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

Cases, hospitalizations, and deaths remain well below last winter’s peak, and vaccines are effective against Delta, but CDC director Dr. Rochelle P. Walensky, urged people to get fully vaccinated for robust protection, pleading, “Do it for yourself, your family, and for your community. And please do it to protect your young children who cannot be vaccinated at the moment. “

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

“Our greatest concern is that we will continue to see preventable cases, hospital admissions and, unfortunately, deaths among the unvaccinated,” said Dr. Walensky. According to a New York Times database, the nation exceeded 34 million cumulative cases as of Friday.

Delta now accounts for more than half of the new infections across the country, and the number of cases has increased in all states. Around 28,000 new cases are reported every day, up from just 11,000 per day less than a month ago.

So far, data suggests that many of the vaccines – including Pfizer-BioNTech, Moderna, and Johnson & Johnson vaccinations – offer good protection against Delta, especially against its worst outcomes, including hospitalization and death. (Receiving a single dose of two-shot therapy, however, offers poor protection against the variant.) Nearly 60 percent of US adults were fully vaccinated, but fewer than 50 percent of Americans were vaccinated; only people aged 12 and over are eligible to participate.

“We have come a long way in our fight against this virus,” said Jeffrey D. Zients, the government’s Covid-19 response coordinator, at the briefing.

The rate of vaccination has slowed considerably since the spring and the rate of vaccination remains very inconsistent. Delta is already skyrocketing case numbers in undervaccinated areas, including parts of Missouri, Arkansas and Louisiana.

The World Health Organization recently reiterated its recommendation that vaccinated people should continue to wear masks, also because of the global spread of Delta.

Updated

July 16, 2021 at 1:16 p.m. ET

However, the CDC has stood by its mask policy, with Dr. Walensky pointed to WHO’s global jurisdiction and the fact that wealthy nations took so many of the recordings available. She added that local officials in the United States can opt for stricter measures to protect the unvaccinated.

On Thursday, the Los Angeles District said that as of this weekend, indoor mask requirements will be reintroduced for everyone, regardless of vaccination status. On Friday, Dr. Walensky pointed out the heterogeneity of the country and said: “These decisions have to be made at the local level.”

“If you have areas with low vaccination and high case numbers, I would say local politicians are considering whether masking would be helpful for their community at this point,” she added.

In New York City, Mayor Bill de Blasio said Friday there are currently no plans to reinstate a mask mandate for everyone across the city, nor did he consider the move necessary. The city recently reported a streak of more than 400 cases per day, up from an average of about 200 per day a few weeks ago. “We have to see it like a hawk,” he said on a radio broadcast, referring to the Delta variant.

Health officials are focusing on hospital stays that have remained low over the past few weeks. According to the city, about 53 percent of city residents are fully vaccinated. Should hospital stays increase, the city will adapt.

“We currently have no plan to change course,” he said. “When we see something that we need to change, we say it right away and call people to arms.”

After narrowly missing a self-imposed target of at least partially vaccinating 70 percent of adults by July 4, the Biden government is trying again to reach out to those who have still not received their vaccinations. Officials also recently announced the creation of surge response teams to help hard-hit states manage delta-driven outbreaks. Missouri and Nevada have already asked for help.