some mild sniffles.
This model, published Jan. 12 in the journal Science, is based on analyses of other coronaviruses, the majority of which cause only mild symptoms in humans. There are six known coronaviruses that infect humans; four are coronaviruses that are "endemic" or regularly circulate among human populations and cause the common cold.
The other two coronaviruses — the ones that cause severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS) — are more deadly, though the first was eliminated years ago and the latter has been largely contained.
To create their model, a group of researchers analyzed previously published data on the four milder coronaviruses and found that "infection-blocking immunity wanes rapidly, but disease-reducing immunity is long-lived," the authors wrote in the study. In other words, people can get infected over and over again, but rarely get severe disease, lead author Jennie Lavine, a postdoctoral fellow at Emory University in Atlanta, said in a statement.
Almost everyone catches one of these endemic coronaviruses during childhood; and these early infections confer partial immunity to adults who become reinfected. "Reinfection is possible within one year, but even if it occurs, symptoms are mild and the virus is cleared from the body more quickly," Lavine said.
But there's no similar long-term data on the duration of immunity for the novel coronavirus SARS-CoV-2 that causes COVID-19. It's not clear how long immunity, whether from vaccines or natural infection, to SARS-CoV-2 will last in people; it's also unknown to what degree vaccines and natural infections will curb transmission or reduce the severity of the disease.
Some people have already been reinfected with SARS-CoV-2, Live Science previously reported. But these cases have been rare and most of these people have had milder disease the second time around, according to the study.
The model assumes that immunity to SARS-CoV-2 will work similarly to these other endemic coronaviruses, Lavine said. And one of the model's key conclusions is that, for existing coronaviruses, the severity of infection during the endemic phase is directly tied to how severe the disease is when it infects children. Unlike the novel coronavirus, almost no one encounters these widely circulating ones first as an adult. But "we don't really know what it would be like if someone got one of the other coronaviruses for the first time as an adult, rather than as a child," she said. It's possible that if they did, they would experience more severe disease.
Their model predicts that if SARS-CoV-2 becomes endemic, and future generations are primarily exposed during childhood, the virus "may be no more virulent than the common cold," the authors wrote. Once endemic, the virus's infection fatality ratio, or the number of people who die compared with those who are infected, will fall below that of the seasonal flu, the authors wrote.
That's because children are generally less severely impacted by COVID-19 infections and mortality is typically low in kids, so this baseline severity should predict SARS-CoV-2's severity in its endemic phase. But if SARS-CoV-2 severely impacted children, as is the case with the virus that causes MERS, then even during the endemic phase, a relatively high number of people might die, the authors predicted.
If this model holds true, exactly how the world will reach the endemic phase is still up to us: Faster virus spread will result in a quicker transition as people gain herd immunity, but it will result in more deaths. Vaccines are a safer way to achieve such immunity and once widely available, they will also accelerate the transition into a possible endemic phase — a phase in which the coronavirus may be tackled with a box of Kleenex, rather than ventilators and lockdowns.
Originally published on Live Science.
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Yasemin is a staff writer at Live Science, covering health, neuroscience and biology. Her work has appeared in Scientific American, Science and the San Jose Mercury News. She has a bachelor's degree in biomedical engineering from the University of Connecticut and a graduate certificate in science communication from the University of California, Santa Cruz.