Why Do Only Some People Get Sick With COVID?

While researchers continue to learn more about COVID, an important question remains unanswered: Why do some people get very sick while others don’t have COVID symptoms at all?

Certain factors like age and underlying conditions can play a large role. But this does not account for all of the times when some people have severe disease and others don’t.

Some theories to account for this difference include immune status, medical history, and the amount of exposure someone has to the virus.

Can you be immune to COVID?

It’s possible to have genetic features that make you resistant to COVID infection. Researchers have noted this before with viral infections, like HIV. But there’s still more research to be done in this area. Given the high rates of COVID infection, it’s likely that there aren’t many people who are immune. 

It’s very hard to estimate how many people have never had COVID and may be immune to it. That’s because some people have no symptoms with a COVID infection. So many people who think they’re immune to COVID may have had an infection and didn’t know it. 

It’s also difficult to know if you may be truly immune to COVID if you’ve been vaccinated. 

Why do only some people with COVID get sick?

It has to do with several factors, like age, race, and underlying health conditions.

Roughly 30% of COVID infections are asymptomatic. In fact, most people have mild symptoms or no symptoms at all. But about 10% need a ventilator and treatment in an intensive care unit (ICU).

Older adults and those with underlying conditions are more likely to get severe COVID. And these groups make up most of the hospitalizations and deaths from the disease so far. But there are also young and otherwise healthy people with severe symptoms. And some older adults never have symptoms. 

All that said, here are a few theories on why some people don’t have symptoms or only get mild illness.

Demographic factors 

Age

As with other infections, your risk for severe COVID increases with age. About 3 out of every 4 deaths from COVID have been in adults over the age of 65. 

Most COVID cases in young people still tend to be mild or asymptomatic. But researchers have found that high rates of smoking or use of electronic cigarettes in this age group can put them at greater risk for severe illness. 

Race

Studies have also shown that COVID affects certain racial groups more than others. The groups most affected include:

• Black people

• American Indian/Alaska Native people

• Hispanic people

These groups may be infected with the virus more often and have a higher likelihood of dying from the disease. On average, they’re dying from COVID at rates twice that of white people. 

Although younger people tend to not have symptoms or have mild cases, this is not necessarily true across all racial groups. These high-risk groups have made up nearly 2 out of 3 COVID deaths in people under age 65.

This may be due to a lack of access to healthcare, higher rates of chronic conditions, and other social barriers to wellness among these groups.

Health factors

Underlying conditions

Regardless of age or race, certain underlying conditions can increase your risk of severe illness from COVID. Conditions that weaken the immune system or affect lung function can increase your risk. Having one of these conditions can put you at higher risk for severe illness:

• Cancer

• Chronic kidney disease

• Lung disease (asthma, COPD, and cystic fibrosis)

• Weakened immune system from organ transplant

• High blood pressure

• Heavier body weight

• Heart conditions (heart failure and coronary artery disease)

• Sickle cell disease

• Type 2 diabetes

Smoking

One clue as to why people who smoke seem to be at increased risk of severe COVID is a receptor on lung cells. The COVID virus uses ACE2 receptors to enter and infect human cells. Research shows that people who smoke have more ACE2 receptors than people who don’t smoke. 

These cells help protect the lungs from damage when you inhale irritants like cigarette smoke. But it may put you at increased risk of infection. 

Check out our resources on the best ways to quit smoking. Or talk with a provider about medications and strategies for quitting.

Exposure factors

How much you’re exposed to the virus

The use of masks has caused a lot of debate. But masking can actually help reduce how much virus you are exposed to. And this may result in more mild or symptom-free cases.

Along with masks, other measures can also help prevent or limit your exposure to the virus, including: 

• Practicing social distancing

• Washing your hands thoroughly and frequently

• Avoiding large crowds, if possible  

Does blood type influence COVID immunity?  

The data is mixed, so it’s hard to say. Some research shows that people with Type A blood may have a higher risk of respiratory failure from COVID. And some studies found that people with Type O blood were less likely to test positive for COVID.

COVID has also been linked to blood clots in some people. Certain blood types may have a higher risk for developing blood clots.

But this does not mean that you won’t get COVID or end up in the hospital just because you have Type O blood. More studies are needed to find if there is a meaningful connection between blood type and COVID.

Do prior vaccinations protect you from COVID? 

There is some evidence that certain non-COVID vaccines may protect you from serious COVID complications. “Live” vaccines, like MMR (measles, mumps and rubella), contain a weakened form of these viruses. This type of vaccine may play a role in training your immune cells to fight off other infections. 

This may be why children and younger people do not get severe COVID infections. They have often received live vaccines more recently than older people.

How much you’re exposed to other coronaviruses

Research shows that exposure to other coronaviruses may keep you from getting severe COVID. Coronaviruses that cause the common cold may offer some protection. This may give a clue as to why some people have severe COVID infection and others don’t. 

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Written by:

Alyssa Billingsley, PharmD

Alyssa Billingsley, PharmD, is the director of pharmacy content for GoodRx. She has over a decade of experience as a pharmacist and has worked in clinical, academic, and administrative roles.

Edited by:

Karla Robinson, MD

Karla Robinson, MD, is a medical editor for GoodRx. She is a licensed, board-certified family physician with almost 20 years of experience in health through varied clinical, administrative, and educational roles.

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