• "To find that strain, what we need to do is to take a percentage of the samples that are diagnosed and do deep genetic analysis, and (in) the U.S., our capacity hasn't been spectacular," said Dr. Nahid Bhadelia.
• A smaller sample of virus sequences means the new mutation could already be present in the country without having been detected, the CDC says.
• The U.S. has only sequenced about 51,000 infections of the 17 million total cases in the country, while the U.K. has completed 125,000 sequences so far.

The new, highly transmissible coronavirus strain could be circulating undetected in the U.S., according to one expert, missed because of COVID-19 testing challenges.

"To find that strain, what we need to do is to take a percentage of the samples that are diagnosed and do deep genetic analysis, and (in) the U.S., our capacity hasn't been spectacular," Dr. Nahid Bhadelia, the medical director of the Special Pathogens Unit at Boston Medical Center, told "The News with Shepard Smith" on Monday.

"If the strain is here, we might just be missing it because the holes in our net are too wide," Bhadelia said.

The new variant, which first emerged in the U.K., has not been detected in the U.S., according to the Centers for Disease Control and Prevention. But a smaller sample of virus sequences — a process through which labs detect virus variations — means the new mutation could already be present in the country without having been detected, the agency says.

The U.S. has only sequenced about 51,000 infections of the 17 million total cases in the country, while the U.K. has completed 125,000 sequences so far — the most of any nation, according to the CDC.

Infections in the U.K. hit their second-highest daily total on Christmas Eve, with the country recording more than 39,000 new COVID-19 cases that day.

Starting Monday, the U.S. will require all travelers flying from the U.K. to show proof of a negative COVID-19 test amid growing concerns about the new variant of the virus.

Bhadelia, an NBC News medical contributor, says the travel restriction doesn't go far enough. 

"You can still have people who were negative when they tested, and get on the plane, and turn out positive later," she said. "So that's why I think testing needs to be linked to some sort of quarantine."

Japan has taken more stringent measures and stopped all foreign national arrivals. More than a dozen countries have reported cases of the fast-spreading mutation including Canada, France, South Africa, Australia, Japan and South Korea. 

Health experts in the U.K. and U.S. noted that while the new variant appears to be more transmissible, there is no evidence yet that it is more deadly. Bhadelia warned that the findings should not make people complacent when it comes to the new strain.

"The troublesome thing is that even though we don't think it increases mortality, the fact that it can be transmitted more easily is also more of a concern because the more people who get infected, the more people eventually end up in the hospitals, and potentially pass away," she added.