A leaked CDC document compared it to the highly contagious children's disease. Data does not support this claim. Nonetheless, the variant is one of the world's most contagious respiratory diseases.



A leaked and then widely circulated document from the CDC claims that the delta variant of COVID is as transmissible as the chicken pox, but is that accurate? NPR's Michaeleen Doucleff looked into it.

MICHAELEEN DOUCLEFF, BYLINE: One way scientists measure how contagious a disease is is by using what's called the R0. Karthik Gangavarapu is a computational biologist at the Scripps Research Institute. He says R0 is the number of people each sick person will infect on average when the entire population is vulnerable to the virus. Nobody has had it. Nobody is vaccinated.

KARTHIK GANGAVARAPU: So given ideal conditions for the virus to spread, if it's a completely susceptible population - no one has any immunity - then yes, that would be the expected number of secondary infections caused by one primary infection.

DOUCLEFF: So, for example, Ebola has an R0 of about two. So...

GANGAVARAPU: One person ends up infecting two other people.

DOUCLEFF: And the flu has an R0 between one and two. So on average, each person passes it on to one or two people. What about SARS-CoV-2? When the coronavirus first emerged in 2019, it was slightly more contagious than flu. Then in December, the delta variant emerged in India. Tom Wenseleers is a biostatistician at the University of Leuven in Belgium. He was one of the first scientists to recognize how dangerous the new variants could be. He says the delta variant is much more contagious than the original strain of SARS-CoV-2. Its R0 is more than double.

TOM WENSELEERS: For the delta variant, it's now calculated at between six and seven, so 6 1/2 is probably about right.

DOUCLEFF: And so what does that mean in practice? First off, it means the delta variant is not as contagious as the chicken pox, as the CDC's document claimed. Wenseleers says that statement was a bit of an exaggeration.

WENSELEERS: Yeah, I didn't find it entirely accurate. Chicken pox's R0 value, yeah, is often put at around 10.

DOUCLEFF: So chicken pox is incredibly contagious. Outbreaks are explosive. There is a big difference between an R0 value of 10 for the chicken pox and an R0 value of 6.5 for delta. So why did the CDC make that claim? A federal official told NPR the claim was, quote, "based on preliminary data with a small sample size. More data are coming, but at the end of the day, the message is the same. The delta variant is much more transmissible than previous versions of the virus," end quote. Wenseleers agrees. He says an R0 of six or seven is still very, very high, especially for viruses that infect the respiratory tract.

WENSELEERS: Yeah, intrinsically, of course, it is true that this delta variant now is - for respiratory virus, it's probably the most contagious one that we know about.

DOUCLEFF: Wenseleers says that with an R0 of six, delta will be extremely difficult to stop. To slow it down, communities need to have a very large percentage of the population vaccinated. Those who are not immunized have a very high risk of catching this version of the virus.

WENSELEERS: Anyone that chooses not to get vaccinated will in all likelihood get infected by the virus in the coming months.

DOUCLEFF: If you are vaccinated, data continue to show that you are well-protected against hospitalization and death, even with the delta variant.

Michaeleen Doucleff, NPR News.

(SOUNDBITE OF MUSIC) Transcript provided by NPR, Copyright NPR.


In an earlier version of this story, evolutionary biologist and biostatistician Tom Wenseleers at the University of Leuven in Belgium stated that the delta variant of the coronavirus was "probably the most contagious respiratory virus that we know, for the moment." Wenseleers clarifies that he was referring to viruses that primarily affect the respiratory tract. Although the primary infection with measles occurs in other tissues besides the respiratory tract, measles can cause a runny nose and sneezing. Its R0 factor makes it more contagious than the delta variant.