Scott Allen, an elementary school teacher in Moore, Okla., started feeling sick at the end of August 2020, just two days before school was set to start. The diagnosis: COVID-19.

“It was like the worst flu I’ve ever had. I was pretty well bedridden,” Scott told Fortune.

While Allen was never hospitalized for his bout of COVID, the symptoms persisted long after his two weeks of paid leave available under federal relief rules ran out. Allen had been a teacher for more than 30 years and had managed to accrue more than 130 days of sick time—more than enough time to see him through his illness, or so he thought.

Yet the symptoms dragged on, including chronic fatigue, tremors, migraines, shortness of breath, and brain fog. So much so that Allen couldn’t return to work. Instead, he spent his days seeing an increasing array of specialists: a pulmonologist, cardiologist, neurologist, and nephrologist.

“Some days just doing the laundry wears me out,” Allen said. “I have days that I think it’s getting better. I think, ‘I’m over the hump.’ Then two days later, something will come up, and I’m shot back down.”

Nearly a year after his initial positive test, Allen is still suffering from what has now been diagnosed as long COVID. His symptoms are so severe, he hasn’t been able to return to work. Instead, he had to rely on donated sick days from other teachers and staff to stretch his paid time off until June 1 of this year, when he officially retired at age 56.

“I wasn’t ready to retire,” Allen said, adding the plan was to wait until his daughter, who will be a senior in high school this year, was through college and his Social Security and retirement investments were vested. Instead, he’s now drawing on his pension and hoping to get approved for Social Security disability insurance to make up the difference.

How many people are suffering from long COVID?

It’s still unclear how many people who contract COVID-19 end up suffering from long COVID. Studies of those suffering from COVID symptoms for at least 12 weeks after the initial infection put the number of patients from 2.3% to 37% of those who test positive, according to a report published last week by the U.K.-based COVID-19 longitudinal health and well-being national core study.

But the percentage of COVID patients who have symptoms so severe they limit daily activities is much smaller, ranging from about 1.2% of 20-year-olds studied to up to 4.8% of 60-year-olds, the U.K. report found.

About 34 million people in the U.S. have been diagnosed with COVID since the start of the pandemic, although some research suggests that number could be nearly twice as high. But even if only a small percentage of those suffer from long COVID, it may mean hundreds of thousands—and possibly up to a couple of million—Americans are experiencing ongoing symptoms.

Long COVID can be difficult for employers to support

One of the reason that estimates vary so widely is the wide array of symptoms and severity that can appear with long COVID. Allen, for example, suffers from neurological effects, asthma, and now even Stage 3 kidney disease—a condition that his doctors are unsure is a direct result of COVID-19 or a side effect of the medication he was prescribed for migraines he started having as a result of long COVID.

The myriad of symptoms and the varying severity of long-COVID patients can also make it challenging to objectively diagnose and prove to an employer, which then needs to determine how to offer the right support.

“There are certain kinds of disabilities people might have from COVID, which are very objectively provable. The challenge is with some of the cases of long COVID, you can’t do that,” said Jeff Levin-Scherz, a physician and managing director and population health leader at Willis Towers Watson. Many times long-COVID patients have brain fog and a hard time concentrating, something that may not show up on a test.

“It could be a cough, it could be headaches, it could be difficulty breathing, it could be fatigue. It’s a multi-symptom diagnosis that the physician would have to make. It’s not like one test that says this is long-term COVID,” said John Dooney, an HR adviser with the Society for Human Resource Management (SHRM).

But more employers may find themselves confronting the issue of long COVID, particularly as more workplaces reopen and more employees work from the office again, Dooney said. “We’re now seeing more issues around long-term COVID, which may not have been as prevalent earlier, because if someone’s working at home, they may not feel the same level of fatigue,” he said.

Working remotely isn’t always a solution for long-COVID patients, but it can help reduce the effect of commuting to the office and sitting at a desk all day, Dooney added. Yet 74% of companies that still have employees working remotely are planning on returning to the office this fall, according to a survey conducted in June by staffing firm LaSalle Network.

What existing support is available for long-COVID patients?

