healthHealth and Medicine

Leaked CDC Report Says COVID-19 Delta Variant As Contagious As Chickenpox


Dr. Katie Spalding

Katie has a PhD in maths, specializing in the intersection of dynamical systems and number theory.

Freelance Writer


We're trying, stop sign, we're trying. Image credit: kumpol.pijadee/

In many ways, the coronavirus pandemic is now more of a delta variant pandemic. Despite being the most recent addition to the WHO’s “variants of concern”, it has now spread to 132 countries and is responsible for the vast majority of new infections in the USA and practically all of them in the UK.

An internal Centers for Disease Control and Prevention (CDC) document obtained by the Washington Post on Thursday, July 29, has revealed the scope of the challenge currently facing national health agencies. Citing new and yet-unpublished data, the document concludes that the delta variant is highly contagious, including among the vaccinated, and notes that it may cause a more severe illness than all other known variants. It's more transmissible than the viruses that cause MERS, SARS, Ebola, smallpox, the common cold, and seasonal flu, and is as contagious as chickenpox.


“I think people need to understand that we're not crying wolf here. This is serious,” CDC Director Rochelle Walensky, who confirmed the authenticity of the document, told CNN. “It's one of the most transmissible viruses we know about. Measles, chickenpox, this – they're all up there.”

In the first half of last year, many of us added a new term to our mental dictionaries: the R number. Simply put, the R, or reproduction, number of a disease gives a rough estimate of how many people one infected individual will pass the illness onto. The original coronavirus infection was estimated to have an R number of around 2 or 3. According to the CDC document, the delta variant is up to four times more infectious than that.

“When you think about diseases that have an R0 of eight or nine – there aren't that many,” Walensky told CNN.

That puts the infection rate of the delta variant at about the same as chickenpox, says the CDC, but that’s not the end of the problem. The delta variant is also more likely to break through the protection provided by vaccination – and if it does, its transmission rate is unaffected by the vaccine status of its host.


“The bottom line was that, in contrast to the other variants, vaccinated people, even if they didn't get sick, got infected and shed virus at similar levels as unvaccinated people who got infected,” explained Walter Orenstein of the Emory Vaccine Center, who viewed the documents, also speaking to CNN.

Despite this, the document confirms the importance of getting vaccinated. While transmission rates of the infection may not have been reduced, the risk of severe illness or death for vaccinated individuals is less than one-tenth that of unvaccinated people. And although the delta variant is more likely than the original virus to infect you even if you’ve had your shot, your risk of infection is still reduced by a factor of three over somebody who hasn’t been vaccinated.

That information is crucial because the CDC is aware that the public is losing confidence in vaccines. The document notes that a change in communication may be needed to combat this, pointing out that the rise of breakthrough cases, as well as the effect of individual circumstances on vaccine efficacy, may have clashed with overoptimistic public health messaging.

“We’ve done a great job of telling the public these are miracle vaccines,” risk communication expert Matthew Seeger told the Washington Post. “We have probably fallen a little into the trap of over-reassurance, which is one of the challenges of any crisis communication circumstance.”


The report comes just days after the CDC formally recommended that even fully vaccinated people should wear masks indoors, a policy reversal that was sparked by the surging delta variant rate and low uptake of vaccines.

“The number of cases we have now is higher than any number we had on any given day last summer,” Walensky told CNN.

“The measures we need to get this under control – they're extreme,” she added. “The measures you need are extreme.”

 This Week in IFLScience

Receive our biggest science stories to your inbox weekly!


healthHealth and Medicine
  • tag
  • pandemic,

  • vaccines,

  • covid-19