Delta Variant as Contagious as Chickenpox

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Internal Centers for Disease Control and Prevention (CDC) documents support the high transmission rate of the delta variant and put the risk in easier to understand terms. The documents also show that breakthrough infections in the vaccinated make people about as contagious as those who are unvaccinated.

In addition to being more transmissible, the delta variant likely causes more severe COVID-19 illness.

Given these recent findings, the document advises that the CDC should “acknowledge the war has changed.”

The internal CDC slide presentation also puts the new transmission risk in simple terms. Saying the delta variant is about as contagious as chickenpox, for example, immediately brings back vivid memories for some of staying indoors and away from friends during childhood or teenage outbreaks.

This also means each person infected with the delta variant could infect an average of eight or nine others.

In contrast, the original strain of the SARS-CoV-2 virus was about as infectious as the common cold. In other words, someone was likely to infect about two other people on average.

In addition to the cold, the CDC notes that the delta variant is now more contagious than Ebola, the seasonal flu, or smallpox.

These delta variant comparisons are one tangible way of explaining why the CDC on Tuesday recommended a return to masking in schools and other indoor spaces for people – vaccinated and unvaccinated – in about 70% of the counties across the United States.

The delta variant is so different that the CDC considers it almost acting like a new virus altogether.

The CDC internal documents were first released by The Washington Post on Thursday. The slides cite communication challenges for the agency to continue promoting vaccination while also acknowledging that breakthrough cases are occurring and therefore the fully vaccinated, in some instances, are likely infecting others.

Trust but Verify

In announcing the new mask guidance on Tuesday, CDC Director Rochelle Walensky, MD, MPH, referred to new science that supports the move. Many experts asked to see the full research for themselves. As a result, the CDC is expected to release the full study details today.

One novel finding is expected to be the evidence for more severe disease associated with the delta variant.  

Novel evidence also came out this week answering a long unanswered question. The CDC explained that the fully vaccinated can be COVID transmitters because people with “breakthrough infections” walk around with the same amount of virus in their noses as the unvaccinated do.

Moving back to science talk, the CDC used a recent outbreak in Barnstable County, Massachusetts, as an example. The cycle threshold or Ct values, a measure of viral load, were about the same between 80 people linked to the Provincetown Fourth of July outbreak, who had a mean Ct value of 21.9, compared to 65 other unvaccinated people with a Ct of 21.5.

Many experts are quick to note that vaccination remains essential, in part, because a vaccinated person also walks around with a much lower risk for severe outcomes, hospitalization, and death. In the internal slide show, the CDC points out that vaccination reduces the risk of infection threefold.

The agency notes next steps for the agency include consideration of prevention measures including vaccine mandates for healthcare professionals to protect vulnerable populations, universal masking for source control and prevention, and reconsidering other community mitigation strategies.

Damian McNamara is a staff journalist based in Miami. He covers a wide range of medical specialties, including infectious diseases, gastroenterology, and critical care. Follow Damian on Twitter: @MedReporter.

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