More than half of Massachusetts residents have been infected with COVID-19, and an even higher percentage nationwide

More than half of Massachusetts residents and nearly 60 percent of all Americans have been infected with COVID-19, and many of those have occurred just since December, the Centers for Disease Control and Prevention said Tuesday.

The new research vividly illustrates the wide path of Omicron as it raced across the country during the winter.

The numbers show that, in December, only one-third of Americans had been infected with COVID-19; by Feb. 22, the share had exploded to 58 percent.

“I definitely expected we would see an increase. I didn’t expect it to increase this much,” said Dr. Kristie Clarke, the agency researcher who led the study.

The most dramatic rise was among children 11 and younger, with the CDC estimating that by February some 75 percent of children in that age bracket had been infected, compared to just 44 percent in December.

It was “quite striking,” Clarke said.

The data show wide variation among New England states, with 53 percent of Massachusetts and Rhode Island residents infected by mid-February, but only 29 percent of those in Vermont.

CDC researchers based their estimates by extrapolating from the results of tests that analyzed antibody levels in people’s blood from across the country. The antibody tests can identify the infection in people for up to two years afterward, Clarke said. The tests use a type of antibody that is produced after infection but not after vaccination.

While infection provides some protection against the virus, it is unclear how long that shield lasts.

For that reason, Clarke urged people to get vaccinated. ”Vaccination remains the safest strategy for preventing complications from [COVID] infection, including hospitalization among children and adults,” she and her colleagues wrote in the report.

While cases are rising in Massachusetts and many parts of the country, hospitalizations are ticking up much more slowly and deaths are declining.

But even among those hospitalized for COVID, fewer are requiring supplemental oxygen and fewer need treatment in intensive care units, CDC Director Dr. Rochelle Walensky said.

“This is something we need to watch carefully,” she said.

Another study to be released shortly by CDC researchers estimates that for every reported case of COVID, there are at least three other infections that are not reported, the highest level it’s ever been, Clarke said. The results of at-home rapid antigen tests, which have become popular, typically are not reported to state health departments.

Researchers are racing to understand the potential impact of new Omicron subvariants that are quickly taking hold nationwide, notably Ba.2 and the latest, Ba.2.12.1 — both widely prevalent in New England.

The CDC reported Tuesday that Ba.2 accounts for 68 percent of the COVID virus circulating nationally, and Ba.2.12.1 makes up nearly 30 percent of samples that have been sequenced.

Walensky said that Ba.2.12.1 appears to be spreading even faster in the population than its sister, Ba.2, with a transmission advantage of 25 percent.

“We do not anticipate more severe disease from these subvariants but we are still studying it,” she said.

But already, two even newer Omicron variants are on the horizon, Ba.4 and Ba.5.

Massachusetts researcher Dr. Jacob Lemieux sounded a note of caution Tuesday during a briefing by infectious disease specialists in Boston. The two newest variants are spreading quickly in South Africa — where Omicron was first reported in November before encircling the world — and have shown up in several places in the United States, including Texas, Lemieux said.

He said it’s too soon to know whether the newest variants in South Africa are capable of causing more severe illness or death.

“The question is, what is happening in South Africa, is it going to happen in the rest of the world?” said Lemieux, an infectious disease specialist at Massachusetts General Hospital and co-leader of the viral variants program at the Massachusetts Consortium on Pathogen Readiness.

“It does have a bit of a flavor of, ‘Here we go again,’ ” he said.

Lemieux said it’s also unclear what percentage of people in South Africa who had a prior COVID infection are being reinfected with the newest forms of Omicron. He said Omicron is already known for its wily ability to evade vaccines, and researchers are still trying to disentangle which forms of protection, including vaccines, boosters, prior infection, or a hybrid, provide the best shield.

“It’s getting to be a ridiculous number of life histories that are possible between vaccination and prior infection,” he said. “Hybrid immunity seems to be stronger than natural immunity or vaccine-acquired immunity alone.”

A critical missing piece is information that can help the public easily and readily understand how well they are protected, either from their level of antibodies or other measures of the immune system, such as T-cells, said Dr. Ofer Levy, a pediatrics professor at Harvard Medical School and director of the Precision Vaccines Program at Boston Children’s Hospital.

“We haven’t completely hammered out a gold standard for a correlate of protection against this virus, even this deep into the effort,” Levy said.

“We need a much better organized federal-led effort for the scientists to figure out what kind of immune response gives short term, medium term, and long term protection against this virus,” he said. “It hasn’t been done yet in an organized way.”

Levy, who serves on a vaccine advisory committee to federal regulators, noted that he and others on the panel have been pressing for more data from the government and vaccine companies.

“This is a very, very important point because otherwise we are chasing our tail,” Levy said.

Kay Lazar can be reached at kay.lazar@globe.com Follow her @GlobeKayLazar.