Gabriela Bhaskar for The Wall Street Journal Coronavirus recoveries are undercounted in public-health statistics.

Health researchers collect various pieces of Covid-19 data to get a handle on the spread of the new coronavirus, but one metric has proven tough to pin down: how many infected people have recovered.

Among the 8.8 million coronavirus cases reported in the U.S. so far, some 3.5 million have recovered, according to Johns Hopkins University, which tracks the pandemic.

Yet the tally of recovered Covid-19 patients misses the mark, health experts say, while also failing to capture the many people who are struggling with lingering medical issues from their cases.

The data are so spotty, public-health authorities say they dont know what the true count is. The national figure displayed on trackers likely misses the true count by millions, estimates Ashish Jha, dean of the Brown University School of Public Health.

The spottiness stems from the absence of both an agreed-upon definition for a coronavirus recovery and a standardized way to track the numbers of patients, the health experts say. What constitutes recovery is so nebulous that some states dont even track it, and those that do probably undercount the true number.

Im not aware of anyone using the recovery figures, said Marcus Plescia, chief medical officer of the Association of State and Territorial Health Officials. Ive seen that data, and Ive not paid attention to them.

The lack of clarity means the popular picture of Covid-19s toll misses a datapoint indicating that most infected people have overcome the illness. And it highlights the challenges in assembling data that would help health authorities track the virus and help people fight it.

Like a lot of these issues with data reporting for Covid, theres really not a clear standard for how to do it, said Lauren Gardner, a Johns Hopkins University associate engineering professor who leads the team that built the schools widely cited Covid-19 dashboard.

Both the general public and many health professionals tend to consider patients recovered if they feel the way they did before they became ill. Yet many states define coronavirus recovery differently.

These states count a Covid-19 case as a recovery simply because time has passed since a person developed symptoms or was discharged from the hospital.

Michigan defines recovered cases as the number of people with a confirmed Covid-19 diagnosis who are alive 30 days after getting symptoms. Texas estimates recoveries under a complex formula that subtracts deaths and certain other cases, and assumes it takes 32 days for hospitalized patients to recover and 14 days for nonhospitalized patients.

Such measurements might indicate how many people who tested positive for the coronavirus didnt die, but might miss those who never displayed symptoms and didnt undergo testing.

The metrics also miss the viruss individual impact, according to health experts. While some patients bounce back quickly, others can struggle with side effects for weeks or even months.

There is a variable path after people get sick, said Eric Topol, a cardiologist and professor of molecular medicine at the Scripps Research Institute. There are people who are still ill months after they get sick. We know those symptoms can be very severe, and people can be debilitated.

Given the complexities, some states dont try to count how many have recovered from Covid-19.

The California Department of Public Health said assessing who is recovered is too subjective, and the persistence of side effects in a subset of Covid-19 patients known as long-haulers makes it hard to get a good count.

The Florida Department of Health also doesnt provide a specific recovery metric. It said relying on hospital discharges or estimating length of illness doesnt capture recovery.

Without data from every state and accurate figures from states reporting data, the recovery numbers given by Covid-19 trackers available online are useless for assessing how many people have recovered, said Jennifer Nuzzo, an epidemiologist and senior scholar at the Johns Hopkins Center for Health Security.

Dr. Nuzzo said she doesnt know what the true number of recovered patients is.

Covid-19 trackers get their recovery figures from states that are reporting the numbers. Alexis Madrigal, a founder of the widely cited Covid Tracking Project, said he has sometimes wanted to stop recording recoveries because of too much imprecision in what constitutes a recovery and in tallying them.

Most recovery definitions are not what they purport to be or what people expect, said Mr. Madrigal, who is also a staff writer at The Atlantic magazine.

Donna Bourne, of Shelby, Ohio, got sick and tested positive for Covid-19 in late March. Seven months later, she said, she still struggles with fatigue and severe shortness of breath climbing the stairs in her home. She uses her asthma inhaler every day, as opposed to once a week before getting Covid-19. She now also has an irregular heartbeat, recall problems and sometimes drools because the lower part of her face is partially paralyzed.

Despite her continued health issues, she fits the description of presumed recovered in Ohio, because she is still alive more than 21 days since she first developed symptoms.

Your body just does not recover from Covid the way it would from the flu, said Ms. Bourne, 55 years old, who was recently furloughed from her job assisting workers with disabilities. Its really, really hard on you.

Many state and local health departments ignore recovery tallies in gauging the progress of the pandemic.

Without a standard definition of Covid-19 recovery, there isnt a way to compare data between counties or cities, or to tell how well people living in these places are recovering, said Oscar Alleyne, chief of programs and services for the National Association of County and City Health Officials.

What health authorities could use is a figure capturing how people infected with Covid-19 are faring over time, Dr. Alleyne said. Armed with such data, public-health departments would be better equipped to write guidelines for treatment and allocate the resources that doctors and patients need, he said.

Write to Sarah Toy at sarah.toy@wsj.com

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