As medical experts continue to exert effort in better understanding long COVID, they have identified six groups of people who might be the most at risk of developing the post-COVID syndrome.

Despite the extensive case reports and studies conducted on long COVID, the condition remains a medical mystery. Scientists are still unable to tell what really causes it, so medical experts turned their attention to how they can help prevent the cases from growing.

The latest endeavors of experts led them to determine who are at a higher risk of developing long COVID after battling the initial infection. They have yet to fully establish the bases for these, but some well-informed theories can back up who’s most at risk amid the ongoing coronavirus pandemic. The list (found below) was compiled by Fortune after consulting multiple experts and specialists.

1. Repeat COVID patients. People who have had repeat COVID-19 infections, regardless of severity, are said to be at risk of having long COVID. A preprint of a study found that the long COVID risk tends to increase with each reinfection.

2. Patients with high viral load during infection. It appears long COVID is also likely to develop among people who recorded higher viral loads during their battle with an acute COVID infection, per a recently published study. Fortunately, this at-risk group can turn to treatments like the antiviral Paxlovid to reduce their viral loads and lower their risk level.

3. Patients with circulating autoimmune antibodies. Autoantibodies are immune proteins that mistakenly target and react with a person’s own organs. The same study that covered the second at-risk group discovered how patients with autoimmune antibodies have a higher risk of developing long COVID. Scientists found an increasing number of autoantibodies in their blood, but it’s still unclear how or why this is so.

4. Patients with Epstein-Barr virus. According to the Centers for Disease Control and Prevention (CDC), many people in the U.S. are infected by the virus during childhood without knowing it. Epstein-Barr can cause Chronic Fatigue Syndrome that manifests symptoms similar to long COVID. In addition, researchers found that some long COVID patients have high levels of reacted EBV in their blood.

5. Patients with neurological symptoms during acute infection. There is evidence that people who experience neurological symptoms like bad headaches during their acute COVID infection can develop long COVID. The neurons damaged by the novel coronavirus trigger lingering symptoms such as shortness of breath and drowsiness.

6. Unvaccinated people. Though there is conflicting data on whether or not the COVID-19 vaccines can lower the risk of long COVID, many experts agree that the vaccines help diminish serious complications caused by the virus. Moreover, while long COVID risk factors are out of control, things like vaccination and masking are very doable.

Even though not included on the list, another study published last month suggested that women could be significantly more likely to develop long-term COVID-19 symptoms than men. Johnson & Johnson’s researchers analyzed data from more than 1.3 million patients and found that while females had more rapid and robust immune responses against the initial infection, the same processes rendered them more vulnerable to prolonged symptoms.