After contracting COVID-19, President Biden quickly appeared on video to show Americans that, while he’s fighting off the symptoms, he’s doing fine.

But even if he’s back on the campaign trail in a day or two, that doesn’t necessarily mean the COVID conversation about Biden’s health is over.

Public health officials warn that millions of Americans who contract the virus go on to suffer from what is known as “long COVID.” What is it?

It depends on who you ask.

A pounding heart when simply getting off the couch, difficulty concentrating, difficulty breathing, chest pain, and ongoing fatigue are just a handful of persistent symptoms of long COVID — scientifically known as Post-Acute Sequelae of SARS-CoV-2 infection (PASC), according to the Centers for Disease Control and Prevention (CDC).

“There is no agreement on how to define and diagnose long COVID,” writes Heidi Ledford at Nature. “The World Health Organization’s attempt at a consensus, published in 2021, has not proved popular with patient advocates or researchers, and studies continue to use a range of criteria to define the condition.”

Dr. Alba Azola, Assistant Professor of Physical Medicine and Rehabilitation at Johns Hopkins Medicine, says the inability of health professionals to settle on a definition is hurting efforts to treat it.

“It becomes very challenging at times to be able to support these claims,” Azola said. “I think advocacy regarding understanding the syndrome, the knowledge that we have so far, and the limitations when it comes to certain medical testing is important.”

Long COVID is a problem that is not going away, health experts say. According to the CDC, 60 percent of the U.S. population and 75 percent of children have been infected with COVID-19.

The American Academy of Physical Medicine Rehabilitation (AAPMR) recently released new guidelines for treating the impacts of long COVID. They also urged government and healthcare systems to prepare for the long-lasting consequences affecting people, according to Steven Flanagan, Rehabilitation Medicine Department Chair at New York University Langone Health.

“These are folks that can’t go back to work in the same capacity that they were doing before they became sick with COVID-19 and developed long COVID,” Flanagan said. “We are trying to take the ‘dis’ out of ‘disability.’ We can work with employers and school systems to get folks back, but it’s not going to happen overnight.”

With the unclear definition comes less-than-solid numbers of long COVID’s reach. An estimated eight to 26 million Americans have experienced or are currently experiencing at least one symptom of long COVID, according to the American Academy of Physical Medicine and Rehabilitation Long COVID Dashboard. Estimates that between 5 and 50 percent of COVID cases will lead to long COVID are so vague they tend to undermine trust in the analysis and make it harder to get public officials to commit to taking action, public health advocates say.

Two significant debates continue to rage over long COVID: Is the syndrome connected to the severity of the original “acute” case, and do vaccinations help prevent its onset? President Biden’s case appears to be mild. Will that help?

Some studies have found no correlation between how badly COVID symptoms hit and the likelihood of suffering from long COVID later.

Others, like Dr. Jonathan Whiteson of New York University’s Langone Health Center, say there is some correlation, but it is hardly 100 percent.

“We are seeing many people who had mild symptoms and were never hospitalized in the acute phase develop cardiovascular disorders,” Whiteson said. “Up to 4 percent of individuals who had mild disease have had a stroke or myocardial infarction in the post-acute stage. This is very significant.”

Symptoms of long COVID aren’t just reserved for older individuals, but have also been found in school-age children and adolescents, Whiteson said.

“Because of the uniqueness of this virus, we are projecting that cardiovascular disease could affect a much younger population,” Whiteson said. “Just like diabetes is a risk factor for cardiovascular disease, COVID-19 could be one too.”

As for the impact of vaccinations, Whiteson also sees a correlation.

“When we look into the long COVID period—three to six months out—we are seeing in those people who are unvaccinated more significant cardiovascular consequences and disability,” Whiteson said. “Again, it is an important call to action to get the message out there that vaccinations reduce the risk of severe disease, reduce the risk of hospitalization, and do have a long-term positive impact.”

But a recent study conducted by VA St. Louis Healthcare System, which examined 13 million cases, found the vaccine reduced the risk of long COVID only by 15 percent, substantially less than other estimates.

Currently, 41 post-COVID-19 clinics collaborate with the American Academy of Physical Medicine Rehabilitation as well as practitioners from multiple other healthcare disciplines.

In Ann Arbor, Mich., Dr. James Neuenschwander treats children and adults with complex, chronic health problems who have not responded to conventional therapies. While a handful of his patients suffer from long COVID, Neuenschwander also treats patients with vaccine injury. He said the symptoms for both are very similar. One symptom is myocarditis — an inflammation of the heart muscle reported as a symptom of long COVID and a possible complication of the vaccine by the CDC.

“We know that both COVID-19 and the vaccine can activate or reactivate an autoimmune disease,’ Neuenschwander said.

In addition to releasing a guide on cardiovascular complications, the American Academy of Physical Medicine Rehabilitation has released guides on fatigue, breathing discomfort, and cognitive symptoms.

According to the AAPMR, there is not a one size fits all approach to treating patients. Each treatment plan should be individually tailored.

“I think we’re in a state right now where there’s recognition that long COVID is real—it’s not just patients being anxious. It’s a real condition,” Flanagan said.