For many people, COVID-19 only has mild symptoms, but for some, it can be deadly.

The majority of people—about 80% according to research—only suffer mild symptoms with COVID-19. Others may require hospitalization—but for some patients, the new coronavirus can be deadly.

One life-threatening complication that can arise is acute respiratory distress syndrome, commonly referred to as ARDS. And new research from the JAMA Internal Medicine suggests that more than 40% of individuals in the study hospitalized for severe and critical COVID-19 developed ARDS—and over 50% of those diagnosed died from the disease.

What exactly is ARDS?

The American Lung Association (ALA) says ARDS is a rapidly progressive disease that can occur in critically ill patients—most notably now, in those diagnosed with severe illness resulting from coronavirus.

ARDS is sometimes initially diagnosed as pneumonia or pulmonary edema (fluid in the lungs from heart disease), and, per the ALA, those with ARDS often have severe shortness of breath, cough, and fever. Fast heart rate and rapid breathing also shows up in ARDS patients, along with chest pain, particularly during inhaling. "ARDS is manifested as rapid and progressive shortness of breath," Shervin Takyar, MD, PhD, a Yale Medicine pulmonologist, tells Health.

The disease can be caused by direct injuries to the lungs (pneumonia, aspiration, near drowning, or smoke inhalation), or indirect injuries to the lungs (sepsis, blood transfusions, or medication reactions). While many still won't develop ARDS even with those injuries, some risk factors—like a smoking history, oxygen use, obesity, and recent chemotherapy—can increase a person's risk of developing the disease.

How is ARDS treated and what are the complications?

In most cases of ARDS, patients will have to be supported by supplemental oxygen and mechanical ventilation soon after the start of their symptoms, says Dr. Takyar. When their blood oxygen levels remain low, patients may be flipped on their stomachs—into a prone position—to get more oxygen into their blood, says the ALA.

Proning is literally turning people over on their bellies, instead of having them on their back on a ventilator, Jeanne Marrazzo, MD, professor of medicine in the infectious diseases division at the University of Alabama at Birmingham, explained during a recent media briefing sponsored by the Infectious Diseases Society of America. "And when you do that in a cycle, you get better oxygenation, and the outcomes are incredible," per Dr. Marrazzo.

In some cases, if inflammation and fluid in the lungs persists, some ARDS patients will develop scarring of the lungs (the fibrotic stage of ARDS). In others, the lung or lungs can eventually pop and deflate, leading to a pneumothorax or a collapsed lung.

According to the American Thoracic Society, the death rate for ARDS shows that it has a mortality rate of 30-40% of those diagnosed with the disease. In those who recover, lung function gradually improves over six months to a year, and even then they're left with significant scarring and lower than normal lung volumes. Those who survive ARDS can also go on to suffer from anxiety, depression, and PTSD from the impaired quality of life and lung function, Gregory P. Cosgrove, MD, Chief Medical Officer of the Pulmonary Fibrosis Foundation, tells Health.

How does COVID-19 lead to ARDS?

Similar to many other viruses, coronaviruses—including the novel coronavirus, or SARS-CoV-2—can infect and damage the lung cells, setting the stage for the occurrence of ARDS, explains Dr. Takyar. “It is thought that the infection-induced damage and inflammation cause a malfunction of the lung vasculature [the pulmonary vessels within the lungs]," he says. When that happens, the exchange of oxygen within the body is impaired. Among viruses, the ones that cause pneumonia (like COVID-19) are more likely to cause ARDS.

When patients are in the late stages of novel coronavirus infection—after severe damage has already been done to their lungs—their body will try to fight off the virus by sending immune cells to the lungs, which is what ultimately causes the inflammatory reaction, explains Dr. Takyar. In fact, ARDS in coronavirus “seems to be less directly due to the virus itself and more related to inflammation, or the body’s response to the virus,” adds Jaimie Meyer, MD, a Yale Medicine infectious disease specialist.

While experts are still in the early stages of learning about COVID-19, research surrounding ARDS and coronavirus has found that the two often go hand-in-hand. According to the earlier JAMA study, researchers also concluded that an older age translated to a higher risk factor of developing ARDS and dying, "likely owing to less rigorous immune response," per the study authors. Additionally, the authors noted that if an ARDS patient was running a higher fever, they were more likely to recover, due to the body fighting off the infection.

An alternate study published in January in The Lancet also found that ARDS can develop incredibly quickly in critically ill coronavirus patients. Those researchers found that ARDS developed in 17% of those diagnosed COVID-19, among whom, 11 worsened in a short period of time and died of either ARDS or other serious complications like multiple organ failure.

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.