Experts like Dooney and Levin-Scherz said HR managers and company leaders should be prepared that some of their workers may request additional time off or accommodations because of long COVID in the coming weeks and months, if they haven’t already. That may range from simply taking paid sick leave to filing for disability status.

During the pandemic, requirements for paid sick leave rapidly expanded. About 16 states and Washington, D.C., require (or will soon require) many employers to offer paid sick leave for workers.

Many companies also enacted their own sick leave policies. Of the 100 largest American employers, 36 announced a new paid sick leave policy or updated existing guidelines during the pandemic, according to Just Capital’s COVID-19 Corporate Response Tracker. Dooney added that he has worked with many employers considering implementing open leave policies, which provide unlimited leave.

For those working without any paid sick leave, or those like Allen who have already wiped out their balance, some employees may be able to fall back on short-term disability, although not all companies offer it. Currently, five states—California, Hawaii, New Jersey, New York, and Rhode Island—require employers to offer short-term disability coverage.

Once paid leave is exhausted, employees may be able to take up to 12 weeks of unpaid, job-protected time off under the Family and Medical Leave Act or ask for reasonable accommodations under the Americans With Disabilities Act. Some workers could even ask for leave as an accommodation under the ADA, but that time off, like the kind provided under the FMLA, is usually unpaid and rarely longer than 12 to 15 months.

“Sometimes the ADA is used more frequently because it doesn’t require an employee to have been working at a company for a period of time, and it’s also for companies that are only 15 or more in staff size, so it’s more prevalent. It’s easier to access,” Dooney said.

It’s worth noting that while there’s no definition of the types of disabilities protected under the ADA, they usually need to be severe and not temporary. “That may be the place that we’re struggling at the moment. We don’t know whether this is temporary, or whether it’s permanent, or how long COVID will exist,” said Domenique Camacho Moran, an employment law attorney with New York–based Farrell Fritz.

Yet President Joe Biden recently said that some Americans experiencing long COVID could qualify for federal disability benefits. “We are bringing agencies together to make sure Americans with long COVID, who have a disability, have access to the rights and resources that are due under disability law,” Biden said late last month.

To that end, the Department of Health and Human Services released nonbinding guidance in July that suggested long COVID can substantially limit a major life activity and could be, in some cases, treated as a protected disability.

How employers should approach adding long COVID support for workers

Most companies are taking situations with long COVID one step at time, with many relying on existing time off and disability frameworks, said Moran.

“For the last 17 months or so, employers have been reeling because every day there is a different law, a different threat, a different set of obligations. And so I think the way in which they are managing these things is to take it one step at a time,” she said.

It’s vitally important, from the employer perspective, that HR managers speak to the employees, find out what they need, Moran said. “Find out why they’re asking for what they’re asking for, and then decide whether that’s an accommodation that’s reasonable, or if there’s an alternative accommodation that can meet the employees needs,” she said.

She also warned companies against jumping to conclusions about workers. “We can’t assume that someone is not experiencing long COVID, but we shouldn’t assume they are,” Moran said. Instead, employers should rely on documentation from medical professionals when evaluating whether to grant employees additional leave or accommodations.

And that may go beyond simply allowing employees to continue to work from home. “We have to be careful in thinking that remote work is the answer,” Moran said, adding that employers may need to be prepared to offer shorter workdays and more breaks throughout the workday.

Employers should also plan to handle requests for accommodations consistently, said Moran. “It’s really key that they’re doing the same process every single time,” she said. That means asking for similar forms of documentation from any employee seeking accommodations; determining in advance who gets the requested accommodation paperwork for these requests; and putting the same individuals in place to review and weigh decisions.

Speaking of documentation, Moran also recommends that employees be prepared to provide more than a positive COVID test to gain accommodations for long COVID, or for absences that are more than 14 to 21 days. “If there is something going on that’s going to limit an employee’s ability to work beyond that 21-day window, we’re going to need some other viewpoints or other documentation explaining what it is,” she said.

While long-COVID cases may prompt some companies to review and update their sick leave policies, Levin-Scherz said employers also need to prioritize reducing the chances that their workers will contract COVID in the first place.