Researchers at Yale have been working on an answer to one of the pandemic’s most lingering questions: When will COVID-19, and the coronavirus that causes it, become endemic?
“We wanted to know when it would be over right from the start,” said Caroline Zeiss, a professor of comparative medicine at the Yale School of Medicine.
Now Zeiss and her team think they’ve landed on a fair estimate.
“The median time was approximately 1,400 days from the start of the pandemic,” she said, “which leaves us at just under two years from now.”
Two years – that’s when infection and transmission rates of SARS-CoV-2 could stabilize in the United States, according to the study’s authors. It means the virus would continue to have a constant presence here, but it would be expected and have predictable patterns, much like influenza viruses and rhinoviruses that cause common colds.
“We’re not going to reach an endemic state until everybody has seen it [the virus], one way or another,” Zeiss said, adding that vaccination is the safest method. “And with vaccination and natural infection, and repeated natural infection, we just build this diverse immune repertoire that ultimately will protect us as a population.”
But it’s still a moving target, she cautioned. The exact pathway to an endemic stage can be influenced by a number of factors.
“The virus is so unpredictable and mutable, it could mutate somewhere,” Zeiss said. “It could mutate here, it could mutate in another country, and if it’s transmissible, it could create a version of what we saw earlier on [in the pandemic].”
The Yale study was funded by the National Science Foundation and published Tuesday in the journal PNAS Nexus.
Zeiss and her team found that the best way to predict the future course of the pandemic would be by modeling rates of infection and transmission among animals, since they, like humans, are susceptible to coronaviruses.
“We haven’t seen pandemic-endemic transition of a coronavirus in our lifetimes, in humans,” she said. “We’ve had other SARS [viruses], we’ve had MERS, but those did not spread globally like this virus.”
Zeiss, who is also a veterinarian, said we have seen such transitions occur among animals.
“Production animals, particularly pigs and chickens, are plagued with coronaviruses,” she said, “and a lot more is understood about them, because they’ve been studied for decades.”
In fact, Zeiss said studying coronavirus activity among animals can be useful not just for predicting an endemic stage, but to also help inform people of what to prepare for along the way. The poultry industry, for example, regularly vaccinates chickens for an endemic respiratory coronavirus that dates back to the 1930s.
“What can we expect down the line once most of us are immune?” Zeiss asked. “What they do tell us is, periodically, the virus mutates and then you get a spike in pathogenicity” – the ability of a virus to cause disease – “or sometimes, the virus mutates and it becomes really, really apathogenic and it dies out. That’s happened, too.”
However, it’s unlikely that the coronavirus that causes COVID-19 is going to die out anytime soon, if ever.
Yale researchers experimented with rats. They introduced the animals to a type of coronavirus that spreads very similarly to the coronavirus at the center of the COVID-19 pandemic but causes only mild disease in the rats.
Scientists then mimicked human behavior and created multiple scenarios in which the rats became exposed to the virus. Zeiss said her team also calculated in immunization to simulate COVID-19 vaccine uptake among the human population, and then continued to expose the rats to reinfection.
Sarah Mullin, a postdoctoral fellow in the Yale Center for Medical Informatics, took the experiment data and built a mathematical model to come up with a possible trajectory of the pandemic’s transition to an endemic stage in the U.S.
For public health reasons, Zeiss said it’s important that people understand that endemic is not synonymous with safe.
“Endemic just means that it has reached a fairly stable reproducible rate of recirculating amongst the population,” she said. “If you’re susceptible, you’re still going to get infected, and if you are prone to severe disease, you could still get severe disease.”
The study found that an estimated 15% of the population will remain at risk of becoming infected with the virus, at any time, during an endemic phase.
The timeline also depends on what happens globally. Containing the virus and its variants in just certain regions or parts of the world leaves the global population vulnerable to a prolonged pandemic, Zeiss said.
“I think nobody is safe until everybody is safe,” she said. “Until we have global endemicity, it’s not going to be stable here.”
In the past 28 days, more than 17 million new confirmed and probable cases of COVID-19 have been identified worldwide, according to data from Johns Hopkins University. Of those, nearly 3 million have been in the United States.
