This post has been updated to reflect newly-released data from the CDC.

When it comes to fighting the COVID-19 pandemic, The Centers for Disease Control and Prevention says “the war has changed.” Data released by the agency on Friday demonstrates that while vaccines are still likely to be highly effective in preventing hospitalization and death due to COVID-19, even immunized individuals are capable of catching and spreading the potent new Delta variant. Meanwhile, infection rates and cases of severe illness are skyrocketing among the unvaccinated.

This comes just days after the agency changed its official recommendations on masking. On a Tuesday press call, CDC director Rochelle Walensky advised school-age children to mask up, and suggested people living in areas with significant COVID-19 transmission wear masks indoors even if they’re vaccinated. That accounts for some two-thirds of the US. 

[Related: What the CDC’s new mask guidance means for you]

On Thursday, The Washington Post obtained an internally-distributed CDC slideshow summarizing some of the data behind that pivot. Most crucially, the CDC now considers the Delta variant to be more contagious than the common cold and Ebola—perhaps as contagious as chicken pox, with each unvaccinated and infected individual giving the virus to another 8 or 9 people, on average—and more likely to cause severe illness than previous strains.

“It’s one of the most transmissible viruses we know about,” Walensky told CNN.

That assessment is based in part on a study of 469 cases of COVID-19 reported during July 2021—all of which were associated with various summer gatherings, trips to bars and other indoor venues, and large public events in Barnstable County, Massachusetts. Three-quarters of those cases were in vaccinated individuals. This doesn’t mean the vaccine made them more likely to get COVID-19, of course; vaccination rates across the state were at 69 percent at the time, so most event attendees would likely have had the jab. The CDC was able to obtain genomic data for samples from 133 of the patients, and found that 90 percent of them were infected with a version of the Delta variant. Five individuals in the cluster have been hospitalized so far, including four fully vaccinated people. Thankfully, no one has died.

The report, published on Friday, has several limitations. While 469 subjects may be enough to get some kind of statistical significance, we can’t assume that the results show us how Delta will actually spread in the larger US population, especially since so many people in the sample were vaccinated. The results of the study are also skewed toward transmission in adult men, as these were the primary attendees of the super-spreader events in question. It’s also quite likely that there are some number of asymptomatic cases that the CDC is not aware of, as such individuals may not have seen any reason to get tested.

Vaccinations are key in keeping clusters like the one described above small and contained, but the results do suggest that vaccinated individuals should also take care. While hospitalization is still rare in vaccinated individuals, it’s apparent that even people who’ve received COVID-19 vaccines can catch and transmit the Delta variant, albeit less easily than those without immunization. While researchers have always thought it’s possible for vaccinated people to catch and spread coronavirus, data suggested that immunization made viral loads much lower. In most cases, it probably still does. But the CDC is clearly keeping an eye on whether Delta (and other, future variants) will put that protection in peril.

[Related: Will we need booster shots? And other questions about the Delta variant, answered.]

The amount of virus in your body can affect how sick you get, but even if your symptoms are mild, a high viral load means a higher risk of exposing the people around you to coronavirus. Some research suggests that the Delta variant produces a thousand times as much virus as the original strain can, but more data is needed to know for certain. Even if COVID-19 doesn’t make you sick, your infection puts people at risk beyond those who have willingly refused the vaccine—immunocompromised individuals and the elderly seem to get less protection from vaccines, for starters, and most children are still ineligible for vaccination.

While the new report should make vaccinated individuals think twice about unmasking and returning to “normal” life, this news does not mean the vaccines aren’t effective and crucial. The CDC’s data still shows that vaccination makes a monumental difference in how many individuals get very sick, how crowded hospitals get, and how many people die. If everyone in the world were to get vaccinated, most people who got COVID-19 would experience flu-like symptoms, though some would fare worse and others might suffer debilitating symptoms due to “Long COVID,” which can affect even those who experience mild initial cases. Still, the takeaway is clear: The Delta variant is no joke, but we’re better equipped to weather it if more of the population is vaccinated. And if more people get vaccinated, we’ll be giving the virus fewer opportunities to circulate, which means fewer chances to mutate into even more dangerous strains.

Instead, the billions of coronavirus vaccines distributed worldwide have predominantly been used in a small number of wealthy countries. And in the US, vaccination rates started plummeting long before the country came even close to reaching safe levels of herd immunity. 

It is more crucial than ever that all individuals with the ability to do so get vaccinated as quickly as possible. Use this tool to find a COVID-19 vaccination site near you. COVID-19 vaccines are free of charge to everyone in the US, and are now approved for children as young 12 years old.

Correction: A previous version of this post cited data on how much more virus may be present in vaccinated people after exposure to Delta versus earlier strain, but this information referred to a random sampling of infected individuals. We deeply regret this error and emphasize to our readers that all evidence suggests vaccines are incredibly effective in stopping the transmission of COVID-19, even if masks and social distancing are still crucial components of the fight against coronavirus.