For the most part, strokes affect older adults: The American Stroke Association says a person's risk of stroke nearly doubles every 10 years after the age of 55. But due to the new coronavirus, doctors have seen an increase in strokes among people as young as 30—and doctors are scrambling to find out why.

According to new reporting by the Washington Post, three large US medical institutions—Philadelphia's Thomas Jefferson University Hospitals, and New York City's NYU Langone Health and Mount Sinai Beth Israel Hospital—are preparing to publish data on the stroke phenomenon among younger patients with COVID-19. While the institutions maintain that there are only a few dozen cases per location, the situation is concerning enough among doctors to raise red flags.

One set of data, to be published in the New England Journal of Medicine (NEJM) on April 29, details five cases of large-vessel strokes in patients under the age of 50, all diagnosed with COVID-19, though patients only had mild symptoms—or none at all. Five cases may not sound like a lot, but it's a sevenfold increase of what would normally be expected among patients in that age range. "It's been surprising to learn that the virus appears to cause disease through a process of blood clotting," Thomas Oxley, MD, PhD, a neurosurgeon at Mount Sinai Health System and lead investigator of the research told Medscape Medical News.

The increase in blood clotting among COVID-19 patients itself has also been a concern for doctors and researchers. "The number of clotting problems I'm seeing in the ICU, all related to COVID-19, is unprecedented," Jeffrey Laurence, MD, a hematologist at Weill Cornell Medicine in New York City, wrote in an email to CNN. "Blood clotting problems appear to be widespread in severe Covid."

Dr. Laurence was one of the physicians involved in an April 2020 commentary published in the journal Translational Research, which detailed the cases of five patients—two autopsied patients and three living patients—who had blood clots in the lungs, beneath the surface of the skin, or both. Another recent study published in the journal Thrombosis Research also found "remarkably high" rates of clotting in ICU COVID-19 patients. Some doctors—like Kathryn Hibbert, MD, director of the medical intensive care unit at Massachusetts General Hospital—have even seen blood clot right before their eyes: "It's rare to have that happen once, and extremely rare to have that happen twice," she recently told CNN.

Why are doctors seeing an uptick in blood clots among COVID-19 patients?

That's not an easy question to answer, especially since any information surrounding COVID-19 is so new, but medical experts have some theories.

First, though, a quick explainer on blood clots: According to the US National Library of Medicine's MedlinePlus database, blood is supposed to clot—it's what helps stop the bleeding after you get a cut. But sometimes, blood can clot abnormally and not dissolve properly—in that case, the clot can travel through blood vessels in the body up to the brain, heart, or lungs, and cause serious damage by blocking blood flow.

Those with underlying conditions like diabetes, heart issues, or even those who are hospitalized and rendered immobile, are at a higher risk for blood clots—something that may also be skewing the rates of blood clotting in COVID-19 patients, since many also commonly deal with those preexisting conditions, Behnood Bikdeli, MD, a cardiovascular medicine fellow at Columbia University Irving Medical Center told CNN.

Inflammation may also play a big role in blood clotting among COVID-19 patients. "When there is very active and severe inflammation in the body, the surface of blood vessels can become disturbed and the clotting system can be activated," Kathryn Hassell, MD, a hematologist at UCHealth University of Colorado Hospital, tells Health. She explains that other viral and bacterial infections and severe acute illnesses can also cause this strong inflammatory response.

Still, Dr. Hassell points out that “it's not clear if the SARS-CoV-2 uniquely triggers clotting, or if it is just very good at causing severe inflammation which in turn can trigger vessel changes and an imbalance in the clotting system.” There is a chance that COVID-19 may be no different than other types of viruses, infections, or other severe medical illnesses that can cause clotting, "but it's just that we're seeing tens of thousands of cases all at once so it really comes to our attention and we are revealing the vulnerable people in the population.”

Regardless, blood clotting with COVID-19 is closely associated with mortality. Barry Boyd, MD, RDN, a Yale Medicine hematologist and oncologist, points to some research which found that individuals with a very high level of COVID-related coagulopathy “have up to a 70% mortality in contrast to those with minimal clotting dysfunction where the mortality is much lower,” he explains. “This strongly suggests that while it is not the only or sole cause of mortality, it is very closely associated with multi organ failure and these clotting events themselves will also compromise survival.”

How is an increase in blood clotting related to the rising number of strokes in COVID-19 patients?

There are two types of strokes, according to MedlinePlus—ischemic and hemorrhagic—but ischemic strokes are the most common, and are usually caused by a blood clot that blocks or plugs a blood vessel in the brain. Because of that, many doctors and researchers suspect strokes—most commonly "large vessel occlusions" or LVOs seen in in COVID-19 patients—may be a result of blood clotting issues. But just as doctors aren't sure what's causing clotting issues among COVID-19 patients, they also aren't entirely sure why stroke are being seen among those patients—especially younger patients with coronavirus infections.

While it's still far too early to determine why younger people appear to be having more strokes related to COVID-19, Joseph Schindler, MD, a Yale Medicine neurologist and director of the Acute Stroke and TeleStroke services says that strokes may be limited to patients with more severe infections. "We know that COVID-19 has had effects on heart function which could put patients at risk for stroke from a blood clot developing in the heart and then traveling to the brain,” he says, adding that patients with other comorbidities who have severe infections and are on ventilators could be at higher risk from less blood flow getting to the brain caused by diseased blood vessels.

Dr. Hassell says that "severe infection" may also look different in different patients, explaining that even in those who aren't showing signs of severe pneumonia or lung failure that happens with severe COVID-19 diagnoses, the body may still be responding to the virus with high levels of inflammation. "This probably doesn't hurt most people, but some people, even young people, may have a mild susceptibility to this process that is revealed when a severe level of inflammation sets in," she says. This disparity may also explain why some younger COVID-19 patients who suffered strokes didn't present with known symptoms, as the NEJM study showed.

Overall, much more research needs to be done on why COVID-19 patients—especially those who are younger—may be falling victim to blood clots and strokes, but as of right now, the increased association was enough for researchers and doctors to sound alarms. In referring to strokes among COVID-19 patients, J Mocco, MD, director of Mount Sinai's Cerebrovascular Center, told CNN he was "shocked" at the number and age of COVID-19 stroke patients—many who were 15 years younger than a typical stroke patient. "This is much too powerful of a signal to be chance or happenstance,” he said.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDC, WHO, and their local public health department as resources.