Another 2 cases of British coronavirus variant found in Mass.

Yauwu Tang, 77, of Wayland was vaccinated in Needham this week at Tripadvisor's temporarily vacant main office in Needham.David L. Ryan/Globe Staff

The US Centers for Disease Control and Prevention says another two cases have been found in Massachusetts of the British coronavirus variant that public health officials are worried could sweep across the nation.

The state has now identified a total of seven cases, according to the variant tracking page periodically updated by the CDC.

Nationally, according to the CDC, a total of 618 coronavirus variant cases have been found. That includes 611 cases of the British variant. Five cases also have been found of another worrisome variant, from Brazil, and two cases have been documented of a variant from South Africa.

The arrival of the variants in the United States comes at a time that the coronavirus surge that began in the fall appears to be subsiding, both nationally and in Massachusetts. CDC Director Rochelle Walensky said this week that cases and hospitalizations nationally “appear to be in a consistent downward trajectory,” and officials are hoping deaths will follow suit, beginning to decline in the coming weeks.

But officials are concerned about the variants. The British variant could become the dominant strain in the US by March and lead to a surge of cases and deaths, the CDC has warned. The CDC is promising to step up its efforts to detect the spread of the variants.

The British variant has an infection rate 25 percent to 40 percent higher than that of other forms of the coronavirus, Public Health England has estimated. Adding to the concern: Some preliminary evidence suggests the variant may also may be more deadly. And, at least in a few cases, the variant has picked up another mutation that appears to make it more resistant to vaccines.

The CDC says it would like to know how widely the variants have spread, how the disease caused by the variants is different, and how the variants may affect existing tests, treatments, and vaccines.

It’s crucial, experts say, that millions of Americans be vaccinated before the variants can cause problems.

“It’s going to be a very close one because we have to get a lot of people, especially high-risk, elderly people, vaccinated before these variants take hold,” Dr. Ashish Jha, dean of the Brown University School of Public Health, said Thursday on NBC’s “Good Morning America.” “That’s going to happen over the next six to eight weeks, the variants. And that’s really the time frame in my mind for getting high-risk people vaccinated.”

The impact of the variants is uncertain, putting the country is in an “eerily similar” situation to where it was a year ago when the coronavirus was first arriving in the US, William Hanage, an associate professor of epidemiology at the Center for Communicable Disease Dynamics at the Harvard T.H. Chan School of Public Health, wrote in a Washington Post opinion piece Thursday.

“A year ago, we were nervously tracking a virus with uncertain properties, which the world was not taking seriously enough, waiting to see what it did next. Now we find ourselves nervously tracking variants with properties that are uncertain but which are surely enough to take seriously. It is reasonable to think they might be very serious indeed,” he wrote.

Viruses regularly change through mutation. Scientists are monitoring how the virus that causes #COVID19 is changing and where variants that may spread more easily are found.

See where U.S. cases caused by recent variants have been found: https://t.co/gKlqUvjWKO. pic.twitter.com/9Y2G21G3Hn

— CDC (@CDCgov) February 4, 2021

Material from Globe wire services and previous Globe stories was used in this report.