The University of Oxford study analyzed health records of 236,379 Covid-19 patients infected last year, according to a report in The Lancet Psychiatry journal. As might be expected, anxiety and mood disorders were the most common diagnoses, at 17% and 14% of patients respectively. But the study also found 7% of those made sickest by the virus had a stroke and 2% were diagnosed with dementia.

While the investigators stressed that the specific causes of these long-term affects are largely unknown, they suggested some might be linked to stress, job loss or loneliness during quarantine. More in-depth research is needed on the neurological risks and more resources are needed to address the full range of implications, they said.

“Although the individual risks for most disorders are small, the effect across the whole population may be substantial for health and social-care systems due to the scale of the pandemic and that many of these conditions are chronic,” said Paul Harrison, a professor of psychiatry at the university and the study’s lead author.

The researchers also explored data from 105,579 people with influenza and 236,038 patients with any respiratory tract infection. There was a 44% greater risk of neurological and mental health diagnoses after Covid-19 than after flu, and a 16% greater risk than with respiratory tract infections.

“Although we know that the virus can access the brain, it’s not necessarily only the neurons in the brain that might be effected,” said Masud Husain, professor of neurology and cognitive neuroscience at the University of Oxford. “We need to be careful about what we attribute the effect of the virus to the brain itself.”

Brain Fog

At the start of the pandemic, a number of Covid patients whose symptoms were initially mild developed long-term neurological problems, what survivors refer to as “brain fog.” At the time there was no strong evidence that Covid-19 infects the brain. However, since then there have been several other studies that have made the link between Covid-19 and increased risk of neurological disorders.

A study published last October found that about 4 of 5 patients hospitalized with Covid-19 suffer neurological symptoms, including muscle pain, headaches, confusion, dizziness and the loss of smell or taste. Another study from Wuhan, China, where the virus first appeared, found that 36% of patients had neurological symptoms ranging from headache to impaired consciousness.

A limitation of the Oxford study is that the researchers only analyzed records of patients diagnosed with Covid, which does not take into consideration people who were infected but had no symptoms.

Some of patients in the control cohort “might have also had Covid,” said Max Taquet, a study co-author. “We didn’t know that.” It means the findings could have underestimated the relative risk of developing neurological or mental health disorders.

Still, the overall message is clear, according to Taquet. “We need to be proactive and establish follow-up strategy for patients who have had Covid,” he said, adding that health-care providers should be prepared to address an increased demand for assessment and treatment of these disorders.