Part of what makes the Covid-19 pandemic so unnerving is that it’s hard to know how the virus will affect any individual person. Though most people infected with the coronavirus — 81%, according to the US Centers for Disease Control and Prevention (CDC) — will present with few or mild symptoms, others may find themselves relying on a ventilator to breathe, or no longer breathing at all.

But as researchers continue to study the disease, they have started to understand which populations it tends to hit the hardest. Here are the factors that increase a person’s chances of having severe symptoms from Covid-19, at least as researchers understand them now.


Researchers in China were the first to determine that people over 60 are at the highest risk of severe symptoms from Covid-19. Different studies define severe or critical symptoms a bit differently, but they often include trouble breathing (based on frequency of breathing, blood oxygen levels, and duration of the symptoms), fever, and confusion. Basically, if symptoms seem to require immediate medical attention, they’re severe.

In the US, 31-59% of adults ages 75 to 84 diagnosed with the virus had symptoms severe enough to be hospitalized, compared to 14-21% of confirmed patients ages 20 to 44, according to the CDC.

This is why the CDC encourages people in this age group to take extra precautions to prevent contracting the disease—and why grocery stores are increasingly designating hours for seniors-only shopping.

Other medical conditions

Older folks, too, are more likely to have underlying health conditions that could make Covid-19 symptoms more severe. But they’re not the only ones—anyone of any age can have medical conditions that put them at higher risk.

People with conditions like heart disease, diabetes, and cancer may be more likely to have severe symptoms of Covid-19, which can lead to a higher risk of death, than people with no underlying medical conditions. People with underlying medical conditions are also more frequently hospitalized with the virus.

Some diseases could impair the function of organs like the heart; others alter how the immune system works, which could inhibit the body’s response to the coronavirus. Chronic lung disease, as well as smoking and vaping, can increase lung damage and may make Covid-19 symptoms more severe, though few studies have looked at this.


Several teams of international researchers are working to understand how genetics may affect the severity of a Covid-19 infection. They’re looking in a lot of different places, but they have a pretty good idea of where to start.

SARS-CoV-2, the coronavirus that causes Covid-19, infects humans by attaching itself to a particular receptor on the outside of cells called ACE2, which it uses to invade a cell. It’s like a cellular phishing scam, as my colleague Katherine Foley writes. Slight variations in the genetic blueprint for those receptors might play a role in how sick a person gets.

“We see huge differences in clinical outcomes and across countries. How much of that is explained by genetic susceptibility is a very open question,” geneticist Andrea Ganna of the University of Helsinki’s Institute for Molecular Medicine Finland (FIMM), told Science Magazine.

Other teams are looking at whether blood type, which is determined by genes, can change the severity of a patient’s Covid-19 symptoms. A preliminary study conducted in Shenzhen and Wuhan, China found that patients with type A blood had more severe symptoms than those with type O blood. The study authors warn that it’s too early to make clinical decisions based on this finding.


Researchers in China found that 58% of patients hospitalized with Covid-19 were male; there was a similar imbalance in patients in the US and Italy, too.

Some speculate that this could be because men’s social habits (and the fact that many don’t wash their hands as often) make it more likely for them to contract the disease, which would mean the increased hospitalizations were simply a result of a greater number of overall infections. But it could also be because men are generally thought to have weaker immune systems and tend to have more underlying health issues than women; women’s two X chromosomes may have a protective quality against a number of conditions.

Previous immunizations

Recently, researchers observed something somewhat surprising: Countries in which people were required to receive the bacillus Calmette-Guerin (BCG) vaccine, one of the oldest to combat tuberculosis, had fewer cases and deaths from Covid-19. Countries hit hardest by the disease, such as Italy, the Netherlands, and the US, do not have universal vaccination programs; others, like Iran, have programs that are too new to have reached the elderly population.

It’s not the first time researchers have found that the BCG vaccine works to combat diseases other than TB. It’s used to treat early-stage bladder cancer and prevents leprosy. And, perhaps most relevant to Covid-19, one study found that the vaccine also reduces the severity of lower lung infections caused by viruses.

Now, researchers in countries such as Australia and the Netherlands are beginning trials to see if the BCG vaccine can prevent and reduce the severity of Covid-19.

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