The Delta variant is likely causing more severe infections, even though experts have not been saying this up until now, and the ease with which it spreads even among the vaccinated has the CDC raising internal alarms.

Late Thursday, multiple news outlets obtained leaked internal documents from a presentation at the CDC which go into further detail about the decision by the agency's director, Dr. Rochelle Walensky, to reverse course and announce new mask guidance for the vaccinated on Tuesday. As the Washington Post reports, the documents suggest that the Delta variant is as contagious as chickenpox, and they urge public health officials to "acknowledge the war has changed."

Vaccination remains the best defense against the worst outcomes of COVID-19, even the Delta variant, but new data raises alarm about how much spread may be happening with the variant that appears have viral loads in the nose and throat that are 1,000 times higher than the original Alpha variant. And these viral loads were found to be the same in both vaccinated and unvaccinated people who tested positive, as we heard on Tuesday.

UCSF's Dr. Bob Wachter, who has generally struck notes of optimism about the pandemic response locally, tells the Post, "I finished reading it significantly more concerned than when I began." But, as he says on Twitter today, people still need to have confidence in the vaccines and know that even with this new data, a person's chance of being infected if exposed is "~1/8 as high" if that person is vaccinated, compared to the unvaccinated.

One reason for renewed concern is that data from Delta infections in other countries are showing a pattern of more serious outcomes, with higher odds of hospitalization found in both Canada and Scotland, and higher odds of a need for oxygen intervention seen in Singapore.

Also, we learn via ABC News and the New York Times that the CDC was influenced heavily in its decision about the new mask recommendation by data from the Provincetown July 4th cluster of cases. So far, 882 cases have been connected to that outbreak, with 74% of the cases among the fully vaccinated, and it sounds like the 100 samples Walensky mentioned that showed "indistinguishable" levels of virus in the noses and throats of vaccinated and unvaccinated cases came out of that Massachusetts study.

As the Times notes, thanks to the probably most gay men who had been partying in Ptown and then took part in contact-tracing process when the outbreak took hold, the data was apparently very thorough in showing vaccinated people infecting other vaccinated people.

"This is one of the most impressive examples of citizen science I have seen,” says infectious disease specialist Dr. Celine Gounder at New York's Bellevue Hospital, speaking to the Times. "The people involved in the Provincetown outbreak were meticulous in making lists of their contacts and exposures."

As NPR's Ari Shapiro puts it, "Leave it to the gays, this ain’t our first pandemic."

All the new data from the CDC was set to be published in full on Friday, but the internal document suggested that the agency is already worried about the "communications challenges" involved. They say the biggest fear here is that the "public [will be] convinced vaccines no longer work/booster doses needed."

Wachter suggests that booster shots likely are needed, especially for vulnerable populations, as more data is emerging about vaccine efficacy waning after six months.

But many public health officials have expressed concern about talking too much about the need for booster shots as so many Americans still haven't sought out a single dose.

An anonymous federal health official who wasn't authorized to speak publicly gave a comment to the Post summing up the new data saying, "Although it’s rare, we believe that at an individual level, vaccinated people may spread the virus, which is why we updated our recommendation. Waiting even days to publish the data could result in needless suffering and as public health professionals we cannot accept that."

John Moore, a virologist at Weill Cornell Medicine in New York, tells the Times, after looking at the CDC documents, "Overall, Delta is the troubling variant we already knew it was. But the sky isn’t falling and vaccination still protects strongly against the worse outcomes."

In San Francisco, data has not yet been shared by the Department of Public Health on the number of breakthrough infections among the vaccinated, but in other local counties where the data is being shared, the numbers do appear to be rising.

In Contra Costa County, the rate of infection among vaccinated residents was 1.7 cases per 100,000 on July 6, compared to 13.7 cases per 100,000 among the unvaccinated. As of July 23, that had risen to 8.1 cases per 100,000 vaccinated residents, a more than four-fold increase, but cases among the unvaccinated showed a similar almost four-fold rise to 41.7 per 100,000.

In the Provincetown outbreak, an estimated 66,000 people were visiting the area over the July 4th holiday, so with around 900 documented cases, the overall rate of spread was still somewhat limited. A similar outbreak was recorded in the Netherlands in recent weeks, where an outdoor music festival attended by around 20,000 people led to around 1,000 infections.

Across the country, according to the CDC's data, there are around 35,000 symptomatic infections each week now among 162 million vaccinated Americans, though this is likely an undercount due to much lower rates of testing in some parts of the country.

San Francisco's seven-day average of tests being administered more than doubled between July 10 and July 22, to almost 4,400 per day, and that number likely has risen further in the last week. The rate of test positivity has also spiked again, from a low of 0.5% in early June to 4.8% as of July 22, according to DPH data.

Related: CDC Confirms That Viral Loads In Vaccinated People With Delta May Be Infectious, So Masks Are Necessary

Top image: Pedestrians walk down Commercial Street on May 25, 2020 in Provincetown, Massachusetts. Massachusetts has begun Phase 1 of reopening after the coronavirus (COVID-19) shutdown. Beaches reopened and non essential businesses were allowed to operate with curbside pickup. (Photo by Maddie Meyer/Getty Images)