July 30, 2021 -- Internal 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 nearly as contagious as those who are unvaccinated.

In addition, the agency released a new outbreak study that  shows that breakthrough infections in the vaccinated make people about as contagious as those who are unvaccinated.

The new report, published today in Morbidity and Mortality Weekly Report (MMWR), also reveals the delta variant likely causes more severe COVID-19 illness.

Given these recent findings, the internal CDC slideshow advises that the agency  should "acknowledge the war has changed."

A 'Pivotal Discovery'

CDC Director Rochelle Walensky, MD, MPH, said in a statement today that the MMWR report demonstrates "that delta infection resulted in similarly high SARS-CoV-2 viral loads in vaccinated and unvaccinated people."

"High viral loads suggest an increased risk of transmission and raised concern that, unlike with other variants, vaccinated people infected with delta can transmit the virus," she added. "This finding is concerning and was a pivotal discovery leading to CDC’s updated mask recommendation."

The report investigators analyzed 469 COVID-19 cases reported in Massachusetts residents July 3 -17, 2021. The infections were associated with an outbreak following multiple events and large gatherings in Provincetown in that state's easternmost Barnstable County, also known as Cape Cod.

Notably, 346 infections or 74% of the cases occurred in fully vaccinated individuals. This group had a median age of 42 and 87% were male. Also, 79% of the breakthrough infections were symptomatic.

Researchers also identified the delta variant in 90% of 133 specimens collected for analysis. Furthermore, viral loads were about the same between samples taken from people who were fully vaccinated and those who were not.

Four of the five people hospitalized were fully vaccinated. No deaths were reported. 

The publication of these results were highly anticipated following the CDC's updated recommendations on Tuesday.

Outside the scope of the MMWR report is the total number of cases associated with the outbreak, including visitors from outside Massachusetts, which now approach 900 infections, NBC Boston reported.

'Very Sobering' Data

"The new information from the CDC around the delta variant is very sobering," David Hirschwerk, MD, infectious disease specialist at Northwell Health in New Hyde Park, New York, told Medscape Medical News.

"The CDC is trying to convey and present this uncertain situation clearly to the public based on new, accumulated data," he said. For example, given the evidence for higher contagiousness of the delta variant, Hirschwerk added, "there will be situations where vaccinated people get infected, because the amount of the virus overwhelms the immune protection,"

"What is new that is concerning is that people who are vaccinated still have the potential to transmit the virus to the same degree," he said.

Today's MMWR study "helps us better understand the question related to whether or not person who has completed a COVID-19 series can spread the infection," agreed Michelle Barron, MD, a professor in the division of infectious disease at the University of Colorado School of Medicine in Aurora.

"The message is that because the delta variant is much more contagious than the original strain, that unvaccinated persons need to get vaccinated because is nearly impossible to avoid the virus indefinitely," Michael Lin, MD, MPH, infectious diseases specialist and epidemiologist at Rush University Medical Center in Chicago, said when asked to comment.

Furthermore, Lin added, "This new data highlights that vaccinated persons, if they become sick, should still seek COVID-19 testing and should still isolate, as they are likely contagious."

More Contagious Than Other Infections

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

"A lot of people will remember getting chicken pox and then having their siblings get it shortly thereafter," Barron said.

"The only key thing to note is that this does not mean that COVID-19 delta variant mechanism of spread is the same as chickenpox and Ebola," she added. "The primary means of spread of COVID-19, even the Delta variant, is via droplets."

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 coronavirus 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 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 US.

In comparing the delta variant to other infections, "I think the CDC is trying to help people understand a little bit better the situation we now face since the information is so new. We are in a very different position now than just a few weeks ago, and it is hard for people to accept this," Hirschwerk said.

The Delta variant is so different that the CDC considers it is 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

Moving back to science talk, the CDC used a recent outbreak in Barnstable County, MA, as an example. The cycle threshold, or Ct values, which measure viral load, were about the same between 80 people linked to the Provincetown Fourth of July outbreak, who had an average 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 3-fold.

"Even with this high amount of virus, [the delta variant] did not necessarily make the vaccinated individuals as sick," Barron said.

In her statement, Walensky credited collaboration with the Commonwealth of Massachusetts Department of Public Health and CD for the new data. She also thanked the residents of Barnstable County for participating in interviews done by contact tracers and their wiliness to get tested and adhere to safety protocols after learning of their exposure.

Next Moves by CDC?

The agency notes next steps for the agency that include possible measures including vaccine mandates for health care professionals to protect vulnerable populations and universal masking for source control and prevention.

Asked if this potential policy is appropriate and feasible, Lin said, " Yes, I believe that every person working in healthcare should be vaccinated for COVID-19, and it is feasible."

Barron agreed as well. "We as healthcare providers choose to work in healthcare, and we should be doing everything feasible to ensure that we are protecting our patients and keeping our co-workers safe."

"Whether you are a healthcare professional or not, I would urge everyone to get the COVID-19 vaccine, especially as cases across the country continue to rise," Hirschwerk said. "Unequivocally vaccines protect you from the virus."