1 in 3 young adults are vulnerable to severe coronavirus infections, a new study suggests. Smoking could be to blame.

• One in three young adults have at least one risk factor that could result in severe COVID-19 infections, a new study found.
• The researchers determined that smoking was the most prevalent risk factor for people in their late teens and 20s.
• Other factors like underlying diseases or genetic differences could also put young people at risk of severe infection.

It's become almost common knowledge that young people are less vulnerable to severe coronavirus infections.

Adults from 18 to 49 made up around 25% of hospitalized coronavirus patients in March, whereas those 65 and older represented around 43%, according to the Centers for Disease Control and Prevention. Adults 18 to 44 years old made up just 2% of coronavirus deaths from February to May, while people 65 and above represented nearly 80%.

But certain factors that can put anyone at risk of serious illness, regardless of age. A new study from researchers at the University of California, San Francisco determined that one in three young adults ages 18 to 25 are vulnerable to severe COVID-19 cases.

Patients were considered vulnerable if they had least one risk factor, including a smoking habit or chronic illness like heart disease, diabetes, asthma, obesity, autoimmune disease, or liver problems.

The researchers discovered that smoking was by far the most prevalent risk factor for people in their late teens and 20s. Of the roughly 8,400 young adults in the study, around 25% said they had smoked tobacco, e-cigarettes, or cigars in the last 30 days.

By contrast, only about 16% reported having a chronic illness. Asthma was by far the most common: Around 9% of young adults reported that they were asthmatic. That's compared to around 12% who said they'd smoked tobacco in the last 30 days and around 7% who said they'd used e-cigarettes.

"The risk of being medically vulnerable is halved when smokers, including e-cigarette users, are removed from the sample," the researchers wrote. Only about one in six young adults who didn't smoke were vulnerable to severe COVID-19 illness, the study found.

The findings came just days after the World Health Organization warned about the link between smoking and severe coronavirus cases.

"Smoking kills 8 million people a year, but if users need more motivation to kick the habit, the pandemic provides the right incentive," WHO director-general Tedros Adhanom Ghebreyesus said at a press briefing on Friday. "Evidence reveals that smokers are more vulnerable than non-smokers to developing a severe case of COVID-19."

Smoking habits differ among men and women

The UCSF study found that the risk of severe coronavirus infections from smoking or e-cigarette use was highest among young, white males with lower incomes who were uninsured for at least part of the year.

Research has shown that white people are more likely to be daily smokers compared to other racial groups, though people of color face other coronavirus risk factors that weren't included in the study. Black and Hispanic people, for instance, are more likely to hold service-industry jobs that increase their risk of coronavirus exposure. The results may also be skewed by the fact that the study examined far more white adults (55%) than Hispanic (22%) or Black (13%) adults.

Around 16% of the young adults who reported smoking in the study were men. Only 9% were young women.

But women in the study had higher rates of asthma and autoimmune conditions like lupus or rheumatoid arthritis. On the whole, that mostly offset the fact that fewer women smoke: 30% of young women in the study were vulnerable to severe COVID-19 infections compared to 33% of young men.

Genetic factors could also increase the risk severe infection

Since the coronavirus attacks the respiratory system first, patients who already suffer from smoking-related lung damage or inflammation could develop more severe respiratory problems as a result of COVID-19.

Research also suggests that smokers have higher expressions of ACE-2 receptors — the cell receptors that the coronavirus uses to invade the body — in their airways. People with more ACE2 receptors seem to have a higher risk of severe COVID-19 infection as well.

But even young patients without a smoking habit or underlying health conditions could still be at risk of a serious case of COVID-19. People ages 18 to 29 make up more than four times as many coronavirus hospitalizations as they did a few months ago: around 38 hospitalizations out of every 100,000 people as of July 4, compared to nine hospitalizations out of every 100,000 people on April 18.

Some young, healthy patients have also reported feeling sick for several months, with lasting symptoms like chest pain and shortness of breath. That could be the result of genetic differences that result in a higher expression of ACE2 receptors or that trigger a more aggressive immune response.

But unlike many risk factors, smoking is one that can be prevented.

"Efforts to reduce smoking and e-cigarette use among young adults would likely reduce their medical vulnerability to severe illness," the UCSF researchers wrote. Their findings, they added, underscore "the importance of smoking prevention and mitigation."