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Delta Variant Grows Rapidly Inside A Person's Respiratory Tract, Study Says


We know now that the highly contagious delta variant now accounts for more than 80% of COVID cases in this country, and it appears to spread about twice as fast as the original strain. NPR's Michaeleen Doucleff looks into the first published study that reveals why.

MICHAELEEN DOUCLEFF, BYLINE: The study took place in Guangzhou, China. The researchers monitored about 60 people who had gotten sick with the delta variant and 60 with the original version of the coronavirus. Right after a person was exposed, the researchers carefully measured how much virus was in each person's nose and, therefore, when they became contagious.

PEI-YONG SHI: They sampled those individuals on daily basis to see, after the exposure, when the virus became detectable in the nasal cavity.

DOUCLEFF: That's Pei-Yong Shi. He's a virologist at the University of Texas Medical Branch at Galveston. He wasn't involved in the study but says the data showed two key results. First, the delta variant grows more rapidly inside people. They likely become contagious sooner than with the original strain. The time between exposure and possible spread to other people?

SHI: It's shortened by two days, and that really indicates, you know, this delta variant is more vigorous.

DOUCLEFF: Second, the delta variant, on average, grows to higher levels inside a person's respiratory tract.

SHI: Virus level is over 1,000 times more than compared with the original virus.

DOUCLEFF: A thousand times more virus particles. So the average person infected today with the delta variant is likely sneezing out much more virus than a person with the original strain last summer. Monica Gandhi is an infectious disease doctor at the University of California, San Francisco. She says the way to think about the delta variant is that it's quicker and there's more of it.

MONICA GANDHI: When you see a study like this one where there's a thousand times higher viral load in the nose, that makes you think, yeah, it is really reproducing more quickly. There's a lot more viral particles, and that is the reason for it being so contagious.

DOUCLEFF: And another reason, she says, for people to get immunized.

GANDHI: So this makes me more nervous for the unvaccinated when I see this type of finding. It's just that it seems to zoom in on unvaccinated.

DOUCLEFF: So far, the vaccines appear to offer excellent protection against the delta variant, especially against severe symptoms and hospitalization. The CDC says 97% of people hospitalized with COVID right now were not vaccinated against the coronavirus.

Michaeleen Doucleff, NPR News. Transcript provided by NPR, Copyright NPR.

Michaeleen Doucleff, PhD, is a correspondent for NPR's Science Desk. For nearly a decade, she has been reporting for the radio and the web for NPR's global health outlet, Goats and Soda. Doucleff focuses on disease outbreaks, cross-cultural parenting, and women and children's health.
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