COVID-19 has been linked with persistent cognitive deficits, including symptoms similar to those found in Alzheimer's disease, new studies have found.

Several scientists from Europe, the United States and Latin America recently presented new research at this year's Alzheimer's Association International Conference, highlighting worrying associations between novel coronavirus infection and neurological damage.

One study from the New York University Grossman School of Medicine discovered biological markers in the blood of COVID-19 patients similar to those found in sufferers of brain injury, neuro-inflammation and Alzheimer's disease.

The study looked at 310 people admitted to the New York University Langone Health hospital with COVID-19. Of the patients, 158 were positive for neurological symptoms and 152 had no such symptoms. The researchers said that, among those with symptoms, there was an increased presence in the blood or spinal fluid of biological markers, including defective tau proteins and neurofilament light chain, or Nfl.

A component of brain cells, Nfl can enter spinal fluid when nerves are damaged. Increased levels of defective tau proteins and Nfl are indicators of some degenerative brain disorders, including Alzheimer's.

"These findings suggest that patients who had COVID-19 may have an acceleration of Alzheimer's-related symptoms and pathology," said Thomas Wisniewski, a professor of neurology at New York University, who led the study. "However, more longitudinal research is needed to study how these biomarkers impact cognition in individuals who had COVID-19 in the long term."

A separate study, submitted at the conference by researchers from the University of Gothenburg in Sweden, also found biomarkers of brain damage among COVID-19 patients admitted to the ICU.

The study found that in some patients the biomarkers went back to normal levels in the months following infection, though some patients experienced long-term cognitive issues. Nelly Kanberg, a neuroscientist who co-authored the study, said that the cause of the brain damage remains uncertain, though inflammation and damage to blood vessels might be involved.

A further study followed 300 COVID-19 patients above the age of 60 in Argentina for up to six months after infection. The study, which was led by the University of Texas as well as international collaborators from the Alzheimer's Association, found that almost 60 percent of patients experienced persistent problems with forgetfulness, and roughly one-in-four had additional problems with cognition including language and executive dysfunction.

Heather Snyder, vice president of medical and scientific relations Alzheimer's Association, said that the combined new research points to "disturbing trends" that show that COVID-19 infections lead to lasting cognitive impairment "and even Alzheimer's symptoms".

"It is imperative that we continue to study what this virus is doing to our bodies and brains," said Snyder.