"My wife didn't kill herself because she was clinically depressed, she killed herself because she was in excruciating physical pain," said Nick Guthe who's the partner of a victim of "long COVID" from the U.S.

The non-medically termed health complication was first identified by the World Health Organization (WHO) as a post COVID-19 condition that lasts for weeks or months after the infection has gone. It usually comprises a variety of symptoms that include fatigue, breathlessness and cognitive dysfunction – confusion, forgetfulness or a lack of mental focus and clarity, for example. 

"After three weeks of barely sleeping more than an hour for a night, her brain was really...really compromised," Guthe told CGTN. "And like prisoners of war who are kept awake, it affects their cognitive ability, and she became extremely despondent and hopeless."

Beyond fatigues and headaches, brain damage is one of the myriad complications of long COVID with the hearts, stomachs and lunges.

A year-long study in the U.S. found that brain and other neurological disorders occurred in seven percent more of those who had been infected with COVID compared with a similar group of people who had never been infected. 

That translates into roughly 6.6 million Americans who had brain impairments linked with their COVID-19 infections, the study said as published in Nature Medicine Thursday.

It assessed brain health across 44 different disorders using medical records of 154,000 U.S. veterans who had tested positive for COVID from March 1, 2020, to January 15, 2021.

The team compared these with records from 5.6 million patients who did not have COVID during the same time frame, and another group of 5.8 million people from the period just before the coronavirus arrived in the United States. 

"The results show the devastating long-term effects of COVID-19," Dr. Ziyad Al-Aly, a clinical epidemiologist at Washington University in St. Louis and senior author of the study said in a statement.

Some 70 million or more patients who have contracted COVID-19 could be suffering from long COVID syndromes worldwide, according to a study published in the Lancet in August.

It has also been estimated by the Lancet that one in eight (or 12.7 percent) could have been experiencing at least one symptom of post-COVID syndromes after infection worldwide. 

Read more: Real threat of 'long COVID': Eased control, reinfections and more

Those who had COVID were 43 percent more likely to have mental health issues, such as anxiety or depression, according to the study. 

However, research around long COVID is in its early stages, and little is known about its full impact.

"We don't even know at this point of why exactly it occurs," Dr. John Swartzberg from UC Berkley School of Public Health told CGTN.