Who gets long COVID and why? Here's what new research reveals

COVID-19 hospital admissions are starting to kick up in California just as the height of the holiday season is about to kick in. More people getting infected also means a potential rise in long COVID cases. New research pinpoints who is more likely to get these lingering symptoms.

For one in every five people who get COVID, the symptoms persist for months - if not years.

New research out of Northwestern Medicine finds that millions of people who tested negative for the virus may actually have long COVID.

"Long COVID is a new disease that affects approximately 30% of people who survive COVID-19," said Dr. Igor Koralnik, a neurologist with the Comprehensive COVID Center at Northwestern Medicine.

Brain fog, memory problems, fatigue, anxiety, depression, insomnia, breathing problems, muscle aches and heart issues are some of the long COVID symptoms that can be life-changing.

Koralnik is part of a team who studied more than 1,800 long COVID patients.

"More than 90% of patients that we see in the clinic are people who have never been hospitalized with COVID-19 pneumonia," he said.

MORE: Battling long COVID? About 18 million US adults have had it, CDC says

New data shows nearly 7% of adults, roughly 18 million people, reported ever having long-term symptoms and nearly 9 million say they are still dealing with it.

The study found 83% of patients had abnormal CT chest scans, 51% had cognitive impairment, 45% had altered lung function and 12% had an elevated heart rate. Long COVID has become the third leading neurologic disorder in the U.S. -- 30 million have been affected.

"Among previously hospitalized patients, the average age is 54. But among people who had never been hospitalized, who had a mild case of COVID-19 initially, the average age is 44," Koralnik said.

And surprisingly, long COVID hits women in their 40s, who were never hospitalized earlier due to COVID.

"We think that long COVID is a new autoimmune disease which is caused by the virus," he said.

Women are four times more likely than men to develop autoimmune diseases.

Researchers at Northwestern are looking at biomarkers in the blood to see if they hold answers as to why one person's symptoms linger on, while others recover quickly.

MORE: How has COVID-19 changed our immunity?

The US Centers for Disease Control and Prevention estimates that 97% of people in the U.S. have some immunity to COVID-19 through vaccination, infection or both.