Factors like age, gender, BMI, and pre-existing conditions may put individuals at higher risk for long COVID, according to a new study.

Published on March 23 in the Journal of the American Medical Association Internal Medicine, the U.K.-based study found that certain groups of people are at a significantly higher risk of developing the post-viral condition, thought to affect millions around the world.

Who is most at risk for long COVID?

• Women

• Over 40

• People with obesity

• Smokers

• Those who were immunosuppressed before COVID

• People who were hospitalized with COVID

• People who had the following conditions before COVID:

o anxiety or depression

o Diabetes

o asthma or COPD

Researchers examined the results of 41 published studies, with a combined total of more than 860,000 patients. They found that the aforementioned conditions were strongly associated with a higher risk of long COVID symptoms persisting three or more months after infection.

The results bolster the case that female gender and older age serve as risk factors for developing long COVID. A potential common thread among several risk categories: pre-existing inflammation, which may extend the acute phase of COVID “even after recovery.” In the case of females, hormones might play a role in inflammatory status, while obesity shares a proinflammatory profile with long COVID, the authors write.

That’s not-so-great news for a giant swath of the population. There is good news, however: At least two doses of COVID vaccination seemed to lower the risk of developing long COVID, researchers found. Other studies have come to similar conclusions, they note. They include a recent report from the U.K. Office of National Statistics, which found that those with two doses of COVID vaccine had a 42% lower risk of developing the potentially disabling condition.

With more than 200 symptoms identified—from lingering cough and fatigue to ear numbness and a sensation of “brain on fire”—long COVID is undoubtedly not one but multiple conditions, experts say.

True long COVID, some contend, is best defined as a chronic-fatigue-syndrome-like condition that develops after a COVID infection, similar to other post-viral syndromes that can occur after an infection with herpes, Lyme disease, and Ebola, among others.

Other post-COVID complications like organ damage should not be defined as long COVID and better fit into the larger umbrella category of PASC, some experts say. Also known as post-acute sequelae of COVID-19, the term is used to encompass a wide variety of COVID consequences, from chronic-fatigue-like symptoms and subsequent heart disease to lasting lung damage and odd new symptoms like urinary incontinence, itching, and skin lesions.

As of Jan. 16, 15% of U.S. adults reported having long COVID symptoms at some point in the pandemic, and 6% reported lingering symptoms, according to a Jan. 26 report by the Kaiser Family Foundation, citing data from the U.S. Centers for Disease Control and Prevention.

The percent of Americans who’ve experienced COVID and still report long COVID symptoms dropped from 19% in June to 11% in January, according to the report.