Daily new cases, hospitalizations and deaths from the delta variant of the coronavirus are all surging as millions of Americans remain unvaccinated, federal health officials warned Thursday.

“Across the board, we are seeing increases in cases and hospitalizations in all age groups,” Centers for Disease Control and Prevention Director Rochelle Walensky said at a White House coronavirus briefing where she and other health leaders reiterated pleas for people to get immunized.

The latest slew of stark statistics: Daily cases passing 100,000 — numbers not seen since February. A seven-day average of hospital admissions up by more than 40 percent from the week before and deaths trending up by roughly the same rate.

There is a bright spot, officials said — a significant boost in the numbers of Americans getting vaccinated. They cited a 90 percent jump in first shots in Tennessee over the last two weeks; an 82 percent increase in Oklahoma and a 66 percent rise in Georgia, among other hopeful signs. Nearly 650 colleges and universities are now requiring shots for students and staff on campus, and more than 100 health care systems are embracing similar mandates, they said.

Also Thursday, the Biden administration announced a new push to vaccinate young people as they head back to school, backing initiatives such as hosting pop-up clinics on campus; sending pediatricians to back-to-school nights to discuss the shots with parents; and incorporating vaccination against covid-19 into physicals for student athletes.

The United States has been vaccinating children as young as 12 since May, but that group remains less likely than adults to have their shots.

“The resources are there and the urgency is there,” Secretary of Education Miguel Cardona said at a news briefing where he said the administration is partnering with groups such as the American Academy of Pediatrics and the National Parent Teacher Association. “Now is the time to get our students back into the classroom, not to be complacent or let politics get in front of what is best for our students across the country.”

Starting Saturday, a White House news release said, a “week of action” will enlist school districts, celebrities, businesses and others to “kick off the school year by encouraging young people to get vaccinated and offering accessible ways to do it in their community.”

The administration continues to focus on communities with low vaccination rates, which officials say are driving the pandemic. White House coronavirus coordinator Jeff Zients pointed to seven Southern states — Florida, Texas, Missouri, Arkansas, Louisiana, Alabama and Mississippi — as accounting for about half of all new infections and hospitalizations over the past week, even as these states represent less than a quarter of the American population.

Here are some significant developments:

The vast majority of people dying, hospitalized and infected with covid-19 are unvaccinated, but Walensky acknowledged Thursday that the numbers she and others have been citing are based on January through June data — a period when most people were unvaccinated and the delta variant was just taking hold. She said officials are working to update the numbers to include the period since the variant became dominant, which has fueled renewed restrictions, including changed mask guidance.

On Wednesday the seven-day average in daily coronavirus infections topped 95,000 for the first time since Feb. 12, according to data tracked by The Washington Post.

White House medical adviser Anthony S. Fauci warned the virus’s resurgence, which is pushing some hospitals to the brink again, also threatens the country’s long-term ability to fight the pandemic. The more the coronavirus spreads, he said, the more likely it is to mutate into forms that might be resistant to the current vaccines.

“You prevent [its continued mutation] by not allowing the virus to freely circulate, finding vulnerable targets,” Fauci said at the briefing. “You protect the vulnerable targets, who are unvaccinated people, by vaccinating them.”

He acknowledged that some people who are immunocompromised have not mounted a full response from the standard vaccine regimen, and said that making booster shots available to them is a “very high priority.”

“It is extremely important for us to move to get those individuals their boosters,” he said. “And we are now working on that, and will make that be implemented as quickly as possible.”

Infectious-disease expert Anthony S. Fauci on Aug. 5 said the U.S. is working to give additional coronavirus booster shots to at-risk Americans. (Reuters)

The World Health Organization is calling for a moratorium on booster shots to focus resources on poorer nations with dismal vaccination rates. But WHO officials said they do not necessarily oppose additional doses for certain populations who are poorly protected by the standard protocol.

Vaccine maker Moderna said Thursday that the protection offered by its coronavirus shots remains strong — 93 percent effective — six months after full vaccination.

Although the vaccine maintained a high level of protection over six months, executives said the emergence of the delta variant combined with the erosion of immunity over time would probably mean boosters would be needed to keep people safe this winter.

While the decision about when to give boosters resides with government officials, the company announced it has tested three potential booster shots, which have demonstrated “robust antibody responses” and have topped off immunity, bringing antibodies back to the protective levels triggered by full vaccination.

“Our approach is … to defer to public health on when boosters are going to be necessary, but to bring forward the best option,” said Stephen Hoge, Moderna’s president, noting that a third shot of the original formulation seems effective against the known variants. He said the company would wait a few weeks for confirmation that a half-dose that showed promise in studies is the optimal formulation before seeking regulatory clearance.

Pfizer and BioNTech made a similar announcement last week, stating that their vaccine remains 91 percent protective six months after the second dose. The Moderna and Pfizer-BioNTech vaccines are based on messenger RNA technology.

Moderna’s vaccine was authorized for emergency use in adults in the United States in December and has since been cleared for emergency or conditional use in adults in more than 50 countries.

The announcement is encouraging news as the United States prepares to welcome back international travelers — when the time is right. A White House official told The Washington Post on Wednesday that the Biden administration is working on a plan to require nearly all incoming foreign travelers to be fully vaccinated against the coronavirus.

The Biden administration wants to reopen international travel, the White House official said. But after more than a year of barring non-U. S. travelers from dozens of countries including China, Britain, India and South Africa, no timeline has yet been announced.

Zients said Thursday that travel restrictions will remain in place for now given the widespread delta variant, but confirmed that working groups are planning the best way to eventually reopen the country’s borders. He said that foreigners may need “some type of a vaccine requirement” but that “that’s not a decision at this point.”

Bans on international travel to the United States because of the coronavirus were first put in place by President Donald Trump in early 2020. A similar approach was taken by many other countries, including China, Chile and Australia, which all utilized border controls as a tool to keep the virus at bay.

The U.S. government under President Biden has maintained a cautious approach to reopening foreign travel despite pressure from the U.S. airline and tourism industries to reopen travel with low-risk countries.

European lawmakers and business groups have also voiced criticism of the Biden administration for a lack of reciprocity. Vaccinated U.S. tourists have been allowed to return to much of Europe for weeks, while most Europeans continue to be unable to travel to the United States.

The United Kingdom announced last week that it would reopen its borders to fully vaccinated travelers from the United States and Europe. On Thursday, it further relaxed its quarantine requirements and increased the number of countries on its welcome list.

The hyper-transmissible delta variant of the coronavirus has left would-be travelers uncertain. The Post spoke to an expert about how to safely make that call. (The Washington Post)

Canada has said it will begin allowing fully vaccinated American tourists into the country starting Aug. 9, after an unprecedented 16-month ban. Vaccinated travelers from countries other than the United States will be permitted to enter starting on Sept. 7, Prime Minister Justin Trudeau said.

However, some countries are moving the other way. The Israeli Health Ministry said Tuesday that it would require both vaccinated and unvaccinated Americans to quarantine for a week upon arrival, as Israel added the United States to a growing list of “red countries” to which Israeli residents were discouraged from traveling. The regulations are slated to take effect Aug. 11.

The United States still warns against travel to Israel and several European countries — among them Spain, Portugal and Cyprus — as the more contagious delta variant fuels coronavirus outbreaks, mostly among the unvaccinated. The Centers for Disease Control and Prevention and State Department advised Monday against travel to places including Greece, Ireland, Iran and the U.S. Virgin Islands due to rising cases.

Jacqueline Dupree contributed to this report.