Older adults appear to be more severely at risk from the new coronavirus, while young children seem to be largely spared — and understanding why could be crucial to treating people with the illness it causes, according to scientists.
Much remains unknown about COVID-19, the disease caused by the coronavirus that is rapidly spreading around the world, but researchers have seized on a factor that seems to influence the severity of infections: the patient's age.
People over age 60, and particularly those with pre-existing health conditions, appear to be most vulnerable to the virus, which has spread to more than 110,000 people in at least 97 countries.
While that is not particularly surprising, the statistics show that young children have made up very few of the confirmed cases so far, a divergence that isn't true for every illness.
While the immune systems of older people are typically not as robust as those of younger people, leaving them more vulnerable to a wide variety of illnesses, scientists say they can't definitively say why the coronavirus has been harder on people of advanced ages.
"We're trying to figure out why age is a primary feature of this infection, but from a biological perspective, we don't have that answer," said Dr. Srinivas Murthy, a clinical associate professor in the department of pediatrics at the University of British Columbia in Vancouver.
Understanding that question could help researchers figure out how to treat the illness, particularly in the older populations that appear to be more susceptible to it.
Surgeon General Jerome Adams, speaking Monday afternoon at a news conference, confirmed that the virus had been more severe for older people based on the data currently available.
The first death in the U.S. from COVID-19 was that of a Washington state man in his 50s with underlying health conditions. Since then, the state's health officials have also been battling the spread of the respiratory illness at a nursing facility in Kirkland, where 19 people have died.
In China, where the coronavirus first emerged, early research also suggests that the coronavirus may pose a graver risk to some populations over others. In a report released last month by the Chinese Center for Disease Control and Prevention, an analysis of 1,023 deaths out of 44,672 confirmed cases diagnosed through Feb. 11 found that 21.9 percent of deaths occurred among patients who were over 80 years old.
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Most people who have been infected have experienced mild to moderate symptoms, which Murthy said likely means either that the virus is not penetrating beyond the upper respiratory tract or that patients' immune systems are preventing it from reaching deep into the lungs.
It's thought that the virus spreads through close contact, traveling through tiny droplets and secretions when a patient coughs, sneezes or breathes.
Typically, when a virus infects a cell in the human body, the cell's so-called innate immune system kicks in if foreign genetic material is detected. This is considered the body's first line of defense against invading pathogens. The second line of defense is known as the adaptive immune system, which first has to detect foreign invaders before producing antibodies and T cells to counteract the infection.
But as people age, both of those systems can break down.
"We don't truly know why, but as you get older, the functionality of the innate immune system and adaptive immune system wanes," said Timothy Sheahan, an epidemiologist at the Gillings School of Global Public Health at the University of North Carolina.
This is likely why the coronavirus is having a more severe impact on older adults, which Sheahan said doesn't altogether surprise scientists. Similar patterns emerge every year with the seasonal flu, and researchers have seen the same trend with past coronavirus outbreaks.
"If you go back and look at SARS, the overall mortality rate was about 10 percent," Sheahan said. "There were around 8,000 cases, and about 800 of those people died. But if you break that down by age, the case fatality rate for people over the age of 65 exceeded 50 percent in some countries."
Underlying medical issues also increase the risks of disease because pre-existing conditions influence how a person's immune system functions. The U.S. Centers for Disease Control and Prevention warned that people with serious chronic medical conditions, such as heart disease, diabetes and lung disease, are at higher risk of getting very sick from COVID-19.
Very few children have been diagnosed with COVID-19 so far — and most of the cases involving children have been mild. In China, children made up just 2.4 percent of confirmed cases, and no coronavirus deaths have been reported in young children to date.
Murthy said scientists are still unsure why the virus appears to be sparing the very young, but he said it's an active area of research.
"Part of it may be that the immune systems of children are just better able to cope with it," he said. "Or maybe there's some virus-receptor interactions that mean children are protected from severe disease. There are a lot of theories floating around, but these are just ideas for now."
Cracking the mystery could help doctors figure out how best to treat older adults and other vulnerable populations, Murthy said.
In otherwise healthy adults, human genetic variation may also play a role in the severity of the disease, according to Sheahan.
"If every healthy adult on Earth got infected with this virus, because of human genetics, there would be a spectrum of disease severity because everybody is different," he said.
Sheahan added that some of the unknowns may become clear as scientists better understand the virus and its structure. For now, the outbreak and its long-term consequences are still a "moving target."
"Not enough is really known right now," Sheahan said. "I don't think there's enough data out there to really get a good handle on all this."
Editor's note: This story was updated to clarify the case fatality rate during the SARS outbreak in 2003.