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At least one of two or more new SARS-CoV-2 variants have likely made it into community spread in the United States, but they're unlikely to have much impact on vaccine effectiveness or standard precautions, CDC officials said at a briefing.

The first reported U.S. case of the highly transmissible coronavirus variant first seen in Great Britain (dubbed SARS-CoV-2 VUI 202012/01) was confirmed on Tuesday in a Colorado man with no travel history. The strain may be and up to 70% more transmissible than the "wild type" virus, although not more severe or more deadly. The British variant has likely been circulating there since September 2020.

Another extra-transmissible variant from South Africa arose independently and appears to be now, and has reported another variant arose there. Neither has yet been isolated in the United States.

"We expect to see new variants emerge over time," Henry Walke, MD, MPH, CDC's COVID-19 incident manager, said during the press briefing Wednesday.

Exactly how many variants have emerged so far is "impossible to answer," because it depends on how you set the threshold of how different a virus needs to be to represent a variant, said Gregory Armstrong, MD, director of the CDC's Advanced Molecular Detection Program.

The government has been working on a strain surveillance system since November with two national labs to analyze genetic sequences of SARS-CoV-2 samples from around the country to determine penetrance of the newly identified variants. Armstrong said findings are expected over the next few days.

How well the vaccines and other treatments will work against these variants can't be known for sure yet. suggested that convalescent plasma doesn't work as well against variants.

However, "from what we know from experience with this mutation and other mutations is it is unlikely to have a large impact on vaccine-induced immunity or existing immunity from previous strains," Armstrong said.

"We do know that some of these mutations can result in reduced efficacy of monoclonal antibodies," which bind one particular portion of the target antigen, Armstrong noted. But immune responses stemming from infection or vaccines recognize multiple parts of the spike protein, he added.

"Experts are generally in consensus that mutations like this are unlikely to cause a large impact on the neutralization," he said. "It may cause a small impact, but keep in mind it's likely that the amount of immunity that is induced by either natural infection or by vaccine is great enough that slightly decreased titer may not have any noticeable effect at all."

In vitro studies of the U.K. variant are underway and should provide a more definitive answer, Armstrong said.

As well, Walke said, there's no reason to think that the recommended measures to reduce spread -- washing hands, maintaining distance, wearing a mask, and avoiding large gatherings -- won't work against these variants.

"We're pushing our same messages that we've been pushing all along," he said. "Even though we do think this variant that originated in the U.K.... potentially may transmit more than the wild type, we firmly believe that our mitigation measures in our guidance now will work."

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