A leaked, unpublished document from the U.S. Centers for Disease Control and Prevention suggests that the delta variant of COVID-19 is highly contagious, can lead to serious illness, and may be transmitted as easily by vaccinated people as it is by those who are unvaccinated.

An important component was new data published Friday from a July outbreak in Massachusetts, which CDC Director Rochelle Walensky called “concerning” and “a pivotal discovery.” The new findings prompted the agency to revise its mask guidance earlier in the week and is now leading scientists to urge the U.S. to adjust its approach to containing the virus, according to the Washington Post, which first reported on a leaked copy of the document.

Details and guidance are evolving as scientists learn more about the delta variant. Here’s what we know so far:

What do I need to know about the new CDC findings?

New CDC estimates suggest that the delta variant is highly contagious, may spread as fast as chickenpox, and can lead to more severe illness than the original COVID-19 strain, according to a leaked copy of the unpublished report obtained by the Washington Post. The document also says the delta variant may be more transmissible than earlier strains, and, if infected with the delta variant, even people who are vaccinated may harbor enough virus to spread it to others. Earlier studies of previous strains suggested that people who were vaccinated were unlikely to transmit the virus to others. At the same time, it is still unlikely a vaccinated individual will become so sick they will need to be hospitalized or die from the virus, according to the CDC. The new findings may be particularly concerning for parents of children too young to be vaccinated and caretakers of individuals with compromised immune systems.

Where are the data from?

The CDC document draws from its own COVID-19 tracking program, as well as many new and some unpublished studies that analyze national and international outbreaks of how the delta variant may differ from earlier strains. The findings were so troubling to CDC scientists that they adjusted the agency’s mask guidance in advance of the report’s publication, according to the Washington Post.

How worried should we be about the increase in infections among vaccinated people?

A small but increasing percentage of COVID-19 hospitalizations and deaths is occurring in vaccinated people. But contrary to what some anti-vaxxers have claimed on social media, that in itself does not mean the vaccines are becoming any less effective. Math alert: The reason has to do with denominators.

The vaccines continue to prevent most hospitalizations and deaths in people who are fully vaccinated. In the leaked document, the CDC estimates an eight-fold decrease in COVID-19 disease and a 25-fold decrease in hospitalizations and deaths among those who are vaccinated.

But because more and more people are getting the shots every day — more than 160 million fully vaccinated, at last count — even the small percentage who develop severe COVID-19 will still amount to thousands of people. It’s a small percentage of a big number.

So it is no surprise that this growing number represents an increasing share of those who are hospitalized with COVID-19, as it simply reflects the much larger (and growing) number of people who are getting vaccinated overall.

The key is that among all those who are vaccinated, the vast majority will avoid hospitalization or death. And among all those who are not vaccinated, the chance of hospitalization and death remains far higher.

What does it mean if the delta variant spreads as fast as chickenpox?

By the CDC’s estimate, the delta variant is more transmissible than original strains of SARS-CoV-2. That means that for every person infected with the delta variant, they may spread the virus to more people than if they had the older strain. To put this in the context of more familiar viruses, the transmissibility of the older strain is similar to that of the common cold, while that of the delta variant may be several times higher — approaching the transmissibility of chickenpox.

“That means you’re more likely to catch it if you come in contact with someone,” said Paul Offit, director of the Vaccine Education Center at the Children’s Hospital of Philadelphia. He added that reports of increased viral shedding — the amount of virus that an infected person spreads — suggest you don’t need to be in contact for as long to catch it, compared to other diseases.

Does the delta variant produce different symptoms?

The most common symptoms of COVID-19 are fever or chills, cough, shortness of breath or difficulty breathing, fatigue, and body aches. Doctors and researchers are still learning about the delta variant, but there have been some reports that headache, runny nose, and sore throat are more common with this strain, while cough and loss of smell are less common.

Does the delta variant create more virus in the body, even in people who have been vaccinated?

The amount of virus is known as the viral load. While there are different ways to measure the amount of virus in an infected person, the studies in the CDC report mostly rely on an approach called the cycle threshold. This is a way of estimating the amount of virus by measuring how many times you need to copy the viral RNA with PCR to reach a detectable quantity of it. The higher the starting viral load, the fewer copies you need to make, resulting in a lower cycle threshold.

Studies have estimated that the delta variant leads to 1,000-fold higher viral load. The CDC document takes a closer look at some case studies. For example, a study from India found that among breakthrough cases — infections in vaccinated people — those involving the delta variant had signs of higher viral loads than non-delta cases. The CDC’s own preliminary data from monitoring cases nationwide echo this, showing that delta breakthroughs have around 10-fold higher viral load than breakthrough cases with the alpha variant. And there may be signs that delta sticks around for longer: a study from Singapore measured high viral loads for a longer period of time in people infected with the delta variant: a median of 18 days, compared to 13 days for the original 2020 strain.

What do I need to know about spreading the virus if I’m vaccinated?

In the data released Friday from Massachusetts, the cycle thresholds were similar in vaccinated and unvaccinated people, suggesting they carried similarly high levels of virus. In a statement, Walensky said this suggests a high risk of transmission.

Offit said it’s important to remember that these are viral loads in people who were infected, which is less likely to occur if you have been vaccinated.

This may allude to another factor at play in transmission: where the virus is thriving in the body. A recent study from the National Institutes of Health’s Vaccine Research Center suggests that in an animal model, the Moderna vaccine prompts an immune response in both the upper airway (nose) and lower airway (lungs), but the virus was cleared faster in the lungs than nose. This makes sense, Offit said, because of how shots work.

“The goal of the vaccine is to keep you from being hospitalized or dying which typically is due to pneumonia,” Offit said, which it accomplishes by triggering an effective immune response in the lungs. With virus lingering in mucosal surfaces like the nose and other parts of the upper airways, it can still spread to others, but Offit said the current vaccines are achieving their main goal of averting potentially fatal lung complications.

Michael LeVasseur, a Drexel University epidemiologist who alerted the CDC to the Massachusetts data, acknowledged that some of the emerging evidence is indirect, and that some of the studies are fairly small. But because the findings are consistent with each other, the CDC was on solid ground to revise its recommendations and again encourage mask wearing, he said.

“For the past year and a half, I’ve been preaching ‘better safe than sorry,’ ” he said. “Putting a mask on certainly doesn’t hurt.”

The CDC now recommends that even vaccinated people mask up indoors in public spaces in high-transmission areas. This is also important for vaccinated people living in a household with unvaccinated or immunocompromised people, regardless of transmission in the area.

Is getting over a delta infection the equivalent of a booster shot for vaccinated people?

Possibly, although it may depend on how the person responded to the initial vaccine. But Offit expects such a breakthrough infection would boost immunity. “It would broaden their immune response and make for a longer duration of immunity,” he said, similar to what has been seen with chickenpox in the past.

In other words, a person infected with the delta variant could develop antibodies and other defenses specific to that strain of the virus. In theory, this would constitute an extra layer of protection for a vaccinated person who has developed antibodies against the original strain, although evidence is yet to come.

Unpublished data released by Pfizer earlier this week suggest that even a booster dose of their vaccine — developed for earlier strains of the virus — can prompt a five-fold increase in antibodies that respond to the delta variant and even larger increases for older strains, although this has been demonstrated in only around 20 people.