As the Delta variant fuels rising U.S. cases, the C.D.C. director warns of a ‘pandemic of the unvaccinated.’

U.S.|As the Delta variant fuels rising U.S. cases, the C.D.C. director warns of a ‘pandemic of the unvaccinated.’

• July 16, 2021, 1:16 p.m. ET

As the highly contagious Delta variant of the coronavirus fuels outbreaks in the United States, the director of the Centers for Disease Control and Prevention warned on Friday that “this is becoming a pandemic of the unvaccinated.”

Cases, hospitalizations and deaths remain far below last winter’s peak and vaccines are effective against Delta, but the agency’s director, Dr. Rochelle P. Walensky, urged people to get fully vaccinated, pleading for people to do it for their family and community.

The number of new virus cases is likely to increase in the coming weeks, and those cases are likely to be concentrated in areas with low vaccine coverage, officials said at a White House briefing on the pandemic.

Dr. Walensky added that “we are seeing outbreaks of cases in parts of the country that have low vaccination coverage, because unvaccinated people are at risk, and communities that are fully vaccinated are generally faring well.”

Delta now accounts for more than half of new infections across the country, and case numbers have been ticking up in every state. There is an average of roughly 28,000 new cases a day, up from just 11,000 less than a month ago.

So far, data suggests that many of the vaccines — including the Pfizer-BioNTech, Moderna, Johnson & Johnson shots — provide good protection against Delta, especially against the worst outcomes, including hospitalization and death. (Receiving a single dose of a two-shot regimen provides only weak protection against the variant, however.) Nearly 60 percent of American adults have been fully vaccinated.

“We have come a long way in our fight against this virus,” Jeffrey D. Zients, the administration’s Covid-19 response coordinator, said at the briefing. “Because we have fully vaccinated more than 160 million Americans, we’ve fundamentally changed the course of this pandemic, from one that puts the lives and livelihoods of all Americans at risk, to one that predominantly threatens unvaccinated people.”

Vaccination rates remain highly uneven, and Delta is already driving case numbers up in areas with low vaccine coverage, including in parts of Missouri, Arkansas and Louisiana. Even Los Angeles County is reinstating a mask mandate for everyone; beginning this weekend, masks will once again be required indoors.

The C.D.C., however, has repeatedly stated that people who are fully vaccinated do not need to wear masks in most situations. But on Friday, Dr. Walensky noted that local officials may want to implement more stringent measures.