Here is what we know:
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1. Delta variant is highly contagious
The CDC said the main difference between the delta variant and previous strains is that delta is highly contagious and likely more severe. Plus, breakthrough infections caused by delta may be as transmissible as unvaccinated cases.
"This is an incredibly, incredibly contagious version of virus, it's almost like a whole different virus," Dr. Ashish Jha said. "And CDC is estimating that it is probably also more deadly."
2. Symptoms are the same
The symptoms are generally the same as other COVID-19 strains: fever, cough, shortness of breath, loss of taste or smell, fatigue, respiratory congestion, nausea or vomiting, and diarrhea. However, physicians are seeing people getting sick quicker, especially younger people. Vaccinated people who contract the variants are either asymptomatic or have mild symptoms.
3. Unvaccinated people are at risk
Nationally, 97% of patients hospitalized with COVID-19 are unvaccinated, as of July 22.
4. Vaccination is the best protection but breakthrough cases are still possible
Vaccines prevent more than 90% of severe diseases, but may be less effective at preventing infection or transmission of the delta variant, the CDC said. Therefore, there could be more breakthrough infections and more community spread despite people's vaccination status.
"Current vaccines continue to provide strong protection against severe illness and death, but the delta variant is likely responsible for increased numbers of breakthrough infections -- breakthroughs that could be as infectious as unvaccinated cases," Dr. John Brownstein, an ABC News contributor, said. "This internal document highlights the challenge we all now face in combating the most transmissible variant of COVID so far."
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5. Vaccinated people can spread the variant
The CDC cites new evidence that vaccinated people with breakthrough infections could carry enough virus in their noses and throats to infect others. COVID-19 vaccines greatly reduce the chance of severe illness and death and remain effective against variants, including the delta variant. But it's still possible to get infected. Masking could prevent the spread of the virus to children too young for vaccination and people with weak immune systems.
6. Experts recommend wearing a mask even if vaccinated
The CDC on Tuesday, citing new science on the transmissibility of the delta variant, changed its mask guidance to now recommend everyone in areas with substantial or high levels of transmission -- vaccinated or not -- wear a face covering in public, indoor settings. In short, the vaccine protects you. A mask protects others in case you are carrying the virus without knowing it.
7. More COVID-19 variants are likely
Four notable variants already exist in the United States:
B.1.1.7 (Alpha): This variant was first detected in the United States in December 2020. It was initially detected in the United Kingdom.
B.1.351 (Beta): This variant was first detected in the United States at the end of January 2021. It was initially detected in South Africa in December 2020.
P.1 (Gamma): This variant was first detected in the United States in January 2021. P.1 was initially identified in travelers from Brazil, who were tested during routine screening at an airport in Japan, in early January.
B.1.617.2 (Delta): This variant was first detected in the United States in March 2021. It was initially identified in India in December 2020.
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The lambda variant out of South America is also emerging. Health experts urge that if people want to get back to normal, a significant portion of the population needs to be vaccinated. As long as a chunk of people across the world are unvaccinated, new strains of the virus will continue to develop and cause problems.
8. COVID-19 cases are surging in the U.S. largely due to the highly contagious delta variant
Check out the map below to see how the CDC is tracking COVID-19 transmission levels by county.
Information from ABC News, Associated Press, Centers for Disease Control and Prevention, UC Davis Health and the World Health Organization