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Some people who've had COVID-19 have yet to feel like their old selves. They live with long COVID, a condition that includes a number of new or returning health problems that emerge well after an initial COVID-19 infection has ended. The symptoms range from mildly bothersome -- like fatigue, headaches and insomnia -- to more debilitating, including organ damage, blood clots, "brain fog" and problems with mental health.

Even people with mild cases can be affected by long-haul COVID.

As more research comes out about the different ways COVID-19 impacts some people's bodies, the criteria for long COVID is adjusted. A new study published this week in the journal Nature suggests that COVID-19 causes changes in the brain structure, including a reduction of gray matter, which has a variety of processing functions in the brain. 

As The New York Times reported, it isn't clear if the people in the study on virus-related brain changes had long COVID, so it's unclear if the findings link to the condition. But whether future research proves the changes are short or long-term, the newest research adds to evidence that the virus that causes COVID-19 impacts some people's bodies in ways that are difficult to pinpoint and don't necessarily require hospitalization. 

Here's everything we know about long COVID, including its symptoms, frequency among COVID patients and potential treatments.

What are the symptoms of long COVID?

Some lingering symptoms of COVID-19, per the Centers for Disease Control and Prevention, include:

• Difficulty breathing or shortness of breath
• Tiredness or fatigue
• Symptoms that get worse after physical or mental activities
• Difficulty thinking or concentrating ("brain fog")
• Cough
• Chest or stomach pain
• Headache
• Fast-beating or pounding heart (also known as heart palpitations)
• Joint or muscle pain
• Pins-and-needles feeling
• Diarrhea
• Sleep problems
• Fever
• Dizziness on standing (lightheadedness)
• Rash
• Mood changes
• Change in sense of smell or taste
• Changes in menstrual period cycle

Dr. Nasia Safdar, medical director of infection control at the University of Wisconsin, told CNET that the key to discerning long COVID is to pay attention to new symptoms that develop or ones that never go away, after about 30 days post-infection. 

"The most common ones that we're seeing are those that are dealing with what's called higher executive functions," Safdar said. "Concentration, memory, being able to do your job the way you could before. Those kinds of symptoms are hard for people to describe, but they've clearly noticed a change from the way they were before." 

As of July last year, long COVID has been classified a disability under the federal Americans with Disability Act. 

At the end of 2021, fatigue and persistent brain fog were still among the most common symptoms of COVID-19: A study in the journal Brain, Behavior, and Immunity found that as many as one in three people were fatigued 12 weeks or more after a COVID-19 diagnosis, and one in five had cognitive impairment.

One set of symptoms of COVID-19 that affected many people sick from earlier variants is the loss of their sense of taste and/or smell. For some, ageusia (loss of taste) and anosmia (loss of smell) don't just affect how they enjoy their food or a favorite scent, but can meddle with their memories and mental health. 

Other viruses have caused loss of smell prior to the coronavirus pandemic, but the sheer number of people affected by it because of COVID-19 is a bit of a phenomenon. According to the Mayo Clinic, the reasons for loss of smell and taste aren't entirely understood, but it's likely due to damage to the cells that support olfactory neurons. 

More serious long COVID symptoms

A January report by the CDC found that kids under 18 who had COVID-19 more than 30 days prior were more likely to be diagnosed with diabetes compared to kids who didn't have COVID-19. The report is consistent with previous research "demonstrating an association between SARS-CoV-2 infection and diabetes in adults," the CDC said. 

A large study in The BMJ, a peer-reviewed journal from the British Medical Association, found that people who test positive for COVID-19 were also more likely to report new mental health issues, including anxiety and depression.

Researchers looked at data from the US Veteran Affairs national health care database, comparing those who tested positive for COVID-19 between March 2020 and January 2021 to those who didn't. While the likelihood of developing long COVID was highest for those who needed to be hospitalized, even people with milder cases were at increased chance of psychological and cognitive problems -- including sleep disorders and cognitive decline. 

The study published this week in the Nature journal found structural changes in the brain, including an overall decrease in volume in COVID-19 patients aged 51 to 81 compared to people who didn't have COVID-19. Grey matter thickness was affected in the orbitofrontal cortex and parahippocampal gyrus, which process emotions and memory, in part. There was also tissue damage to regions "functionally connected" to the primary olfactory cortex, which processes smell.

Researchers can't definitively link these changes to long COVID yet, and the changes to some people's brains don't necessarily mean that's what's causing brain fog or concentration issues in some younger COVID-19 patients, as The New York Times reported. But whether the changes are longer-term or improve with time, COVID-19's impact on the mind calls for further study.

