CDC: 44 cases of UK variant, two cases of South Africa variant now detected in Mass.

CDC officials say "rapid and widespread uptake of effective vaccines" is key to stay ahead of emerging variants.Erin Clark/Globe Staff

The number of confirmed coronavirus variant cases in Massachusetts continues to grow, as public health officials warn that the variants could silently sweep across the nation and revive a pandemic that, for now at least, appears to be on the decline.

The US Centers for Disease Control and Prevention on Thursday evening reported 44 cases in Massachusetts of the variant that emerged in the United Kingdom, and two cases of the variant that emerged in South Africa. That was up from 34 cases of the UK variant plus one case of the South Africa variant reported as of Tuesday.

Nationwide, the CDC has tallied 1,523 cases of the UK variant, 21 cases of the South African variant, and five cases of a worrisome variant that first emerged in Brazil.

Experts say the numbers don’t reflect the true extent of the variants’ spread because of the nation’s underdeveloped genetic surveillance system.

“That is an undercount,” Johns Hopkins epidemiologist Caitlin Rivers said of the CDC numbers earlier this week.

The CDC says the variants “seem to spread more easily and quickly than other variants, which may lead to more cases of COVID-19. An increase in the number of cases will put more strain on health care resources, lead to more hospitalizations, and potentially more deaths.”

The agency says it is “monitoring the situation closely.”

Daily averages for both coronavirus cases and deaths have dropped by about half nationwide over the past two weeks, according to data from Johns Hopkins University. And as of Wednesday, more than 40 million people — about 12 percent of the population — had received at least one dose of a vaccine.

But experts including Dr. Anthony Fauci, President Biden’s chief medical adviser on the pandemic, and CDC Director Dr. Rochelle Walensky say the downward trend could reverse itself if new variants take hold.

“I know these variants are concerning, especially as we’re seeing signs of progress,” Walensky said during a White House briefing on Wednesday.

“While cases and hospitalizations continue to move in the right direction, we remain in the midst of a very serious pandemic, and we continue to have more cases than we did even during ... last summer’s peak,” Walensky said. “The continued spread of variants that are more transmissible could jeopardize the progress we have made in the last month if we let our guard down.”

In a Viewpoint article posted online Wednesday in the Journal of the American Medical Association, Walensky, Fauci, and Dr. Henry Walke, another top CDC official, said “a concerted and well-coordinated public health effort, together with rapid and widespread uptake of effective vaccines, is essential to remain ahead of the inevitable evolution of variants that could dangerously accelerate the trajectory of the pandemic.”

Material from Globe wire services was used in this report.