Ongoing, and in some cases debilitating, problems with thinking affect a large majority of Covid-19 patients who were never sick enough to be hospitalized, according to a study published Tuesday in the Annals of Clinical and Translational Neurology.
The research, from Northwestern Medicine in Chicago, is based on an analysis of 100 Covid-19 "long-hauler" patients whose symptoms have lasted at least six weeks. All originally had a mild illness: sore throat, cough, low-grade fever.
But after the acute infection cleared, 85 percent reported at least four lasting neurological problems that have affected their daily lives.
By far, the most commonly reported symptom was "brain fog," with 81 percent of respondents reporting ongoing issues with memory and thinking.
Sixty-eight percent said they had headaches, and more than half said they had problems with loss of taste and smell, numbness or tingling, and muscle pain.
Most were women, with an average age of 43. Less common symptoms included dizziness, blurred vision and tinnitus (ringing in the ears).
Symptoms tended to come and go over a period of months. Most patients improve over time, but not all. Just 64 percent said they felt better after five months, but the vast majority still reported "intense fatigue."
Patients "also had some respiratory problems, despite the fact they never had pneumonia or were hospitalized for low oxygen levels," said Dr. Igor Koralnik, a study author and chief of neuro-infectious diseases and global neurology at Northwestern Medicine. Koralnik also oversees the facility's post-Covid clinic.
While this study was small, it illustrates what millions of people may be facing in the coming months. Of the more than 30 million Americans diagnosed with Covid-19, it's estimated that up to 30 percent could experience long-term symptoms. That's as many as 10 million people.
Download the NBC News app for full coverage of the coronavirus outbreak
And a review of research published Monday found that long-term Covid-19 has the ability to affect many organs in the body, including the lungs, heart, kidneys and skin, as well as the gastrointestinal, nervous and endocrine systems.
Before their Covid-19 long-haul symptoms, 42 percent of the respondents had reported depression or anxiety, and 16 percent reported having had an autoimmune disease.
"We want to understand the most effective interventions for these patients," Koralnik said. "We're trying to devise the best ways to manage and treat these patients, but further research is necessary."