How common is long COVID? 

Exactly how many people develop long COVID -- referred to scientifically as post-acute sequelae of COVID-19, or PASC -- is still a lingering question, with different experts coming to diverse conclusions.

Some researchers have put the figure at 10% to 30% while other studies say close to one-half of COVID survivors have lingering symptoms six months after their initial infection.

Safdar told CNET that the varying numbers are likely caused by differences in the population examined and who was enrolled in the study. 

The large number of COVID-19 infections caused by the omicron variant will mean more research into how many people will go on to develop long COVID: As the New York Times reported, omicron's decreased severity compared to delta doesn't necessarily mean a decreased likelihood of developing long-term symptoms.

More than 415 million cases of COVID-19 have been reported worldwide, according to Johns Hopkins University, though many believe that's an undercount. Of people who've been sick, a significant portion has yet to feel like their old selves. More research is needed to understand why long COVID manifests, and organizations including the National Institutes for Health have launched efforts to try to understand more about the syndrome.

What causes long COVID? 

Because of the huge number of people living with post-COVID-19 symptoms, we can expect research into its origins to continue for years to come. Certain demographics appear to be more susceptible: A September report from the CDC found Black people, women, people age 40 and up and those living with a preexisting medical condition were all more likely to get long COVID. 

Type 2 diabetes, in particular, appears to be a major factor, according to research in the journal Cell. Other research has also pointed to lower levels of some antibodies in people who develop long COVID. 

Another theory on what causes the syndrome involves microscopic blood clots: South African scientist Resia Pretorius found inflammatory molecules trapped in these microclots, which prevented cells from getting enough oxygen to perform bodily functions. 

This, Pretorius wrote in an op-ed for The Guardian, "may be central to the numerous reported debilitating symptoms."

It's also possible, or even likely, that long COVID isn't caused by just one thing. The inflammation COVID-19 causes in the body can have a myriad of effects, as can each individual damaged organ from the illness. There has also been some evidence to suggest the virus can hide in the body, as seen in the immune system's T-cell activity.

Can vaccines help prevent or treat long COVID? 

COVID-19 vaccines reduce the likelihood of long COVID by lowering your chance of getting infected in the first place. But, according to a growing body of research, even in breakthrough infections the chance of symptoms that last for a month or more is lowered by roughly 50% in people who've had the primary two shots of an mRNA vaccine like the ones offered by Pfizer and Moderna.

A February report by the UK Health Security Agency corroborated that people who received both doses are less likely than unvaccinated people to report dizziness, fatigue, persistent muscle pain, hair loss, shortness of breath, loss of sense of smell and other symptoms in the short, medium and long term.

But the meta-analysis, compiled from 15 global studies, also found many people who developed long COVID before vaccination "reported an improvement in symptoms after vaccination, either immediately or over several weeks."

Some individuals did, however, report a worsening in symptoms after vaccination.

Researchers have hypothesized that the reason some people with long COVID report feeling much better after getting the vaccine is due to a "reset" of their immune system. It's also possible the vaccine is helping fight off the lingering virus, though that's not the case for everyone. 

What other treatment options are there?

In a Feb. 7 report in the Journal for Nurse Practitioners, researchers at the University of California, Irvine, provided anecdotal evidence that over-the-counter antihistamines may also help relieve the debilitating symptoms of long COVID for some people.

They related the cases of two middle-aged women diagnosed with the coronavirus in 2020 who developed a laundry list of lingering effects months after their initial infection cleared -- including rashes, bruising, chest pain, headaches, fatigue and cognitive impairment.

Many months after these new symptoms emerged, both women took antihistamines for unrelated allergies and said their long COVID symptoms improved dramatically.

One patient stopped taking antihistamines for 72 hours and found her symptoms reappeared, only to lessen when she took the medication again. Now on a doctor-prescribed daily regimen of antihistamines, she reports regaining 90% of her pre-COVID-19 functionality.

In a control group, only six of 23 patients not given the drug reported an improvement in their condition. 

Dr. Lawrence Afrin, a senior consultant in hematology and oncology at the AIM Center for Personalized Medicine, told Live Science he believes mast cells, a type of immune cells that release histamine in the body, may go into overdrive in some people with COVID-19 and contribute to long COVID.

He added that there is evidence antihistamines can quiet mast cells, but more research is needed.

The information contained in this article is for educational and informational purposes only and is not intended as health or medical advice. Always consult a physician or other qualified health provider regarding any questions you may have about a medical condition or health objectives.