What should I make of the “leaked” CDC slides last week? Is Delta worse than we thought?!

COVID Variants Data and Metrics Infection and Spread


Just when we thought our summer might be more relaxing than last year, SARS-CoV-2 is humbling us again. But despite the emotionally amped up headlines (which are, after all, designed to make you click), we’re here to reassure you that in countries with access to vaccinations, the trajectory is still good. There will be bumps in the road, but the vaccines really are our way out of this pandemic. While it may feel like déjà vu (or pan-doomerang?!), we are not back to square one.

Last week the U.S. Centers for Disease Control and Prevention (CDC) released updated guidance for vaccinated individuals in the US. The bottom line was a recommendation to return to masking in public indoor spaces where community transmission is high, due to concerns about more breakthrough infections with the Delta variant. Masks were also recommended for the return to school (summary here)

The next day, the Washington Post published some internal slides from the CDC that helped inform this guidance, and a bit of online panic ensued.

Some of the highlighted points included that Delta is “as contagious as chicken pox,” and “may be spread by vaccinated people as easily as the unvaccinated.”

And finally -“The war has changed.”

What’s the Nerdy Girl take?

We’re watching carefully AND keeping calm. The data presented in these slides is mostly not new, and some of it is highly preliminary. PowerPoint bullet points taken out of context are not the best way to communicate evidence. The important facts about Delta remain quite consistent:

➡️ Delta is *likely* more contagious than previous variants.

This is based on epidemiological evidence of how quickly Delta has become the predominant variant in many places, as well as the “secondary attack rate” among contacts of confirmed cases. We also have data on higher viral loads among those infected with Delta.

Is Delta as contagious as the chicken pox? This claim is more speculative and TBD in our opinion.

Transmissibility estimates are highly variable because they depend not just on the virus, but human behavior and the environment. Early estimates of transmissibility of other variants such as Alpha have come down over time, and the same has been happening for Delta. The CDC slide that equates Delta with the chicken pox has no specific citation and a wide estimate for the Reproductive rate of Delta (5-10), reflecting a high level of uncertainty. So, while it’s likely more transmissible, Delta has not achieved superpowers just yet. Unfortunately a bit more transmission can quickly add up to big trouble with cases rising exponentially as we’ve seen in the UK and now in parts of the US.

➡️ The vaccines are still highly effective (>90%) at preventing severe disease and death from Delta. THIS IS SUCH GOOD NEWS.

BUT—they appear slightly less effective at preventing infection or transmission compared to earlier variants. This means “breakthrough” infections among the vaccinated are more likely than before and vaccinated people can in theory transmit to others.

The CDC reported that breakthrough infections “may be” as transmissible as unvaccinated cases, based on a study from Massachusetts showing similar viral loads among vaccinated and unvaccinated cases. Several other studies have shown lower viral loads and shorter duration of symptoms among cases who were vaccinated, so these new results are intriguing but not conclusive.

The vaccines still dramatically reduce the chance of getting infected in the first place, so vaccinated people are MUCH less likely to transmit the virus to others than unvaccinated people- just more likely with Delta than previous variants. This small but increased likelihood of transmission from vaccinated people is the main reason for a move towards more vigilance in areas with high transmission.

➡️ Does Delta lead to more severe disease? Possibly, but we haven’t seen strong evidence for this from England which has given us much of the best Delta data so far.


💥Vaccines work *really well* to reduce hospitalizations & death, even with Delta.

💥They are also good but less effective at preventing infections & transmission with Delta.

💥The sky is not falling, but we can still take sensible precautions to protect our unvaccinated children and other vulnerable citizens while this Delta wave crashes ashore.

Those Nerdy Girls


“‘The war has changed’: Internal CDC document urges new messaging, warns delta infections likely more severe”

Link to CDC slides

Thoughts on the CDC slides from Dr. James Hamblin

5 Things To Know About the Delta Variant

Morbidity and Mortality Weekly Report (MMWR), Massachusetts outbreak

Previous DP posts on the Delta variant:


If the Delta variant is more transmissible, are normal precautions (masks, 6 foot distance, being outside) less effective?

What should I do about the large discrepancies in vaccine efficacy against the Delta variant being reported across studies?


Link to Original FB Post