RALEIGH, N.C. — One of the most confusing aspects of COVID-19 is its tendency to spread when people don’t have symptoms, but a new study from researchers at the University of North Carolina at Chapel Hill may explain why that happens and help doctors prevent the worst cases of the disease.
The study found that the coronavirus infects the cells in the nose much more easily than those in the throat and lungs.
Scientists are still working to understand the ways people can catch COVID-19. An important clue to Dr. Richard Boucher, one of the senior researchers at the UNC School of Medicine, is that many people with COVID-19 lose their sense of smell. Previous research suggests that over half of patients with COVID-19 partially or completely lost their sense of smell while sick.
This suggested that the coronavirus could be especially active in the nose even when people don’t show classic nasal symptoms like sneezing, runny nose, or congestion.
As a pulmonologist, Boucher sees many people with chronic lung problems get sick when germs from the mouth or nose accidentally fall into the lungs with fluids like saliva or mucus. This could mean that instead launching a direct assault on the lungs, the coronavirus camps out in the nose, building up and waiting for some lucky drips to carry it down the sinuses and throat.
The UNC team compared how easily the coronavirus infects the cells lining different parts of the respiratory tract, from the nose to the lungs. The cells in the nose turned out to be the easiest to infect, followed by cells in the throat and then those in the lungs. This mostly matched up with the relative concentrations of a protein in our bodies that the coronavirus uses to get inside cells.
The researchers also examined the lungs of people who had died from COVID-19. If they got the coronavirus directly from breathing it in the air, it would be evenly spread out in the lungs. “You would expect the lung to look like it had one big veil of COVID-19 coming down over it,” Boucher said.
Instead, spots of severe infection were next to healthy tissue. This supports the theory that the lungs are infected when mucus and other liquids are aspirated into the lungs.
“If you get a teaspoon down the wrong way, it’s going to go preferentially down a few airways,” Boucher said.
Most of the infected patches were in the lower lungs, another sign of a falling liquid. These findings matched with X-ray observations of COVID-19 patients showing more diseased patches lower in the lungs.
The UNC study helps explain why it’s so easy to spread COVID-19 even without showing symptoms, Boucher said.
“Your nose is used to filtering out a lot of things and seeing a lot of things. It’s the first line of defense” and catches things like viruses in hair and mucus. You might feel fine if the coronavirus is only in your nose, but breathing or sneezing can send coronavirus particles into the air.
Some people are more likely to aspirate fluid into their lungs, which can explain why certain groups are at higher risk for severe illness from COVID-19.
“As you get older, as you get chubbier, or if you have diabetes,” Boucher said, the reflexes that prevent aspiration tend to be less effective. As a result, higher amounts of coronavirus get into the lungs, where the most harmful symptoms develop.
If the nose is where the coronavirus prepares to attack the rest of the body, we can fortify our nasal defenses to stop it from gaining ground in the first place.
Clinical trials are looking into whether rinses can flush the coronavirus out from the nose. With less virus, aspiration is less likely to infect the lungs.
It may also be possible to trick the nose into thinking it’s already under attack before a person is exposed to the coronavirus. This primes a broad immune response that can help fight off viruses generally before the body is able to produce targeted defenses against COVID-19.
Wearing a mask, even a non-medical one, blocks most large virus-carrying droplets that come out of the nose, like from a sneeze. And if the coronavirus is most likely to infect someone by getting into the nose, covering it makes the nose a harder target.
Wearing masks “really is a two-fer,” Boucher said. “You’re protecting yourself and you’re protecting somebody else from you transmitting something in an asymptomatic phase.”