The study examined the de-identified electronic health records of nearly 2 million patients, matching those with a documented COVID diagnosis to similar patients without documented infection. The health records examined were from March 2020 through November 2021. 

The study looked at both sets of patients for nearly 30 conditions that have previously been associated with post-COVID illness, also known as long COVID. Those conditions included heart disease, kidney failure, anxiety, asthma, bleeding disorders, gastrointestinal disorders, fatigue, heart failure, pain, heart attack, dementia, sleeping disorders, and diabetes. Those who had a history of such conditions before the study were excluded. Patients were followed until their first qualifying medical incident or through Oct. 31 of last year, whichever came first. 

The study found that roughly one in five COVID survivors ages 18–64, and one in four ages 65 or older, experienced a medical event or diagnosis that could be attributable to COVID in the weeks and months after infection.

Among adults of all ages, the most commonly observed new complaints were respiratory symptoms and musculoskeletal pain. Seniors who survived COVID were at increased risk for neurologic conditions and many mental health conditions when compared to peers who didn’t have COVID. Post-COVID medical issues could hasten seniors’ entry into care centers and nursing homes, the authors stated.

The findings are “consistent with those from several large studies that indicated that post-COVID incident conditions occur in 20%–30% of patients,” the authors wrote. They noted that COVID can have long-term effects on individuals’ health care needs and economic well-being, as some are too sick to return to work. They also noted the potential impact on the health care system, which could find itself under strain after “heavy” COVID surges.

The study didn’t take into account vaccination status, and may not be reflective of post-COVID complication waves from other variants like Omicron, which may differ, the authors wrote.

The authors call for the use of COVID-19 prevention strategies and the monitoring of all COVID survivors, especially seniors, for post-COVID medical conditions.

A new condition, difficult to define

Long COVID is poorly understood, and definitions vary.

The World Health Organization defines long COVID as a condition that occurs in someone who had COVID, with symptoms that cannot be explained by another diagnosis, that last for two months or more. The symptoms can persist following the initial onset, or come and go over time, the organization says, adding that a diagnosis of long COVID usually wouldn’t be made until three months after acute illness.

The Mayo Clinic defines long COVID as a set of symptoms stemming from COVID that persist for more than four weeks after diagnosis.

In reality, long COVID is likely an umbrella term for a combination of issues and conditions: people who have long-term COVID infections who are able to continue to spread the disease; people whose COVID aftereffects clear up after a few weeks; and people with long COVID itself who aren’t infectious but experience all kinds of symptoms for much longer. 

What’s more, COVID patients whose disease was severe enough to require ICU admission may suffer post-ICU complications like muscle weakness, shortness of breath, cognitive issues, anxiety, and depression—symptoms that look a lot like Long COVID, but are not. Those issues might occur due to extended periods of immobility and ventilator use, and other traumatic medical events.

Long COVID may already affect between 7 million and 23 million Americans who previously had the virus, or up to 7% of the U.S. population, according to the U.S. Government Accountability Office. It could potentially impact over a billion worldwide in just a few years, says Arijit Chakravarty, a COVID researcher and CEO of Fractal Therapeutics, a drug development firm. Experts say that it’s quickly growing into a major public health concern already overwhelming primary-care physicians. 

It seems like most any ailment—from ear numbness, a sensation of “brain on fire,” and hallucinations—could be a symptom of long COVID, according to a landmark July study published in British medical journal The Lancet.

The study identified more than 200 potential long COVID symptoms in 10 organ systems, with 66 symptoms typically lasting more than seven months. Researchers surveyed nearly 4,000 sufferers from nearly 60 countries with confirmed or suspected COVID, with illness of a month or longer.