As Covid cases rise in more than half of the United States, concerns about troubling new variants loom.

"States are seeing a growing proportion of their Covid-19 cases attributed to variants," Centers for Disease Control director Dr. Rochelle Walensky said that during a White House briefing Monday. At the same time, many states are loosening restrictions, which is "a serious threat" that could undo the progress the U.S. has made, she said.

In Europe, a third surge is being driven by the more virulent variant that was first discovered in the U.K., called B.1.1.7, leading parts of France, Germany and Italy to reintroduce lockdowns and restrictions. (Many experts believe that the U.S. is a few weeks behind Europe.)

Walensky said she is worried that if action is not taken now, "we will have another avoidable surge," during the briefing Monday. Here's what you need to know about the variants in the U.S. and what to do about it:

Which Covid variants are in the U.S.

B.1.1.7: Initially identified in the U.K. in the fall of 2020, the CDC projects that this more contagious (it's 50% more transmissible compared to SARS-CoV-2) and potentially more deadly variant could become the dominant strain in the U.S. by the end of March.

There are currently more than 6,300 reported cases of the B.1.1.7 variant in the U.S., with the most cases in Florida (1,040 reports), Michigan (616 reports) and California (471 reports), according to CDC data. "The B.1.1.7 variant is estimated to be responsible for 9% of cases in New Jersey and 8% in Florida," Walensky said Monday.

B.1427/B.1429: This newly identified variant, aka the "California" or "West Coast" variant, is estimated to account for 52% of Covid cases in California, 41% in Nevada and 25% in Arizona, Walensky said Monday. It is 20% more transmissible than SARS-Cov-2, according to the CDC.  

The CDC has officially classified B.1427/B.1429 as a variant of concern, which means there is evidence that it leads to an increase in transmissibility and more severe disease, as well as a significant reduction in neutralizing antibodies generated by vaccines, and reduced effectiveness of treatments and vaccines.

B.1.351: Originating in South Africa, this strain is also 50% more transmissible, and accounts for nearly 200 Covid cases in the U.S., primarily in South Carolina, Maryland and Virginia, according to the CDC.

Data suggests that people who have had Covid can be reinfected with this strain: Researchers in South Africa found that 2% of people who had a previous version of Covid were reinfected with a variant.

P.1: This strain was first detected in travelers from Brazil, who were tested during a routine screening at an airport in Japan in January, according to the CDC.

"This variant contains a set of additional mutations that may affect its ability to be recognized by antibodies," the CDC says. There is also evidence that this strain is more transmissible and could make vaccines less effective.

On Saturday, the strain from Brazil was detected in New York City. There are 21 cases of the P.1 strain in Florida and 54 nationwide, according to data from the CDC.

B.1.526: In early March, Dr. Anthony Fauci said that the Biden administration would be taking a new Covid strain that likely originated in the Washington Heights neighborhood of New York City "very seriously."

Early data suggests that this variant can elude protection provided by vaccines and therapeutic antibody treatments.

A concern is that the New York City mutation, called B.1.526, could cause reinfection in people who have already had Covid, former FDA commissioner Scott Gottlieb said during an interview with CBS' "Face the Nation" Sunday. Citing surging cases in "pockets of New York City," Gottlieb said that it's too soon to tell whether people are being reinfected or if people who have yet to be vaccinated are getting infected.

What you can do to avoid the variants

New variants drive home the importance of safety measures, even as people get vaccinated.

"The one thing you want to do is to make sure you do not allow that to continue to spread," Fauci told CBS' "Face the Nation" on March 7. When the virus cannot spread, it also can't mutate further.

The ways to do that include getting vaccinated and maintaining the public health measures that have been in place for over a year, such as social distancing, mask-wearing and practicing hand hygiene. "That's what you can do to prevent the spread of a worrisome variant," Fauci said.

This holds true even if you're already fully vaccinated, because although early data suggests that the vaccines may work against some variants, they could be less effective against others. Plus, it's still unclear the effect that vaccines will have against transmission of the virus.

To check your vaccine eligibility status, you can use NBC News' plan your vaccine tool.

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