Researchers at Yale have been working on an answer to one of the pandemic鈥檚 most lingering questions: When will COVID-19, and the coronavirus that causes it, become endemic?
鈥淲e wanted to know when it would be over right from the start,鈥� said Caroline Zeiss, a professor of comparative medicine at the Yale School of Medicine.
Now Zeiss and her team think they鈥檝e landed on a fair estimate.
鈥淭he median time was approximately 1,400 days from the start of the pandemic,鈥� she said, 鈥渨hich leaves us at just under two years from now.鈥�
Two years 鈥� that鈥檚 when infection and transmission rates of SARS-CoV-2 could stabilize in the United States, according to the study鈥檚 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.
鈥淲e鈥檙e not going to reach an endemic state until everybody has seen it [the virus], one way or another,鈥� Zeiss said, adding that vaccination is the safest method. 鈥淎nd with vaccination and natural infection, and repeated natural infection, we just build this diverse immune repertoire that ultimately will protect us as a population.鈥�
But it鈥檚 still a moving target, she cautioned. The exact pathway to an endemic stage can be influenced by a number of factors.
鈥淭he virus is so unpredictable and mutable, it could mutate somewhere,鈥� Zeiss said. 鈥淚t could mutate here, it could mutate in another country, and if it鈥檚 transmissible, it could create a version of what we saw earlier on [in the pandemic].鈥�
The Yale study was funded by the National Science Foundation and .
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.
鈥淲e haven鈥檛 seen pandemic-endemic transition of a coronavirus in our lifetimes, in humans,鈥� she said. 鈥淲e鈥檝e had other SARS [viruses], we鈥檝e had MERS, but those did not spread globally like this virus.鈥�
Zeiss, who is also a veterinarian, said we have seen such transitions occur among animals.
鈥淧roduction animals, particularly pigs and chickens, are plagued with coronaviruses,鈥� she said, 鈥渁nd a lot more is understood about them, because they鈥檝e been studied for decades.鈥�
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.
鈥淲hat can we expect down the line once most of us are immune?鈥� Zeiss asked. 鈥淲hat they do tell us is, periodically, the virus mutates and then you get a spike in pathogenicity鈥� 鈥� the ability of a virus to cause disease 鈥� 鈥渙r sometimes, the virus mutates and it becomes really, really apathogenic and it dies out. That鈥檚 happened, too.鈥�
However, it鈥檚 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鈥檚 transition to an endemic stage in the U.S.
For public health reasons, Zeiss said it鈥檚 important that people understand that endemic is not synonymous with safe.
鈥淓ndemic just means that it has reached a fairly stable reproducible rate of recirculating amongst the population,鈥� she said. 鈥淚f you鈥檙e susceptible, you鈥檙e still going to get infected, and if you are prone to severe disease, you could still get severe disease.鈥�
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.
鈥淚 think nobody is safe until everybody is safe,鈥� she said. 鈥淯ntil we have global endemicity, it鈥檚 not going to be stable here.鈥�
In the past 28 days, more than 17 million new confirmed and probable cases of COVID-19 have been identified worldwide, according to . Of those, nearly 3 million have been in the United States.
