The C.D.C. now recommends that people stay ‘up to date’ with Covid vaccine boosters.

Wednesday’s announcement by the Centers for Disease Control and Prevention indicated that they are not changing their definition of full vaccination against coronavirus. However, the agency shifted its focus to the correct regimen and changed how it referred about the shots.

The agency said that three doses of Pfizer-BioNTech or Moderna’s vaccines should be considered “up to date” inoculations, and that Johnson & Johnson recipients should receive a second dose, preferably of Moderna or Pfizer-BioNTech, to also be considered “up to date” with Covid vaccines. Federal law allows people to mix and match brands to boost their immunity.

Federal health officials recommended that Americans change the way they talk about vaccination schedules. Boosters are now recommended for all adults. People 16 years and older are eligible for boosters, a group that could expand as early as Wednesday after the C.D.C.’s vaccine advisory committee weighs whether to advise 12- to 15-year-olds to get them.

“Consistent with how public health has historically viewed or even talked about how we recommend vaccines, we are now recommending that individuals stay up to date with additional doses that they are eligible for,” Dr. Rochelle P. Walensky, the C.D.C. At a White House news conference on Wednesday, the director said.

The C.D.C. did not change the definition of what qualifies as full vaccination — a subject of intense interest to corporations, schools, state health departments and professional sports leagues, which have themselves been reconsidering what it means to be “fully vaccinated.”

“The technical definition of ‘fully vaccinated’ — two doses of an mRNA vaccine or one dose of the J & J vaccine — has not changed,” Kristen Nordlund, a C.D.C. In a statement, a spokeswoman said. “Individuals are considered fully vaccinated once they have received their primary series.”

She added that the agency recommend that people “stay ‘up to date’ by receiving any additional doses they are eligible for, according to C.D.C.’s recommendations, to ensure they have optimal protection.”

Federal officials have typically referred to people being considered “fully vaccinated” as two weeks after a first dose of Johnson & Johnson or a second dose of the Moderna and Pfizer vaccines. While studies have showed that protection against infection has waned in fully vaccinated people and can be strengthened by a booster, two doses still offer strong protection against severe Covid-19 — the true point of vaccination, some vaccine experts have argued.

It is not clear what impact the change will have on institutions. Many schools, businesses and governments have relied on the C.D.C.’s definition of “fully vaccinated” to establish mandates, requiring people to complete a two- or one-dose series in order to go to school, eat at a restaurant or stay employed.

The move on Wednesday, Ms. Nordlund said, was intended to make Covid-19 vaccines “align with standard language C.D.C. uses about other vaccinations.” It also accounted for differences in eligibility for booster shots, since younger adolescents and children are not yet recommended by the C.D.C. for booster doses. Some people are also still not five months out from receiving a second dose of the Pfizer vaccine, six months from a second dose of Moderna or two months from a first dose of Johnson & Johnson, the authorized intervals for boosters.

“C.D.C. recommends that individuals stay ‘up to date’ by receiving any additional doses they are eligible for, according to C.D.C.’s recommendations, to ensure they have optimal protection against Covid-19,” Ms. Nordlund said.

Dr. Anthony S. Fauci and other top federal health officials had urged the administration to alter how it discussed vaccine schedules. They argued that Moderna and Pfizer should be considered three dose vaccines. Officials wanted to preserve what is officially considered a complete vaccination schedule.

That change could have carried significant legal implications, potentially intensifying challenges to vaccination requirements, as the Biden administration’s attempt to mandate that large employers require employees to be vaccinated is already bogged down in the courts.

“If you think about the different requirements,” Jeffrey D. Zients, the White House Covid-19 response coordinator, said at the Wednesday news briefing, “that has not changed, and we do not have any plans to change that.”