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You've been hearing it a lot lately: the Delta variant is 1,000 times more infectious than the original version of SARS-CoV-2.

So where does that figure come from?

It's from a study by the Guangdong CDC in China -- published earlier this month as a preprint on both and -- that experts are touting for its solid science.

Baisheng Li, MD, and colleagues studied 62 people and their close contacts who were infected in the initial Delta outbreak in Guangzhou from May 21 to June 18. These close contacts were tested daily via PCR, and data were compared with similar sampling from 63 people infected with an earlier version of the virus from the first wave in 2020.

Ultimately, they found that the viral load for the first positive test was 1,260 times higher for Delta compared with the variant in the initial wave of infections -- hence, the "1,000 times" higher estimate going around social and other media.

Indeed, the PCR cycle threshold value for the first detected infection was just 24 with Delta, compared with 34 for the earlier variant, they reported.

The time from exposure to the first positive PCR test was also shorter with Delta, at 4 days compared with 6 days.

This all suggests a faster replication rate, a reduced incubation period, and greater viral shedding -- all factors that contribute to Delta's increased infectiousness and transmissibility, said Angela Rasmussen, PhD, of the Vaccine and Infectious Disease Organization at the University of Saskatchewan in Canada, in a twitter thread.

"If people are shedding 1000X more virus, the probability that a close contact will be exposed to an infectious dose is much higher," . "If people become contagious more quickly after exposure, they can have more opportunity to infect others."

She noted that she's seen more "Delta is more airborne!" commentary, which is "neither helpful nor accurate. ... I'm not even sure what that means," as a virus must still obey the laws of physics, she said.

"Delta hasn't learned how to fly or morphed into an ACE2-seeking virus missile," . "It's not 'more airborne.'"

She said the that the mechanism behind Delta's increased transmissibility "is just that there's a lot more of it, sooner. If there's more virus around, it's going to be easier to spread."

The increased transmissibility for Delta is also supported by epidemiological evidence from the U.K., which found Delta to be than the Alpha variant (B.1.1.7). Alpha was already estimated to be 50% more transmissible than the wild-type virus, or the D614G strain -- making Delta about twice as transmissible as wild-type virus.

Rasmussen said the findings from Li's group only make the need to get vaccinated , since the fully vaccinated "have high titers of neutralizing antibodies and mature memory B and T cell responses. It's a lot harder for Delta to overwhelm these defenses by just throwing more virus at them."

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