The Centers for Disease Control and Prevention (CDC) has quietly updated its list of conditions that put people at risk of developing severe illness due to COVID-19. The update, which appears to have happened on October 14, now includes certain "mental health conditions."

"Having mood disorders, including depression, and schizophrenia spectrum disorders can make you more likely to get severely ill from COVID-19," the CDC says online.

While the CDC offers up brief explanations on why people with other health conditions on the list are at a higher risk for severe COVID-19, there is nothing more here other than links to information on mood disorders, depression, and coping with COVID-19.

According to the National Alliance on Mental Illness (NAMI), one in five US adults experience some form of mental illness every year, and one in 20 experience a serious form of mental illness-meaning a lot of people are potentially impacted by this.

The CDC's move raises a lot of questions, but there is data to suggest having certain mental health conditions can raise your risk of severe COVID-19. Here's what you need to know.

What does science tell us about mental health and COVID-19 risk?

There's not a lot of data, but there are some trends that have been picked up. "Mental health diagnoses have been associated with higher risks of severe COVID in observational cohorts," David Cennimo, MD, an infectious disease expert at Rutgers New Jersey Medical School, tells Health.

A research letter published in JAMA Network Open in September 2020 analyzed data from 1,685 people who were hospitalized with COVID-19 between February 15 and April 25, 2020. Of those, 473 (or 28%) had received a psychiatric diagnosis before they were hospitalized. Patients who had a psychiatric diagnosis were also more likely to have underlying medical comorbidities like malignant cancer, cerebrovascular disease, congestive heart failure, diabetes, kidney disease, liver disease, heart issues, or HIV. Overall, 318 patients (about 19%) died from COVID-19. But those who had a psychiatric diagnosis were more likely to die of the virus than those who didn't have a previous psychiatric diagnosis—specifically 35.7% vs. 14.7% after two weeks and 44.8% vs. 31.5% after four weeks.

"After controlling for demographic characteristics, other medical comorbidities, and hospital location, the risk of death remained significantly greater among patients with a psychiatric disorder," the researchers wrote.

A study published in The Lancet Psychiatry in November 2020 analyzed data from 62,354 people with COVID-19 and found that patients were more likely to be diagnosed with a mental health disorder after having the virus than the general population. The researchers also included this note: "A psychiatric diagnosis might be an independent risk factor for COVID-19."

Another study published in JAMA Psychiatry in January analyzed data from 7,348 adults with COVID-19 and found that those who were diagnosed with schizophrenia had a nearly three times greater risk of dying from COVID-19 than others. However, the researchers found, people who had mood and anxiety disorders were not more likely to die of the virus.

Why might certain mental health conditions raise your risk of severe COVID-19?

It's not entirely clear at this point. "We're doing real-time data collection with COVID and trying to interpret these findings," clinical psychologist Thea Gallagher, PsyD, an assistant professor at NYU and co-host of the Mind in View podcast, tells Health. "But we can stand confident that there is a link between our mental and physical health."

Gallagher points out that conditions like schizophrenia and depression can cause patients to struggle with symptoms like sleeplessness and eating less—and that could potentially impact the immune system. Gail Saltz, MD, clinical associate professor of psychiatry at New York Presbyterian Hospital and host of the How Can I Help? podcast from iHeartRadio, agrees.

"It is not known what exactly about schizophrenia and mood disorders leads to the increased risk of severe COVID," she says. "Possibilities are that severe psychiatric illness has an impact on the immune system, and the significant toll of chronic stress placed on the body by psychiatric illness—but there may be another unknown cause."

She continues, "COVID infects the entire body, including the brain and while we don't know the mechanism of how COVID is becoming more severe in the severely mentally ill, it is not surprising because there are neurological consequences of COVID and physical ones and those additionally affect each other."

Having a mental health condition upsets "the basic healthy 'balance' of the body's function," clinical psychologist John Mayer, PhD, author of Family Fit: Find Your Balance in Life, tells Health. "In the case of COVID-19, this change also includes the lowering of the body's resistance to illness, thus, an increase in the risk of getting COVID-19," he adds.

People with mental health conditions also tend to have more underlying health conditions and poorer health habits than the general population, Alex Young, MD, professor and interim chair of the Department of Psychiatry and Biobehavioral Sciences, and interim director of the Semel Institute for Neuroscience & Human Behavior at the David Geffen School of Medicine at UCLA, tells Health. "There is a very high rate of smoking in people with serious mental illness—that has direct health consequences, including for COVID," he says. "Other problems like having obesity are very common in people with serious mental illness, and obesity is a high risk factor for severe COVID."

People with mental health conditions also typically don't receive as good care as those without those conditions, Dr. Young says, and they may become sicker than others before actually getting care. "Their physical problems may not rise to the level of importance to them due to other daily problems they may face because of their mental health," he says.

But Gallagher also notes that there is a range with mental health conditions. "These conditions are bundled together under mental health but they have very different presentations and they way they may impact a person's COVID-19 risk can also be different," she says. However, she says, "there needs to be more studies analyzing the link."

Timothy Murphy, MD, senior associate dean for clinical and translational research at the University at Buffalo Jacobs School of Medicine, agrees. "There is a clear statistical association in these large meta analyses—people with mood disorders and particularly schizophrenia have a higher rate of death," he tells Health. "We don't know why. Our knowledge of this is at an elementary level."

What does this mean for booster shots?

People with mental health conditions are now eligible for a booster shot based on the latest addition to the CDC's high-risk category, Dr. Cennimo says. This broadens the number of people who are now eligible for booster shots.

"You now basically have 85% of the US population eligible for a booster," infectious disease expert Amesh A. Adalja, MD, senior scholar at the Johns Hopkins Center for Health Security, tells Health.

Some infectious disease physicians point out that the data linking severe COVID-19 and mental health conditions is preliminary and that adding all people with a mental health condition to the list of those at risk for severe COVID may be premature. "There are some questions about mental illnesses and whether they qualify," Dr. Adalja says. "There may be some indication with schizophrenia and metabolic changes that may take place when someone is on medication, but I wouldn't necessarily group those people in with someone who has generalized anxiety."

Still, if you have a mental health condition and want to get a booster shot, you don't need a doctor's note to do so, says William Schaffner, MD, an infectious disease specialist and professor at the Vanderbilt University School of Medicine. "It's all self-attestation," he tells Health. "You don't need any objective validation, whether you qualify because you're immunosuppressed or some other reason."