The coronavirus delta variant may be as contagious as chickenpox and cause more severe illness than previous variants, according to an internal presentation from the Centers for Disease Control and Prevention (CDC).
Vaccines continue to be highly effective, especially at preventing severe illness and death, but may be less effective at preventing infection or transmission of the delta variant, according to a CDC slide deck first obtained by The Washington Post.
The slides were shared within the CDC and cite some of the data — both published and unpublished — that drove the recent shift in the CDC's masking recommendations. "Acknowledge the war has changed," the CDC wrote in the report.
Related: Delta variant: Your questions answered
One major takeaway from the slides is that at current levels of vaccination in the U.S., delta will continue to spread exponentially without other mitigation measures in place, such as masking of vaccinated people. "Given higher transmissibility and current vaccine coverage, universal masking is essential to reduce transmission of the delta variant," according to the presentation.
On Tuesday (July 27), the CDC updated its mask guidance to say that fully vaccinated people should resume wearing masks in public indoor spaces in areas with substantial coronavirus transmission, Live Science previously reported. The delta variant is "different from previous strains," according to the slides. It is "highly contagious," "likely more severe," and "breakthrough infections may be as transmissible as unvaccinated cases," according to the summary slide.
There are currently about 35,000 symptomatic breakthrough infections (with any variant) per week among 162 million vaccinated Americans, according to the report. Currently, the risk of symptomatic disease is reduced eightfold and the risk of hospitalization and death is reduced 25-fold among those who are fully vaccinated versus those who are unvaccinated, those national estimates suggest.
However, the risk of infection with the delta variant is likely only reduced threefold in those who are vaccinated, according to the slides.
The delta variant is more transmissible than the viruses that cause Middle East respiratory syndrome (MERS) and severe acute respiratory syndrome (SARS), Ebola, the common cold, seasonal flu, the 1918 flu and smallpox, and just as transmissible as chicken pox, according to the slides.
What's more, people who are infected with the delta variant may carry a higher viral load than people infected with other variants (even in breakthrough cases) and shed virus — thus being able to spread it — for longer, according to the slides. A small preliminary study found that people infected with delta may be carrying more than 1,000 times more virus particles and test positive two days earlier than those infected with the original virus, Live Science previously reported.
More preliminary data from an outbreak of delta in Barnstable County, Massachusetts, suggests that there was no difference in viral load among those who were vaccinated but had a breakthrough case and those who were unvaccinated, which suggests that vaccinated breakthrough cases may be able to transmit the delta variant as easily as unvaccinated cases. (In contrast, previous variants could not easily spread from vaccinated people with breakthrough infections, according to The New York Times).
However it's not clear whether all of those virus particles are infectious, and whether the fraction that can infect others is the same for vaccinated versus unvaccinated people.
The CDC slides say that the "delta variant may cause more severe disease than alpha or ancestral strains," according to published data from Canada, Singapore and Scotland.
Data from England, Scotland, Canada and Israel suggests that the Pfizer-BioNTech vaccine is somewhere between 93% and 100% effective in preventing hospitalization or death but 64% to 88% effective in preventing symptomatic disease from the delta variant.
What's more, breakthrough cases will occur more frequently in congregate settings and in groups for whom the vaccine works less robustly, including people who are immunocompromised or elderly, according to the slides. The risk of hospitalization and death is higher among older adults compared to younger populations, regardless of vaccination status, according to the presentation.
Originally published on Live Science.