The Delta (B.1.617.2) coronavirus variant originally discovered in India last December has now become the most dominant — and worrisome — strain of the coronavirus circulating globally.

The most transmissible variant yet, Delta is roughly twice as contagious as the original COVID strain, and as much as 60 percent more contagious than the Alpha (U.K./B.1.1.7) variant, which itself caused numerous waves of the pandemic around the world. As of the end of July, Delta has been detected in nearly every country in the world and has fuelled rapid outbreaks in places where vaccination rates are low.

Delta has also quickly become the dominant strain in the U.S., where it is estimated to account for more than 80 percent of all new sequenced cases in the country. Delta’s rapid spread among the unvaccinated in the U.S. has prompted the CDC to once again advise that everyone wear face masks indoors in public wherever there is significant or higher transmission of the variant. And there is evidence that Delta may come with a higher risk of severe illness for the unvaccinated.

Though COVID vaccines continue to offer strong protection against Delta — and particularly against severe illness and death from the strain — the variant is believed to be at least partially able to avoid antibodies produced by a previous coronavirus infection or vaccination. Below is what we know about the Delta variant. Here are five things you need to know about the Delta variant.

1. Delta is more contagious than the other virus strains.

One thing that is unique about Delta is how quickly it is spreading, says F. Perry Wilson, MD, a Yale Medicine epidemiologist. Around the world, he says, “Delta will certainly accelerate the pandemic.” The first Delta case was identified in December 2020, and the variant soon became the predominant strain of the virus in both India and then Great Britain. By the end of July, Delta was the cause of more than 80% of new U.S. COVID-19 cases, according to CDC estimates.

The July CDC report confirmed this after an outbreak that occurred in Provincetown, Mass., after a crowded July 4 weekend, which quickly turned into a cluster of at least 470 cases. While the number of reported breakthrough cases in general has been very low in the U.S., three quarters of those infected in Provincetown were people who had been immunized. According to the CDC, even people with “breakthrough cases” carry tremendous amounts of virus in their nose and throat, and, according to preliminary reports, can spread the virus to others whether or not they have symptoms.

The CDC has labeled Delta “a variant of concern,” using a designation also given to the Alpha strain that first appeared in Great Britain, the Beta strain that first surfaced in South Africa, and the Gamma strain identified in Brazil. (The new naming conventions for the variants were established by the WHO as an alternative to numerical names.) “It’s actually quite dramatic how the growth rate will change,” says Dr. Wilson, commenting on Delta’s spread in the U.S. in June. Delta was spreading 50% faster than Alpha, which was 50% more contagious than the original strain of SARS-CoV-2, he says. “In a completely unmitigated environment—where no one is vaccinated or wearing masks—it’s estimated that the average person infected with the original coronavirus strain will infect 2.5 other people,” Dr. Wilson says. “In the same environment, Delta would spread from one person to maybe 3.5 or 4 other people.” “Because of the math, it grows exponentially and more quickly,” he says. “So, what seems like a fairly modest rate of infectivity can cause a virus to dominate very quickly.”

2. Unvaccinated people are at risk.

People who have not been fully vaccinated against COVID-19 are most at risk. In the U.S., there is a disproportionate number of unvaccinated people in Southern and Appalachian states including Alabama, Arkansas, Georgia, Mississippi, Missouri, and West Virginia, where vaccination rates are low. (In some of these states, the number of cases is on the rise even as some other states are lifting restrictions because their cases are going down).

Kids and young people are a concern as well. “A recent study from the United Kingdom showed that children and adults under 50 were 2.5 times more likely to become infected with Delta,” says Dr. Yildirim. And so far, no vaccine has been approved for children 5 to 12 in the U.S., although the U.S. and a number of other countries have either authorized vaccines for adolescents and young children or are considering them. “As older age groups get vaccinated, those who are younger and unvaccinated will be at higher risk of getting COVID-19 with any variant,” says Dr. Yildirim.  “But Delta seems to be impacting younger age groups more than previous variants.”

3. Delta could lead to ‘hyperlocal outbreaks.’

If Delta continues to move fast enough to accelerate the pandemic, Dr. Wilson says the biggest questions will be about the heightened transmissibility—how many people will get the Delta variant and how fast will it spread? The answers could depend, in part, on where you live—and how many people in your location are vaccinated, he says. “I call it ‘patchwork vaccination,’ where you have these pockets that are highly vaccinated that are adjacent to places that have 20% vaccination,” Dr. Wilson says. “The problem is that this allows the virus to hop, skip, and jump from one poorly vaccinated area to another.”

In some cases, a low-vaccination town that is surrounded by high vaccination areas could end up with the virus contained within its borders, and the result could be “hyperlocal outbreaks,” he says. “Then, the pandemic could look different than what we’ve seen before, where there are real hotspots around the country.”

Some experts say the U.S. is in a good position because of its relatively high vaccination rates—or that conquering Delta will take a race between vaccination rates and the variant. But if Delta keeps moving fast, multiplying infections in the U.S. could steepen an upward COVID-19 curve, Dr. Wilson says.

So, instead of a three- or four-year pandemic that peters out once enough people are vaccinated or naturally immune (because they have had the virus), an uptick in cases would be compressed into a shorter period of time. “That sounds almost like a good thing,” Dr. Wilson says. “It’s not.” If too many people are infected at once in a particular area, the local health care system will become overwhelmed, and more people will die, he says. While that might be less likely to happen in the U.S., it will be the case in other parts of the world, he adds. “That’s something we have to worry about a lot.”

4. There is still more to learn about Delta.

One important question is whether the Delta strain will make you sicker than the original virus. But many scientists say they don’t know yet. Early information about the severity of Delta included a study from Scotland that showed the Delta variant was about twice as likely as Alpha to result in hospitalization in unvaccinated individuals, but other data has shown no significant difference.

Another question focuses on how Delta affects the body. There have been reports of symptoms that are different than those associated with the original coronavirus strain, Dr. Yildirim says. “It seems like cough and loss of smell are less common. And headache, sore throat, runny nose, and fever are present based on the most recent surveys in the U.K., where more than 90% of the cases are due to the Delta strain,” she says. It’s unclear whether Delta could cause more breakthrough cases—infections in people who have been vaccinated or have natural immunity from a prior COVID-19 infection.

This question is being revisited after the Provincetown outbreak. “Breakthrough is a big question,” Dr. Wilson says. “At least with immunity from the mRNA vaccines, it doesn’t look like it will be a problem.” A Public Health England analysis (in a preprint that has not yet been peer-reviewed) showed that at least two vaccines are effective against Delta.

5. Vaccination is the best protection against Delta.

The most important thing you can do to protect yourself from Delta is to get fully vaccinated, the doctors say. At this point, that means if you get a two-dose vaccine like Pfizer or Moderna, for example, you must get both shots and then wait the recommended two-week period for those shots to take full effect. Whether or not you are vaccinated, it’s also important to follow CDC prevention guidelines that are available for vaccinated and unvaccinated people.

Face masks can provide additional protection and the WHO has encouraged mask-wearing even among vaccinated people. The CDC updated its guidance in July to recommend that both vaccinated and unvaccinated individuals wear masks in public indoor settings in areas of high transmission to help prevent Delta’s spread and to protect others, especially those who are immuno-compromised, unvaccinated, or at risk for severe disease. The agency is also recommending universal indoor masking for all teachers, staff, students, and visitors to K-12 schools.