The CDC's leaked document is 'largely reassuring' for vaccinated people, says a Brown University public-health expert

• An internal CDC document suggests the Delta variant poses a mounting threat in the US.
• But Ashish Jha, dean of the Brown School of Public Health, called the document "reassuring."
• The bottom line, Jha wrote, is that Delta is "really bad," but our vaccines are "really good."

First, the bad news: People who are fully vaccinated can spread the Delta variant just as easily as those who are unvaccinated, according to an internal presentation from the Centers for Disease Control and Prevention obtained by The Washington Post.

A chart from the presentation suggests that each person infected with Delta may spread the coronavirus to between five and 9.5 others, on average, making it more transmissible than the viruses that cause Ebola, the common cold, and smallpox - and about as transmissible as chickenpox.

But there's plenty of positive news in the document, too, according to Ashish Jha, dean of the Brown School of Public Health.

"I find the slides insightful and largely reassuring," Jha wrote Thursday on Twitter.

He pointed to data indicating that just 35,000 out of 162 million vaccinated Americans are developing symptomatic COVID-19 cases per week - a rate of just 0.02%.

That number is probably a bit too low, Jha said. The CDC stopped tracking asymptomatic, mild, or moderate breakthrough cases at a national level in May, making its data limited. So Jha estimated the figure may be closer to 300,000 breakthrough cases per week, or 19%.

Still, given that half of Americans are fully vaccinated, Jha said, that would mean the US's current shots are roughly 88% protective against COVID-19 and more than 90% effective against severe infection.

That's consistent with data from other countries: Studies from England and Scotland indicate that Pfizer's two-dose vaccine reduces the risk of a symptomatic Delta infection by 88%, down from 95% for the original strain. The vaccine also lowers the risk of any type of Delta infection by 79% and of hospitalization from Delta by 96%, according to the studies.

Additionally, a Canadian study that's still awaiting peer review found that after both doses of Pfizer's shot, the risk of a symptomatic Delta infection drops by 87% and the risk of hospitalization or death from Delta drops by 100%.

Data from Israel's Ministry of Health, though, suggests a bit less protection: 64% against a Delta infection (asymptomatic or symptomatic) and 93% against serious illness or hospitalization from Delta.

The bottom line, according to Jha, is that Delta is "really bad," but: "Our vaccines are good. Like really good."

"Breakthrough infections happen," he wrote. "Sometimes they may spread to others. But if enough people get the shot, the pandemic does come to an end."

The CDC estimated last week that unvaccinated people represent about 97% of hospitalized COVID-19 cases in the US. The agency's leaked presentation also shows that vaccines reduce the risk of hospitalization or death 25-fold.

Delta led the CDC to recommend masks again amid lagging vaccinations

It's hard to say how many Americans need to get vaccinated to force the coronavirus to circulate at low levels instead of in dramatic surges.

Dr. Monica Gandhi, an infectious-disease expert at the University of California, San Francisco, recently told Insider that COVID-19 case and hospitalization trends may no longer be closely linked once 75% of the population has received at least one vaccine dose.

Just 57% of the US population has received at least one dose so far, but the vaccination rate continues to taper off nationally. The number of daily vaccinations in the US has declined 25%, on average, in the last month - from 573,000 to 431,000 per day. However, states like Arkansas and Louisiana have seen slight upticks in their vaccination rates as of late.

Given the transmissibility of the Delta variant, the CDC recommended Tuesday that fully vaccinated people wear masks indoors "in areas with substantial and high transmission." That constitutes around 70% of US counties right now.

Still, "the vast majority of transmission, the vast majority of severe disease, hospitalization, and death is almost exclusively happening among unvaccinated people," CDC Director Rochelle Walensky said on Tuesday.