DAVID GURA, HOST:
Explosive growth - that's how the spread of omicron is being described over and over again. It sped around the world in a matter of weeks. Here in the U.S., new daily case numbers exceeded half a million several times recently, smashing records. Scientists are trying to figure out exactly what makes this new version of the virus so contagious. NPR's Will Stone joins us now to explain more. Hey, Will.
WILL STONE, BYLINE: Hello.
GURA: So I'm old enough to remember that before omicron, there was delta. Do we know why this new variant is overtaking delta as the most widespread?
STONE: Omicron's best trick by far is how well it dodges our pre-existing immunity, so that comes from either the vaccines, getting infected or both. And Dr. Joshua Schiffer says you can think of this built-up immunity kind of like the playing field on which the virus has to compete.
JOSHUA SCHIFFER: On that playing field, the majority of the variants that we've seen couldn't survive. And the delta virus could survive, but really at a tie, where it was not really growing very rapidly or decreasing very rapidly.
STONE: Schiffer's at the Fred Hutchinson Cancer Research Center. And he says, now along comes omicron, and it's thriving because it has these mutations that let it get by the vaccines pretty easily. So all these people that delta couldn't get to, omicron can. But what we're trying to figure out is whether omicron has other things going for it, aside from breaking through the vaccines, that explain why it's spreading so fast.
GURA: Like what?
STONE: Yeah, we don't know yet, but we've got some hints. And one is a study from Hong Kong that looked at samples of human tissue taken from the bronchus. And these are the airways connecting your windpipe to the lungs. And scientists found that omicron multiplies 70 times faster than delta in that tissue. And at the same time, they also found it does not do a great job infecting cells that are deeper in the lungs. I spoke to Angie Rasmussen, who's a virologist, about the implications here.
ANGIE RASMUSSEN: You're shedding more virus in your upper respiratory tract, meaning out your nose. So potentially, you could be shedding more than you would be if most of the replication was happening deep in your lungs.
STONE: So if these findings hold up, it's possible people infected with omicron are essentially filling the air with a lot more virus.
GURA: OK, so some scientists think it multiplies in some tissues faster. Are there any studies that have compared spread in the real world to try to get at this question of whether omicron is inherently more contagious than delta?
STONE: Yeah, and the real world can make this story a bit messier. So scientists in Denmark looked at how well omicron spreads in households, and they compared that to delta. And first, they found omicron is substantially better at infecting vaccinated people, so that's no surprise. But when they only looked at unvaccinated people, the interesting thing is they found no difference between delta and omicron. The infection rates were the same. So that suggests that omicron's big competitive advantage over delta is simply that it's able to infect vaccinated people.
GURA: Will, we know a lot of people who are vaccinated are getting infected, even though many of them are not getting really sick. Do we know if they can also be very contagious?
STONE: We actually do have a small study from the University of Maryland that tried to get at this, and this research was done in a lab where you can measure how much virus is being exhaled by people infected with omicron. Dr. Don Milton led the study, and he says four of the five people had detectable virus in their breath.
DON MILTON: These were vaccinated people, and they're shedding frequently. But what's striking is I was expecting to see the amounts be much higher, and they're not.
STONE: So that's the good news. Milton says his experiment did not find that vaccinated people are spewing out tons of virus like he had suspected. But the bad news is most people who are vaccinated appear to be very capable of spreading the virus. And this could well be one reason the virus is spreading as quickly as it is, even if it isn't making vaccinated people very sick.
GURA: NPR's Will Stone. Will, thanks very much.
STONE: Thank you. Transcript provided by NPR, Copyright NPR.