The C.D.C. director reaffirms that vaccinated people in the U.S. don’t need masks in most situations.

The director of the Centers for Disease Control and Prevention on Wednesday stood by advice that people fully vaccinated against the coronavirus do not need to wear masks in most situations, but added that there are instances where local authorities might impose more stringent measures to protect the unvaccinated.

The comments came after the World Health Organization recently reiterated longstanding guidance that everyone, vaccinated or not, wear masks and take other precautions, following a global surge in infections of the highly contagious Delta variant. On Monday, Los Angeles County recommended that even fully vaccinated people wear masks indoors.

In May, the C.D.C. advised that fully vaccinated Americans could forgo masks in most settings. The agency’s director, Dr. Rochelle P. Walensky, said during multiple television appearances on Wednesday that local changes to masking and distancing rules were consistent with those guidelines.

“We have always said that this virus is an opportunist, and in areas where we still have rates of low vaccination, that is where the virus is likely to take hold,” she said on “Good Morning America.” “We are still seeing uptick of cases in areas of low vaccination, and in that situation we are suggesting that policies be made at the local level.”

Dr. Walensky added that the W.H.O.’s blanket suggestion that both vaccinated and unvaccinated individuals wear masks was informed by its global purview. “The W.H.O. really does have to make recommendations for an entire world,” and many countries have far less access to vaccines than the United States, she said.

The Delta variant now accounts for about one in every four infections in the United States, according to new estimates this week from the C.D.C. But cases across the country have plummeted in recent months, along with hospitalizations and deaths.

Still, the average rate of U.S. vaccinations a day has decreased by about 75 percent from the peak reported in mid-April. As of Tuesday, about 46 percent of people of all ages and about 57 percent of adults are fully vaccinated, according to federal data.

Vaccines consistently protect people from the variants circulating in the United States, including the Delta variant, Dr. Walensky noted on NBC’s “Today” show on Wednesday. But “local policymakers need to make policies for their local environment,” she said, especially in parts of the country with low rates of vaccination.

“Those masking policies are not to protect the vaccinated — they are to protect the unvaccinated,” she added, noting that “everybody should consider their own situation if they would feel more comfortable wearing a mask.”

In Los Angeles County, where half of the population is fully vaccinated, health officials recommended that everyone wear masks indoors because of an uptick in cases and an increase in infections with the Delta variant. Officials in other large cities, like New York and Chicago, indicated earlier this week that they did not plan to change their guidance on masks.

On Tuesday, Dr. Anthony S. Fauci, the government’s top infectious disease expert, said on CNN that he was particularly concerned about the Delta variant causing outbreaks in states, cities and counties with low vaccination rates.

“It’s almost like it’s going to be two Americas,” he said, noting that areas where relatively fewer people are fully vaccinated — as is the case in many parts of the South — are far more at risk than regions with high inoculation rates.

“If you are vaccinated, you diminish dramatically your risk of getting infected and even more dramatically your risk of getting seriously ill,” Dr. Fauci said. “If you are not vaccinated, you are at considerable risk.”

Dr. Vivek Murthy, the United States surgeon general, said on CNN on Wednesday morning that he saw nothing wrong with local authorities imposing new restrictions if they seemed necessary.

“If I was in a community that had a low vaccination rate, as a number of our communities do around the country, if I was seeing a rise in the Delta variant, I think it’s perfectly reasonable to think about different approaches we could take to limit spread to try to get ahead of it,” Dr. Murthy said.