What have we learned about COVID-19?

When the virus that causes COVID-19 sparked a global pandemic in 2020, questions about this pathogen far exceeded answers. How does it spread? Can we get it from contaminated objects or surfaces? Why do some people get really sick when others don’t?

While some mysteries remain, there is also a lot we have learned about COVID-19 since it upended our lives four years ago — from how it transmits to how long immunity is likely to last. In this two-part series, find out what we know now about COVID-19.

Why COVID-19 affects people differently

Some people with a COVID infection experience cold-like symptoms that last a few days, while others may be ill for weeks or even require hospitalization. There are many factors that influence how sick a person gets.

People 50 and older or those with an underlying medical condition, like diabetes or heart disease, are at higher risk of experiencing severe illness. This is because their immune systems may have more difficulty fighting the virus.

How sick you get also depends on your viral load — the amount of virus particles in your body. If the viral load is minimal, your immune system may be able to clear the virus quickly and you may experience few or no symptoms. If your viral load is high, you’re more likely to experience severe symptoms for longer.

Perhaps the biggest factor shaping our experience with COVID-19 that we have control over is vaccination status.

“Several studies have now shown how effective the vaccines are at reducing the severity of infections and preventing both hospitalizations and deaths,” says Mary Fairchok, MD, a pediatric infectious disease specialist at MultiCare Mary Bridge Children’s Hospital.

A study published in The Lancet that collected COVID-19 data in Europe through March 2023 found that the vaccines saved 1.6 million lives and reduced deaths by 59 percent.

How some people seem to avoid infection

Most of us by now have had a COVID-19 infection at least once, but there are people, sometimes referred to as “superdodgers,” who have never tested positive or developed symptoms. This is likely, in part, due to individual habits and behaviors.

“If you managed to avoid COVID early on in the pandemic and got vaccinated, then there’s a high chance that at some point after vaccination, you may have actually gotten infected, but it was so mild you didn’t notice or test positive,” Dr. Fairchok says.

Biology and genetics may play a role, too. A study published in Nature found that some people who were exposed to the virus experienced abortive infections, meaning they never developed symptoms or tested positive. There were two notable aspects about this group of study participants:

• They had a much faster immune response in their nasal tissue than others did, which prevented the virus from reaching the respiratory tract and other parts of the body.
• They had a specific gene that was expressed at a much higher level in their nasal cells, which likely helped these cells fight the infection.

Why kids tend to have mild COVID-19 symptoms

While some kids experience severe illness from COVID-19, most only develop mild symptoms or have a symptom-free infection. This has led some to wonder whether kids get infected at all.

It turns out — they do. In fact, more than 90 percent of kids 4 and under in the United States have been infected. So what seems to shield many children from severe disease?

The secret may lie in their noses. Similar to findings in the Nature study around nasal cells, a study published in Cell found that kids produced much higher amounts of two proteins in the nose than adults do — one protein stops viruses from replicating and the other protein recruits immune cells to fight off infections.

“This indicates that kids may be better able to nip this particular virus in the bud in their noses, preventing it from traveling down the respiratory tract to other areas of the body,” Dr. Fairchok explains.

How the virus spreads

In the early days of the pandemic, many of us were wiping down groceries and vigilantly sanitizing countertops. Since then, we have learned that you aren’t likely to pick up an infection simply by touching a contaminated surface.

The main way COVID-19 spreads is through aerosols — tiny droplets released when breathing and talking that can hang in the air for a long period of time and travel across distances. Larger droplets emitted by a sneeze or cough can also cause an infection. While standing six feet away from someone (a recommendation early on in the pandemic) may help protect you from inhaling larger droplets, it doesn’t protect you from aerosols.

“This is why ventilation in crowded spaces and the kind of mask worn became so important,” Dr. Fairchok says. “N95 and KN95 masks are the best at keeping these tiny airborne particles from getting into your lungs.”

How long immunity lasts

Immunity varies from person to person, and it also depends on how quickly the virus mutates (changes).

“Immunity, whether from vaccination or a previous infection, tends to wane after about three months,” Dr. Fairchok says. “However, protection from severe illness lasts longer, about four to six months.”

A study in The New England Journal of Medicine indicates that hybrid immunity — from both vaccination and prior infection — provides the best protection and in some cases can last for more than a year.

How long you’re contagious

If you have COVID-19, or another contagious respiratory virus, the CDC recommends staying home and away from others until your symptoms are improving and you’re fever-free — without medications — for 24 hours.* The CDC also recommends taking additional precautions, such as masking, for the next five days.

Dr. Fairchok adds that this guidance doesn’t mean you stop being contagious at that 24-hour mark.

“People can still be contagious and spread the virus for approximately 8-10 days after first experiencing symptoms. This is why the CDC recommends taking added precautions, such as wearing a mask, once you resume normal activities.”

In the next installment of this series, find out what we’ve learned about long COVID, including the results of a MultiCare Institute for Research & Innovation study.

*COVID-19 infection guidance differs for those who work in health care settings. Visit the CDC website to learn more.