(WHTM) — For some, it may feel like the COVID-19 pandemic is already over. For others, it may feel like it will never end. We know from history that pandemics do have to end eventually, but when exactly will this one be over, and what does it take to finally get “back to normal?”

The World Health Organization officially declared COVID-19 a pandemic in March of 2020. In his remarks on March 11, 2020, the WHO Director-General Dr. Tedros Adhanom Ghebreyesus said, “Pandemic is not a word to use lightly or carelessly. It is a word that, if misused, can cause unreasonable fear, or unjustified acceptance that the fight is over, leading to unnecessary suffering and death.”

A specific and consistent definition of “pandemic” is fairly difficult to track down, but sources generally agree that a disease becomes a pandemic if it is widespread across countries, continents, and/or regions and if it can easily spread from person to person, infecting a significant number of people.

Simply put, “A pandemic is anytime a disease spreads rapidly throughout the world,” says Casey Pinto, assistant professor of public health sciences at the Penn State College of Medicine.

So a pandemic starts with a disease — typically a novel disease like COVID-19 — spreading quickly around the globe. But when and how does it end?

“That’s a fantastic question that experts around the world are really kind of figuring out right now because it’s really hard to tell when a pandemic ends,” Pinto said.

Contributing to the challenge of clearly determining the criteria for the end of the COVID-19 pandemic, the WHO Director-General says that there has never before been a pandemic caused by a coronavirus.

The WHO defines several phases of an influenza pandemic, one of which is the post-pandemic period. In this phase, “Levels of influenza activity have returned to the levels seen for seasonal influenza in most countries with adequate surveillance,” according to the WHO.

COVID-19 is caused by a coronavirus, which is different from an influenza virus, but that post-pandemic definition may provide some guidance as to when this pandemic will finally be over.

While the specifics are hazy, “the key takeaway for when a pandemic ends is when we can say the virus stops readily spreading among a population,” Pinto said.

How will the COVID-19 pandemic end?

There are several factors that contribute to reaching the point when the virus stops easily spreading, but one big one is achieving herd immunity, Pinto says, and not just achieving herd immunity in one state or even one country.

“Because we are a society that is just so connected, we fly everywhere in the world, this is going to continue to manifest,” Pinto said.

Achieving herd immunity in one place won’t prevent the virus from continuing to spread in other areas. On top of that, we don’t yet know how long immunity from the COVID-19 vaccines lasts, says Pinto, or whether the virus will eventually be able to circumvent the vaccines or the immunity previously infected individuals developed.

People are going to travel between states and between countries, so herd immunity will need to be reached around the nation and the world in order for the pandemic to end, Pinto hypothesizes. Although the exact threshold for herd immunity is unknown, experts estimate that we will need at least 70% of people to be immune to the virus.

Researchers can approximate the percentage of people who need to be immune to the virus in order to achieve herd immunity by comparing COVID-19 with other better-known diseases that are similarly contagious, explains Pinto.

“So like measles, we know that we need about 93% of the population to be vaccinated or have natural immunity through being exposed in order to prevent an outbreak of measles. With COVID, because it’s nearly as contagious as measles…we know that we need about 90% [of people to be immune],” Pinto said.

According to CDC data from June 23, 45.4% of the total U.S. population has been fully vaccinated, and just over 53% of Americans 12 years and older have been fully vaccinated.

That’s quite a distance from that 70-90% goal, but “it can’t just be 90% in the U.S.,” says Pinto, “and this is where we’re going to run into trouble.”

According to Our World in Data, about 10% of people worldwide are fully vaccinated, and the distribution of these vaccinations is not even across countries. Our World in Data reports that just 0.9% of people in low-income countries have received at least one dose of a COVID-19 vaccine.

Going from 10% to 70-90% of people vaccinated may feel impossible, but if we’ve learned one thing from past pandemics, it’s that they do eventually end.

Something else we can learn from previous pandemics, says Pinto, is that this coronavirus will probably never completely disappear.

Since the COVID-19 pandemic began, people have been comparing it to the 1918 flu.

“That pandemic took about 18-20 months to fully end, but that virus did not go away, and that’s the same thing we think we’re going to see with COVID,” Pinto said.

Much like the flu, experts like Pinto think COVID-19 will become endemic, meaning it will regularly occur in our population. And, just like with the flu, Pinto expects we’ll need to periodically get revaccinated against the coronavirus to prevent serious illness.

However, the virus will hopefully be less deadly if it does stick around. Viruses “want” to live, explains Pinto, so if they quickly kill their hosts, they harm themselves, as well. Pinto anticipates that COVID-19 will mutate to become less deadly in the future.

Regardless of the future of COVID-19, Pinto reminds people that the pandemic is not over yet. Pinto encourages continued social distancing and thorough hand washing, and she notes that individuals who feel unwell should stay home to protect others, such as children who are not yet able to get vaccinated.

Pinto also encourages individuals to get vaccinated in order to protect themselves and to help the world get closer to herd immunity.

“We have no reported deaths from the COVID vaccine, but in the U.S. we have 600,000 deaths from COVID,” Pinto said.

“[The pandemic] will end, but we are not there yet,” Pinto said. “We’re all doing our best. We can get through this together.”