(CNN) Covid-19 is here to stay.
It's highly unlikely that the United States, let alone the world, will be able to completely eliminate the coronavirus that causes Covid-19.
But there will come a day when it's no longer a pandemic, when cases are no longer out of control and hospitals aren't at great risk of overflowing with patients. Many experts predict the spread of coronavirus will look and feel more like seasonal influenza.
What's less clear is how and when that will happen.
"There's not even a measurement to say that something is an epidemic or pandemic. All of this is in the eye of the beholder -- and that's part of the issue," Dr. Arnold Monto, a professor of epidemiology at the University of Michigan and acting chair of the US Food and Drug Administration's Vaccines and Related Biological Products Advisory Committee, told CNN.
"So, all of this is not based on rules. It's based typically on what you have to do to control the outbreak," Monto said. "What is so different here is that our vaccines are much more effective than what we usually see."
The good news, according to Monto, is the power of vaccines. The bad news comes with the power of the virus to change and evolve.
No one can predict what the future of Covid-19 could look like -- and the emergence of coronavirus variants, like Delta, has shifted the trajectory, he said.
"With the change in transmission patterns, as the variants have emerged -- I call it a parade of variants -- we now see much more extensive transmission and much more uniform spread globally. This makes declaring the end of the pandemic more difficult," Monto said. "Because the whole pattern of spread has changed, and there may still be pockets that really haven't gone through the kind of waves that the rest of the world has gone through."
Monto and other public health leaders anticipate that in the future, the world could track the spread of SARS-CoV-2, the coronavirus that causes Covid-19, in ways similar to how the seasonal flu is monitored.
"We have no idea whether we're going to see that kind of seasonal pattern with SARS-CoV-2, but it does remind us that most of our respiratory viruses start behaving as seasonal events," Monto said.
"There is the precedent for a very seasonal pattern for some of the coronaviruses that have been infecting people," he added. "Whether SARS-CoV-2 starts to behave like that, we don't know, but at least it gives us one scenario that it might start to behave like that."
As Monto put it, we have to "wait and see and hold our breath" to unlock what an endemic phase of the coronavirus might look like.
Endemic means that a disease has a constant presence in a population -- but it's not affecting an alarmingly large number of people as typically seen in a pandemic. Even in early 2020, as the pandemic was ramping up, officials at the World Health Organization predicted that the novel coronavirus "may become another endemic virus in our communities" and never go away.
"When you think about pandemics, you're in the pandemic phase and then you have a deceleration phase, then you have a control phase, then hopefully you'll have elimination and maybe eradication," Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told the US Senate Committee on Health, Education Labor & Pensions in a hearing on Thursday.
"What we hope to get it at is such a low level that even though it isn't completely eliminated, it doesn't have a major impact on public health or on the way we run our lives," Fauci said. "So, if we get more people vaccinated globally and more people vaccinated now, hopefully within a reasonable period of time we will get to that point where it might occasionally be up and down in the background but it won't dominate us the way it's doing right now."
While the US Department of Health and Human Services last month renewed its determination that a public health emergency still exists in the United States due to Covid-19, federal health officials already are thinking about how to measure the end of the pandemic and how to continue to track the coronavirus once it becomes endemic.
To transition from pandemic to endemic, the nation has to build up immunity to the coronavirus -- which means many more people need to get vaccinated, Dr. Philip Landrigan, a pediatrician and epidemiologist at Boston College, told CNN.
With some Americans still refusing to get their Covid-19 shots and some refusing to wear masks, the transition could take more time.
Currently, about 58% of the total US population is fully vaccinated against Covid-19.
"We have to get somewhere well north of 80%, possibly even well north of 90% of the population with immunity either through having had infection or through having had vaccinations," said Landrigan, who worked at the CDC for 15 years.
To control the spread of the measles virus in the US population, for instance, "we had to get the immunity rate up above 95%, and even then, we've had sporadic outbreaks. These outbreaks typically occur when you have a cluster of people in a particular place who are not immunized and all of a sudden the virus gets introduced because a traveler has come in with the virus -- and bang, you've got 20 cases of measles in some town," Landrigan said. "But that's not an epidemic. It's an outbreak against a background of almost no cases or scattered endemic cases."
For now, the CDC says there's much work to do to control the current spread of the virus.
"We know there is still much to be done to stop the spread of COVID-19 and end the pandemic. We are still seeing far too many new cases, hospitalizations, and deaths. The daily average of cases is over 70,000 a day with more than 1,000 deaths. This is why we're encouraging everyone 5 years and older get vaccinated to protect them against COVID-19," CDC spokesperson Kristen Nordlund wrote in an email to CNN last week.
"As we look forward to the fall and winter, it's important to continue practicing prevention measures that we know work -- vaccinating, wearing a mask in public, indoor settings, staying home when you are sick, and washing your hands frequently."
Health officials are familiar with the work needed to improve vaccination rates.
The CDC recommends that almost everyone 6 months and older should get a flu shot every year. But during the 2019-2020 flu season, only about half of those people -- 51.8% -- did, according to the CDC. The agency estimates that flu has caused about 12,000 to 52,000 deaths each year between 2010 and 2020.
The coronavirus has killed more than 750,000 people in the United States so far.
The battle to corral coronavirus every year may look very much like the annual fight against the flu.
"We've been thinking a lot about what an endemic phase looks like and the data that we're needing to collect during that phase. Certainly right now we are collecting data on cases, hospitalizations, deaths," Dr. Rochelle Walensky, director of the US Centers for Disease Control and Prevention, said in Thursday's Senate committee hearing. "The question is: What are going to be our best metrics moving forward? And probably modeling it on flu."
The CDC collaborates with health departments, laboratories, hospitals and health care providers to track diagnosed flu cases, determine what influenza viruses are circulating and measure the impact those viruses are having on hospitalizations and deaths.
One idea is that when the coronavirus becomes endemic, a similar tracking system could be used to monitor the pathogen.
"We could handle the cases just like we do with seasonal flu -- where we're able to say we know we're going to see a number of cases in the winter season, and we can have the right staffing, we can have the right supplies ready and we're ready to handle it, as opposed to the surges that we've been dealing with here," Dr. Stephen Parodi, national infectious disease leader for Kaiser Permanente, told CNN.
"I'm still on phone calls talking about what's our ICU bed capacity? What's our supplies chains that we need to provide care to patients? Do we have enough medication? Do we have enough monoclonal antibodies?" Parodi said. "We have a lot more work to still do to get to where we want to be, and I think we're going to see this transition over year 2022. But for some locales, where there's less immunity, it's going to be a longer run."
Even flu is unpredictable, and doctors have seen a lot of flu over the years.
"We know there are going to be cases," Monto said. "With the flu, we've had experience with flu pandemics before. So we know typically the way they behave. This has been an evolving situation with a totally novel pathogen."