While the World Health Organization (WHO) estimates that the new coronavirus is mild for about 80% of patients, the virus so far has killed more than 2,000 people—leaving health experts trying to explain why the virus kills certain patients when tens of thousands of others survive.
About the outbreak
Reports of the new coronavirus first surfaced in early December 2019 in Wuhan, China. According to WHO, the main symptoms of infection from the new virus, called COVID-19, are fever and lesions in both lungs. Some patients also have reported difficulty breathing, WHO said.
As of Friday, officials reported more than 76,900 cases of the virus globally, with the vast majority of those cases occurring in mainland China, according to CNN. Officials have reported more than 1,000 cases of the new coronavirus outside of mainland China. In the United States, CDC as of Wednesday reported 15 confirmed cases of COVID-19.
Officials said as of Friday there have been about 2,247 reported deaths linked to the virus, CNN reports. According to CNN, all but 11 of those deaths occurred in mainland China.
Why do 2% of COVID-19 cases lead to death?
While the death toll linked to the new virus has continued to climb, WHO estimates that about 80% of COVID-19 cases have been relatively mild for patients.
Patients with a mild case of the virus might experience symptoms similar to a common cold—such as a fever, dry cough, and shortness of breath—according to Carlos del Rio, a professor of medicine and global health at Emory University. These patients usually don't require major medical intervention, according to WBUR.
But about 20% of coronavirus infections are more severe—and, about 2% of cases are fatal.
According to the Washington Post, experts aren't entirely sure why COVID-19 can be mild in some patients and lead to death in others—but they have some hypotheses. For instance, some experts think the difference between a lethal coronavirus infection and a mild infection is dependent on how a patient's immune system responds to the virus, which can be impacted by a person's age, genetics, and underlying medical conditions.
The Post reports that the new coronavirus attacks and multiplies in cells in a patient's lungs and, in some cases, the impact of the virus can cause the patient's immune system to overreact.
Matthew Frieman, a virologist at the University of Maryland School of Medicine, explained, "What you get is the initial damage and rush of inflammatory cells, but the damage is so extensive that the body's immune response is completely overwhelmed—which causes even more immune response, more immune cells and more damage."
Anthony Fehr, a virologist at the University of Kansas, noted, "If the virus replicates very quickly, before your body has a chance to try and prevent it with an immune response, or if the immune response comes in too late, then it can't control the virus and starts going berserk."
According to the Post, scientists call such an immune response a "cytokine storm." Essentially, the immune system sends cells to fight off the virus in the lungs but, in doing so, the immune cells also can cause damage to a person's body.
Vineet Menachery, a virologist at the University of Texas Medical Branch, said once a cytokine storm begins, a patient's condition can begin to decline. She explained that if the new coronavirus progresses in a manner similar to SARS, it can damage a patient's alveoli—which are air sacs that take in oxygen in the lungs—and cause the lung tissue to stiffen. As a result, the heart has to work harder to get a limited amount of oxygen to the rest of the patient's organs.
"[Y]ou're losing lung function," Menachery said, "and that puts a strain on every organ in your body."
Del Rio explained, "The lack of oxygen leads to more inflammation, more problems in the body. Organs need oxygen to function, right? So when you don't have oxygen there, then your liver dies and your kidney dies."
Every case is different
But experts cautioned that every patient is different—and there are numerous factors that could play into whether a person develops a mild or severe case of the virus.
For instance, they've noted that the majority of serious COVID-19 cases occurred in patients with weaker immune systems, such as middle-aged or elderly individuals, and those with underlying medical conditions, such as chronic lung disease, diabetes, and heart disease. Yoko Furuya, an infectious disease specialist at Columbia University Irving Medical Center, said long-term smokers also are at a higher risk for serious infection because their airways and lungs are more vulnerable when compared with non-smokers.
Further, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said there have been extraordinary instances where otherwise healthy patients have died from the virus. "Of course, you have outliers—people who are young and otherwise previously healthy who are dying. But if you look at the vast majority of the people who have serious disease and who will ultimately die, they are in that group that are either elderly and/or have underlying conditions."
In patients who recover from the virus, their immune systems worked effectively to clear the virus from their bodies. However, experts are still unsure whether having the virus will impact the patients' health in the future.
"Every individual is different," Fehr said. "There are lots of dynamics at play when you talk about each individual and how they might die from this virus or why they might survive" (Johnson, Washington Post, 2/19; Wescott et al., CNN, 2/21; Wescott et al., CNN, 2/20; Godoy, WBUR, 2/14; CDC website, 2/19).