President Joe Biden lit a firestorm of controversy this week when he said in an interview that "the pandemic is over": Is it really over? How do we know? Who gets to decide?
Pandemics don't have hard edges. Knowing where they start or stop is a judgment call, and there isn't a clear authority that gets to make that decision.
Epidemiologists recognize a pandemic as a disease outbreak that happens in many countries at the same time and affects more people than an epidemic. Pandemics are often caused by new viruses or a strain of a virus that hasn't circulated among humans in a long time. These events are impactful, often leading to large numbers of deaths, social disruptions and economic hardship.
"I think we were all a little bit shocked when President Biden said what he said on'60 Minutes,' but I also think that in many ways, he was reflecting what many Americans already think and feel," said J. Alex Navarro, assistant director of the Center for the History of Medicine at the University of Michigan Medical School.
But because a pandemic is a global event, no single country or leader can decide it's over, he said.
"Declaring a pandemic over is a little different than declaring a local epidemic over," Navarro said. "To declare a pandemic over, you need to have various regions of the world having their epidemics over. So it looks a little bit different, I think."
The world will probably have to reach an consensus, and that's something that may come as a kind of acknowledgment from the World Health Organization-- or it might not.
"There's not an official designation-holder," said Dr. Amesh Adalja, a senior scholar at the Johns Hopkins Center for Global Health Security. "WHO will acknowledge that we're in a pandemic, but it's not as if somebody calls it a pandemic and then says the pandemic is over."
Pandemics vs. public health emergencies
Biden's comment doesn't change how the US or other countries are responding to Covid-19. For now, it remains a public health emergency in the United States, according to the US Department of Health and Human Services, and it's still a public health emergency of international concern, or PHEIC, according to WHO.
A PHEIC creates an agreement between countries to abide by WHO's recommendations for managing the emergency. Each country, in turn, declares its own public health emergency -- declarations that carry legal weight. Countries use them to marshal resources and waive rules in order to ease a crisis.
When these designations are lifted, there will be changes that reverberate through governments and that reach individuals and families. In the United States, for example, the end of the public health emergency will have ramifications for health care coverage and cost-sharing of Covid-19 tests and treatments.
Saying the pandemic is over might influence public perception, but it doesn't materially change how the federal government or states are responding.
"It's separating out what is the formal legal definition versus what is just a popular discussion of saying, 'hey, we kind of think this is over now, and hey, let's move on,' and there are implications for both," said Rebecca Katz, who directs the Center for Global Health Science and Security at Georgetown University.
In March 2020, WHO Director-General Tedros Adhanom Ghebreyesus said he was finally using the term because of the severity of the disease, how fast it was spreading and "alarming levels of inaction."
"WHO has been assessing this outbreak around the clock," Tedros said. "We have therefore made the assessment that Covid-19 can be characterized as a pandemic."
It was a shift in language that put the world on alert, but it came long after many public health experts had reached the same conclusion, and it didn't change what WHO, the US Centers for Disease Control and Prevention or other international health agencies were doing to respond.
At this point, WHO is not saying whether it will recognize an end to the Covid-19 pandemic.
"WHO does not have a mechanism for declaring or ending a "pandemic,' " spokesman Tarik Jasarevik said in an email to CNN. Instead, he said, WHO will continue to assess the need for the public health emergency, and an expert committee meets every three months to do that.
Last week, Tedros said the end of the pandemic "is in sight," but he added that "we are not there yet."
And it's not clear what "there" will look like.
"That's one of the challenges we have is, we don't have a good definition of when the pandemic begins or when it ends," said Michael Osterholm, who directs the Center for Infectious Disease Research and Policy at the University of Minnesota.
How an earlier pandemic ended
In the modern era, there's no real precedent for closing the curtains on a pandemic.
"We're really in some new territory here," University of Michigan's Navarro said.
The last pandemic that approached this scale was the 1918 influenza pandemic. Back then, there was no federal response. President Woodrow Wilson "really didn't say anything on influenza as far as we know," Navarro said, nor was he expected to.
"The pandemic response was then much more state and local, and so, citizens residents looked to state and local health officers for their guidance," Navarro said. That guidance was typically communicated through local newspapers, which were widely read.
During that pandemic, life mostly got back to normal after the devastating wave of fall illnesses in 1918.
With some exceptions, most mask orders, closures and social distancing orders were lifted by the beginning of 1919.
But waves of influenza continued until 1920, Navarro said. The United States continued to see cases and deaths, but "they did not consider that to be sort of an epidemic level."
Pandemics can end when viruses mutate to become less deadly and people build up some immunity against them, Navarro said.
"Eventually, we reach an equilibrium where we just sort of live with these microbes," he said.
Whether the world is there with Covid-19 still remains to be seen. Vaccines and treatments now offer some protection from severe disease and death, and cases are declining in most parts of the world.
But in the United States alone, there are still about 65,000 new cases reported daily, according to data from Johns Hopkins University, and an average of about 400 people die from Covid-19 every day.