According to the study, conducted by researchers from Oxford University, a diagnosis of COVID-19 was associated with increased incidence of being diagnosed with several psychiatric conditions and disorders including anxiety, insomnia and dementia.
The researchers said the mental health consequences of having COVID-19 have not yet accurately been measured — but have been widely predicted.
"We were interested primarily in the question if people who were diagnosed with COVID-19 were going to be at greater risk of developing mental health disorders," University of Oxford professor Paul Harrison, who worked on the study, told CBS News.
The researchers sought to answer this question by looking at the rates of psychiatric diagnoses in patients in the three months after they were diagnosed with COVID-19, Harrison said.
The group used the TriNetX Analytics Network, which collects data from electronic medical records from 54 health care organizations in the U.S. This includes data on 69.8 million patients — 62,354 of whom were diagnosed with COVID-19 between January 20 and August 1. They then looked how many of these patients had been diagnosed with a psychiatric disorder.
For comparison, the researchers also measured the rate of psychiatric disorder diagnoses in patients who experienced six other health events like the flu, respiratory tract infections, or a large bone fracture.
"That's important to try and tease out what might be due to COVID itself or what might be due to the environment we're living in," Harrison explained.
The researchers looked for diagnoses of dementia, insomnia, anxiety and mood disorders in the COVID-19 patients as well as in those diagnosed with the six other health conditions, who served as a control group.
"In the 90 days after diagnoses, about 18% or almost 1 in 5 of people who had COVID received a psychiatric diagnosis of one kind or another — the highest being an anxiety disorder," Harrison said.
He said about 6% of COVID-19 patients who had never been diagnosed with a psychiatric disorder before were diagnosed with one within three months of getting COVID-19. The most common, again, was anxiety.
Harrison said there are two probable causes of psychiatric disorders associated with COVID-19. "One possibility is that there could be some direct effect of the virus producing the symptoms. Perhaps what the virus might be doing in the brain in some people, or the way some people's immune systems have responded to fight the virus," he said.
"There's some hint that that might be a biological explanation for some people."
The other possible cause may be the psychological reactions people have to being diagnosed with the virus, Harrison said.
"The psychological reactions people have to having COVID, having worried about their life perhaps for the first time, having worried about the future — in a way, COVID is simply acting as another major stressor event, rather than some direct viral effect," he said.
Compared to the six other illnesses and health events, COVID-19 led to more first-time diagnoses of psychiatric disorder, according to the study.
"It's possible to speculate that in the moment, you're more stressed when someone tells you you've got COVID than you would be if someone tells you that you have most other things wrong with you," Harrison said, adding that it is too difficult to know whether this is a definite reason.
Harrison said there are still more questions to be answered after the Oxford study. He also said that he suspects people diagnosed with a psychiatric disorder after a COVID-19 diagnosis can recover from it.
"If you look at almost every other cause of anxiety and depression I can think of, a significant proportion of people do get better, either with time and/or with treatment," he said. "In my own clinical practice, in the COVID survivors I've seen, I've certainly seen some people who are clearly getting better and some who clearly aren't."
"I suspect there will be a range of outcomes and we'll need longer-term studies where we can actually measure that with time," he continued.
Harrison said the second part of the Oxford study was to ask: Are people who already have a psychiatric diagnosis more at risk of getting COVID-19 than people without a psychiatric diagnosis?
"To my surprise, the answer is yes," Harrison said. "People who had a psychiatric diagnosis in the year before COVID came along ... they were about 65% more like to receive a diagnosis of COVID than people without psychiatric diagnosis."
Harrison said he and his colleagues were puzzled by that finding. "Is that just because people with psychiatric problems also tend to have worse physical health problems?" he questioned.
To determine this, the group of researchers controlled for the physical health factors — and the effect didn't go away. "So, there does seem to be some connection between having a psychiatric diagnosis and your risk of COVID-19," Harrison said.
"A bit like the first part of the study, there's really two explanations for that," he continued. "One is that there is a connection between the factors that make you vulnerable to psychiatric illness and the factors that make you vulnerable to COVID." For example, he explained, inflammation might play a role in both those conditions.
But other factors that are hard to measure, like lifestyle or behavior, may play a role in both psychiatric illness and other illnesses like COVID-19. "That might explain why they are more vulnerable to catching COVID," he said. "To take a simple possibility, it might be that they live in slightly more crowded circumstances and therefore there's more chance of catching the virus."
"The important thing is that it may or may not be having a psychiatric illness that puts you at risk of COVID, it might be one of these related factors," Harrison said.
Another study published in The Lancet Psychiatry journal in June looked at 153 patients who were hospitalized with severe cases of COVID-19 to see if they experienced any neurological and psychiatric complications associated with the virus. The study found that 125 of the patients did.
Over 60% of the patients had strokes, a majority of which occurred in patients age 60 or older.
About a third of patients experienced "an altered mental state such as brain inflammation, psychosis and dementia-like symptoms," according to the June study. This includes signs of confusion or changes in behavior.
Almost a quarter of patients with an altered mental state were diagnosed with psychiatric conditions, the "vast majority" of which were found to be newly developed. However, researchers say it is possible some may have been simply undiagnosed before the patient developed the virus.