Hundreds of thousands of Americans have sought medical care for post-COVID health problems that they had not been diagnosed with before becoming infected with the coronavirus, according to the largest study to date of long-term symptoms in COVID-19 patients.

The study, tracking the health insurance records of nearly 2 million people in the United States who contracted the coronavirus last year, found that one month or more after their infection, almost one-quarter — 23% — of them sought medical treatment for new conditions.

Those affected were all ages, including children. Their most common new health problems were pain, including in nerves and muscles; breathing difficulties; high cholesterol; malaise and fatigue; and high blood pressure. Other issues included intestinal symptoms; migraines; skin problems; heart abnormalities; sleep disorders; and mental health conditions like anxiety and depression.

Post-COVID health problems were common even among people who had not gotten sick from the virus at all, the study found. While nearly half of patients who were hospitalized for COVID-19 experienced subsequent medical issues, so did 27% of people who had mild or moderate symptoms and 19% of people who said they were asymptomatic.

“One thing that was surprising to us was the large percentage of asymptomatic patients that are in that category of long COVID,” said Robin Gelburd, president of FAIR Health, a nonprofit organization that conducted the study based on what it says is the nation’s largest database of private health insurance claims.

More than half of the 1,959,982 patients whose records were evaluated reported no symptoms from their COVID infection. Forty percent had symptoms but didn’t require hospitalization, including 1% whose only symptom was loss of taste or smell; only 5% were hospitalized.

Gelburd said the fact that asymptomatic people can have post-COVID symptoms is important to emphasize, so that patients and doctors can know to consider the possibility that some health issues may actually be aftereffects of the coronavirus. “There are some people who may not have even known they had COVID,” she said, “but if they continue to present with some of these conditions that are unusual for their health history, it may be worth some further investigation by the medical professional that they’re working with.”

The report, which will be posted publicly on Tuesday morning on the organization’s website, analyzed records of people diagnosed with COVID-19 between February and December 2020, tracking them until February 2021. It found that 454,477 people consulted health providers for symptoms 30 days or more after their infection. FAIR Health said the analysis was evaluated by an independent academic reviewer but was not formally peer-reviewed.

“The strength of this study is really its size and its ability to look across the range of disease severity in a diversity of age groups,” said Dr. Helen Chu, an associate professor of medicine and infectious diseases at the University of Washington School of Medicine, who was not involved in the report. “This is a hard study to do with that much data.”

The report “drives home the point that long COVID can affect nearly every organ system,” said Dr. Ziyad Al-Aly, chief of the research and development service at the VA St. Louis Health Care System, who was not involved in the new study.

“Some of these manifestations are chronic conditions that will last a lifetime and will forever scar some individuals and families,” added Al-Aly, who was an author of a large study published in April of lingering symptoms in COVID patients in the Department of Veterans Affairs health system.

In the new study, the most common issue for which patients sought medical care was pain — including nerve inflammation and aches and pains associated with nerves and muscles — which was reported by more than 5% of patients or nearly 100,000 people, more than a fifth of those who reported post-COVID problems. Breathing difficulties, including shortness of breath, were experienced by 3.5% of post-COVID patients.

Nearly 3% of patients sought treatment for symptoms that were labeled with diagnostic codes for malaise and fatigue, a far-reaching category that could include issues like brain fog and exhaustion that gets worse after physical or mental activity — effects that have been reported by many people with long COVID.

Other new issues for patients, especially adults in their 40s and 50s, included high cholesterol, diagnosed in 3% of all post-COVID patients, and high blood pressure, diagnosed in 2.4%, the report said. Al-Aly said such health conditions, which have not been commonly considered aftereffects of the virus, make it “increasingly clear that post-COVID or long COVID has a metabolic signature marked by derangements in the metabolic machinery.”

Relatively few deaths — 594 — occurred 30 days or more post-COVID, and most were among people who had been hospitalized for their coronavirus infection, the report found.

The study, like many involving electronic records, only addressed some aspects of the post-COVID landscape. It did not say when patients’ symptoms arose or how long the problems persisted, and it did not evaluate exactly when after infection patients sought help from doctors, only that it was 30 days or more.

The database included only people with private health insurance or Medicare Advantage, not those uninsured or covered by Medicare Parts A, B and D, Medicaid or other government health programs. Chu said people without insurance or with incomes low enough to qualify for Medicaid are often “more likely to have worse outcomes,” so the findings may understate the prevalence of some post-COVID health problems or may not represent the full picture.

In addition, diagnostic codes in electronic records are “only as good as what is documented by the provider who saw the patient,” said Chu, a co-author of a smaller study of post-COVID symptoms among patients at the University of Washington.

For example, neurological or cognitive issues like brain fog may be underreported because doctors may not find an appropriate diagnostic code or patients may not be seeking medical help for that specific issue, FAIR Health said.

It’s also possible that some people classified as having asymptomatic COVID-19 infections developed symptoms after they tested positive. And some people who received their first diagnosis of a medical issue like hypertension or high cholesterol post-COVID might have previously had those issues but never sought or received treatment.

Another limitation of the study is that it did not compare people who had COVID-19 with those who did not, making it unclear if rates of post-COVID symptoms were higher than in a more general population. Al-Aly’s study, which made such a comparison, found that between one and six months after becoming infected with the coronavirus, patients who’d had COVID had a 60% greater risk of death and a 20% greater chance of needing outpatient medical care than people who had not been infected.

The FAIR Health report excluded patients with certain serious or chronic preexisting conditions like cancer, kidney disease, HIV, liver disease and stroke because researchers said it would be difficult to separate their previous health status from post-COVID symptoms.

The report did not explore connections between other preexisting conditions patients had and their likelihood of developing post-COVID symptoms. But it said that people with intellectual disabilities or those with Alzheimer’s disease or dementia had a greater risk of dying 30 days or more after their infection.

Overall, experts said, the report’s findings underscore the widespread and varied nature of post-COVID symptoms.

“People with long COVID need multidisciplinary care,” said Al-Aly, “and our health systems should adapt to this reality and develop capacity to deal with these patients.”

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