New research from the Centers for Disease Control and Prevention has found that long COVID is more common than most people realize. The study, which was published earlier this week, found that one in five adults under the age of 65 has the condition.

For the study, researchers analyzed electronic medical records of nearly 2 million people and compared people who were diagnosed with COVID-19 in the first 18 months of the pandemic to those who had never had the virus. Researchers looked for 26 different symptoms that could be linked with long COVID and found that the most common long COVID-19 symptoms were respiratory issues and musculoskeletal pain, i.e. pain in the bones, joints, ligaments, tendons, or muscles.

The researchers discovered that, between 30 and 365 days after people were diagnosed with COVID-19, 38% developed one or more new health issues (compared to 16% of people who didn’t have COVID-19 but saw a doctor). In people aged 65 and up, 45% who had COVID-19 developed new health issues, compared to 19% of those who didn’t have the virus.

The researchers discovered that the risk of long COVID symptoms in people who had COVID-19 who were 65 and up was between 20% and 120% higher than in people who never had the virus. People between the ages of 18 and 64 had an up to 110% higher risk of developing most of the symptoms of long COVID.

“These findings are not at all surprising,” says Thomas Russo, M.D., professor and chief of infectious disease at the University at Buffalo in New York. “This is going to be the next phase of the pandemic. It’s quite clear that long COVID is real. A significant portion of individuals are affected and for a significant period of time.”

Though it is important to note that COVID-19 vaccination status was not considered in the CDC’s analysis, another large study published Wednesday says that your risk of long COVID-19 is only reduced by about 15% if you’re vaccinated. Though, the study did show that being vaccinated does appear to lower your risk of lung and blood clot disorders.

Vaccination status aside, there’s a lot scientists still don’t know about long COVID in particular. But, there are some answers. Here’s what experts know right now.

What is long COVID, again?
Long COVID, aka post-COVID-19 conditions, is an umbrella term used to describe a wide range of new, returning, or ongoing health problems that people experience after first being infected with SARS-CoV-2, the virus that causes COVID-19, according to the CDC.

There is no test for long COVID and symptoms can overlap with those of other health issues, making the condition tricky to diagnose, per the CDC.

What are some of the potential long-term effects of COVID-19?
There is a laundry list of potential symptoms of long COVID. Per the CDC, they most commonly include:

Tiredness or fatigue that interferes with daily life
Symptoms that get worse after physical or mental effort
Fever
Difficulty breathing or shortness of breath
Cough
Chest pain
Fast-beating or pounding heart
Difficulty thinking or concentrating
Sleep problems
Dizziness when you stand up (lightheadedness)
Pins-and-needles feelings
Change in smell or taste
Depression or anxiety
Diarrhea
Stomach pain
Joint or muscle pain
Rash
Changes in menstrual cycles

But these can be symptoms of a range of other health conditions, which makes things tricky, says infectious disease expert Amesh A. Adalja, M.D., a senior scholar at the Johns Hopkins Center for Health Security. “It’s important to really delineate which symptoms are actually disabling and interfere with activities of daily life versus people who have a cough that persists post-infection,” he says. “Many of the long COVID studies do not use control groups, so you really cannot determine the true prevalence of some of the symptoms.”

Why are older people at more risk for long COVID?
The study didn’t explore this, but Dr. Russo speculates that it’s likely because older people are at greater risk for severe forms of COVID-19. “If you’re asymptomatic or have mild disease that doesn’t result in hospitalization, by no means does it preclude you from the possibility that you can develop long COVID,” he says. “But most studies suggest that the more severe disease you have, the greater the risk you’ll develop long COVID.”

People over 65 were at an increased risk of developing neurological and mental health conditions, the CDC study found. “Post-COVID conditions affecting the nervous system are of particular concern because these conditions can lead to early entry into supportive services or investment of additional resources into care,” the researchers wrote.

How long do lingering symptoms last after COVID-19 and before it’s considered long COVID?
There have been different definitions for this, Dr. Russo says, noting that some define someone as having long COVID when it’s been at least 30 days since their COVID-19 diagnosis and they’re having symptoms, while others go up to 90 days.

“We need a uniform definition for study purposes so we can define a population and move forward,” Dr. Russo said.

As for how long symptoms of long COVID can last, Dr. Russo says it really depends. “Some people have had symptoms for over a year at this point,” he says.

If you’re having unusual symptoms and you suspect you have long COVID, Dr. Russo recommends doing your research to find a specialized center near you that treats these conditions. “There are an increasing number of post-COVID recovery centers,” he says. And, if you can find a current study of people with your long COVID symptoms, he suggests trying to get included. “It will not only enable you to be part of a solution, you can also be at the front end of some treatment modalities,” he says.