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Why The Delta Variant Is So Contagious: A New Study Sheds Light

By: Michaeleen Doucleff | NPR
Posted on: Thursday, July 8, 2021

WASHINGTON, D.C. (NPR) — After months of data collection, scientists agree: The delta variant is the most contagious version of SARS-CoV-2 worldwide. It spreads about 225% faster than the original version of the virus. And it is currently dominating the outbreak here in the U.S.

The numerals in the illustration show the main mutation sites of the delta variant first detected in India, which is likely the most contagious version of the coronavirus. Here, the virus' spike protein (red) binds to a receptor on a human cell (blue). New research sheds light on what makes this variant so transmissible. — The numerals in the illustration show the main mutation sites of the delta variant first detected in India, which is likely the most contagious version of the coronavirus. Here, the virus’ spike protein (red) binds to a receptor on a human cell (blue). New research sheds light on what makes this variant so transmissible. [Juan Gaertner | Science Source]

A new study, published online Wednesday, sheds light on why. It finds that the variant grows more rapidly inside people’s respiratory tracts and to much higher levels, researchers at the Guangdong Provincial Center for Disease Control and Prevention report.

On average, people infected with the delta variant had about 1,000 times more copies of the virus in their respiratory tracts than people infected with the original strain of the coronavirus, the study reports.

In addition, after a person catches the delta variant, they likely become infectious sooner. On average, it took only about 4 days for the delta variant to reach detectable levels inside a person versus 6 days for the original coronavirus variant.

In the study, scientists analyzed COVID-19 patients involved in the first outbreak of the delta variant in mainland China, which occurred between May 21 and June 18 in Guangzhou, the capital city of Guangdong province. The researchers measured the levels of virus in 62 people involved in that outbreak and compared them to the levels in 63 patients infected back in 2020 with the early version of the virus.

Their findings suggest that people who have contracted the delta variant are likely spreading the virus earlier in the course of their infection.

And they underscore the importance of quarantining immediately for 14 days after coming into contact with someone diagnosed with COVID-19, as the CDC recommends.

Or even better, getting fully vaccinated. Preliminary data show that in some U.S. states, 99.5% of COVID-19 deaths in the past few months were in people who weren’t vaccinated, the CDC director Dr. Rochelle Walensky said Thursday at the White House.

“We know that the delta variant … is currently surging in pockets of the country with low vaccination rates,” she said. “We also know that our authorized vaccines prevent severe disease, hospitalization and death from the delta variant.”