Experts agree that Covid-19 will not be completely eradicated but will instead transition from a "pandemic" phase to an "endemic" phase. Writing for Vox, Sigal Samuel outlines the many factors that determine endemicity—and what might happen as Covid-19 becomes endemic.

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When is a disease classified as endemic?

According to Samuel, an infectious disease cannot be classified as endemic until—at the very least—the rate of infections "more or less" stabilizes over a multi-year period.

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"A disease is endemic if the reproductive number is stably at one. That means one infected person, on average, infects one other person," explained Eleanor Murray, an epidemiologist at Boston University. "Right now, we are nowhere near that [with Covid-19]. Each person who's infected is infecting more than one person."

According to Samuel, this is primarily because of the delta variant, which is particularly contagious, and the fact that most people around the world do not yet have immunity, either via vaccination or prior infection. And while experts hoped that the arrival of vaccines would help the world reach overall herd immunity, an insufficient number of people have been vaccinated to reach that goal, Samuel writes.

According to Murray, getting a virus's reproductive number down to one is "the bare minimum" factor in a disease becoming endemic—and the other factors are much more subjective in nature.

Key factors to monitor

For Covid-19 to become endemic, health experts, governmental bodies, and the public must collectively agree that the level of impact the virus has is acceptable. But people don't usually agree on what constitutes as an acceptable level of impact, Samuel writes.

For example, CDC estimates that the flu kills between 12,000 and 52,000 Americans each year. And while there has been continual disagreement on whether this is an "acceptable" level of mortality, Samuel writes, many experts believe that Covid-19's mortality rate would have to more closely resemble influenza's before it could be classified as endemic.

"I am not prepared to say what the appropriate benchmark is yet, but it certainly is much, much lower than where we are, and much closer to where the flu is," said Jen Kates, director of global health and HIV policy at the Kaiser Family Foundation.

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However, some experts have argued that even the flu's estimated mortality rate is too high to be considered an acceptable risk level. "The way I think about it, even with influenza, that's too much," said Joshua Petrie, an epidemiologist at the University of Michigan School of Public Health.

And there are other factors to consider when classifying an infectious disease as endemic, Murray said, such as whether there are treatments available for early-stage patients and individuals with long-term symptoms. Additional factors, according to Murray, include ICU and hospital bed capacity, supply-chain issues, and whether the use of medications used to treat Covid-19 would take away from the supply of those medications for other chronic conditions they are intended to treat.

How ending the state of emergency could unfold

According to Murray, since Covid-19 vaccines are expected to soon be authorized for use in children ages five to 11, Covid-19 could start to transition to an endemic stage in the United States by 2022.

And the process likely won't be a one-and-done announcement, Samuel writes. As the United States moves toward endemicity, experts said people will first see more individual states declaring an end to the emergency—with states that have higher vaccination rates likely reaching endemicity first. According to Petrie, those state and local decisions will likely be governed by the virus's reproductive number and more subjective requirements.

Then, "CDC may pull back our state of emergency in the U.S. if cases remain low at some point in the future," said Tara Smith, an epidemiologist at Kent State University.

Eventually, the World Health Organization will likely make a declaration saying the global state of emergency has ended, like it did in the past for the H1N1 "swine flu" pandemic, Samuel writes. However, this declaration will probably not happen soon.

But until that point, Petrie advised people to monitor local transmission rates and make subjective choices about their own level of risk tolerance based on local factors. "As we're transitioning to a more endemic level," he said, "I think adjusting your behavior based on what's happening locally makes a lot of sense." (Samuel, Vox, 10/22)