The Centers for Disease Control and Prevention says "the war has changed" againstand that the agency should acknowledge that in communications, according to an internal presentation by the agency.
Data in the document, which was first obtained by The Washington Post, underscores the danger posed by the highly-contagious Delta variant of the virus that was first spotted in India.
In the presentation, dated July 29, the agency does not estimate that vaccinated Americans are at a significantly greater risk of so-called "breakthrough" infections. In fact, it cites recent unpublished data from several of the CDC's ongoing cohort studies that have scrutinized large groups of Americans suggesting vaccine effectiveness remains high months after their second shot, suggesting the Pfizer and Moderna vaccines remain 65-75% effective even against asymptomatic infection.
Public health officials have repeatedly emphasized that the vaccines provide strong protection against serious illness, hospitalization and death, even if a vaccinated person does contract the virus.
The CDC's presentation also points to new "preliminary data" from its COVID-NET system that suggests vaccinated people remain a minority of COVID-19 hospitalizations in the United States. In May, fully vaccinated people accounted for 9% of all hospitalizations according to the document, which the presentation says reflects "increases in vaccine coverage" that are higher "in older adults."
The CDC had previously disclosed that less than 3% of hospitalizations had occurred in fully vaccinated people.
However, the CDC also now estimates in its presentation that the Delta variant could be as transmissible as Chickenpox and far more than other diseases like Ebola or SARS-CoV-2's original ancestral strain.
A CDC spokesperson declined to comment on the leaked document.
The agency cites data — expected to be released on Friday — from a July 4th outbreak of cases among residents and visitors to Provincetown, Massachusetts, showing the amount virus in samples collected from vaccinated and unvaccinated cases were virtually identical. That echoes previous reports from India, which the agency cites in the presentation, suggesting vaccine breakthrough cases with the delta variant could be far more transmissible than with previous mutant strains, spreading even as much as in some unvaccinated cases.
The research on the July 4th outbreak prompted federal health officials to urge even fully vaccinated Americans to wear masks indoors in areas of "substantial" or "high" spread of the virus, urge all people in schools to wear masks this fall, and revise its earlier guidance exempting many fully vaccinated people from COVID-19 testing recommendations.
Local health authorities have since tracked the outbreak from the popular Cape Cod retreat to hundreds of confirmed cases, many among fully vaccinated residents.
CDC Director Rochelle Walensky and Dr. Anthony Fauci, the president's chief medical adviser, also briefed members of Congress on Thursday about the new data, according to a release from the House Select Subcommittee on the Coronavirus. Fauci warned the group that the Delta variant is "considerably more transmissible" with a viral load "about a thousand times higher" than the original strain.
Briefing reporters earlier this week about the shift in the agency's guidance, Walensky acknowledged that their science on the risk fully vaccinated people had of spreading the virus to others — in the rare cases they had a "breakthrough" infection — had shifted.
"Public health experts, scientific experts, medical experts, when we have shown them these data have universally said that this required action. I thought and I felt that when I saw the data myself," Walensky said.