WASHINGTON — A week of public health reversals from the White House and the Centers for Disease Control and Prevention has left Americans with pandemic whiplash, sowing confusion about coronavirus vaccines and mask-wearing as the delta variant upends what people thought they knew about how to stay safe.

Vaccines remain effective and highly protective against hospitalization and death, even among those infected with the extremely contagious delta variant. Mask-wearing prevents transmission of the virus to those most at risk.

But the crisis President Joe Biden once thought he had under control is changing shape faster than the country can adapt. An evolving virus, new scientific discoveries, deep ideological divides and 18 months of ever-changing pandemic messaging have left Americans skeptical of public health advice. So although the White House had promised a "summer of joy," the nation is instead caught in a summer of confusion.

"While we desperately want to be done with this pandemic, COVID-19 is clearly not done with us, and so our battle must last a little longer," Dr. Rochelle P. Walensky, the director of the CDC, told reporters on Monday. "This is hard. This is heavy. But we are in this together."

Monday was another day that underscored the crosscurrents for the nation's leaders as their efforts at a disciplined public health campaign collided yet again with the chaotic nature of the pandemic. Instead of a consistent message, the result was another dizzying jumble of news stories and divergent announcements.

In Louisiana, a state with one of the lowest vaccination rates, Gov. John Bel Edwards reinstated an indoor mask mandate, as did health officials in San Francisco and six other Bay Area counties. But in New York City, Mayor Bill de Blasio declined to do so, even though such a move would have been in line with CDC guidelines.

The virus continued to scramble traditional politics. In left-leaning Chicago, city officials announced that more than 385,000 people had attended the four-day Lollapalooza music festival — and Mayor Lori Lightfoot defended it. In Washington, Sen. Lindsey Graham, R-S.C., a longtime supporter of former President Donald Trump, announced that he had tested positive for the coronavirus, but said his symptoms had been mild, which he attributed to having received the vaccine.

"I feel like I have a sinus infection," Graham wrote on Twitter. "Without vaccination I am certain I would not feel as well as I do now. My symptoms would be far worse."

Nationally, caseloads continued to climb. The country reported a daily average of nearly 80,000 new infections on Sunday, up from about 12,000 in early July, according to a New York Times database. A spate of scary news stories about unvaccinated people dying from COVID-19 appears to have accomplished what Biden could not: The nation finally reached the White House's target, initially set for July 4, of having 70% of American adults at least partially vaccinated.

Some experts say the CDC is to blame for some of the confusion. After saying in May that vaccinated people could go maskless both indoors and outdoors, the agency did an about-face last week, once again recommending indoor masking for everyone — vaccinated or not — in places where the virus is spreading rapidly.

Only days later did a leaked document deliver the grim reasoning: Delta is as contagious as chickenpox and spreading even among the fully vaccinated, which puts unvaccinated people at risk and poses the threat of yet another viral mutation that could evade vaccines.

"The delta variant is different from prior strains," Walensky said. "I understand this is all frustrating news, and I share this frustration."

A senior administration official, who spoke on the condition of anonymity to discuss the administration's thinking, conceded on Monday that many Americans remained perplexed after the flurry of sometimes difficult and seemingly contradictory information.

Another administration official said Biden would address the nation later this week — the second time in less than a week — to reiterate and clarify his main takeaway points: The vaccines are safe and effective. The reason even vaccinated people have to mask up again is that so many people are unvaccinated. So go get your shots and tell your friends and neighbors to do the same.

"The thing that hasn't changed is the need to get vaccinated; the thing that hasn't changed is that masks do work and they protect you," said Dr. Carlos del Rio, an infectious disease specialist at Emory University who lectures on crisis communications.

But del Rio said the CDC made a misstep in May when it told vaccinated Americans they did not need to wear masks — not because the science behind the recommendation was wrong, but because the move led everyone to doff their masks and prompted states, localities and retail businesses to abandon their mask requirements, which enabled the delta variant to flourish.

"That was scientifically correct from a virology standpoint," del Rio said of the earlier recommendation. "It was wrong from a behavioral science standpoint."

The new recommendation — that even vaccinated people wear masks in areas of the country where the virus is spreading rapidly — is far more nuanced, leaving state and local leaders to navigate their own paths and making it difficult for residents to know how to behave. Republican opponents of the administration, meanwhile, have lampooned the shifting advice.

In the House, Republican lawmakers revolted against a mask requirement even as Sean Hannity of Fox News urged his viewers to get vaccinated. Yet former President Barack Obama plans to go ahead with a star-studded party on Martha's Vineyard to celebrate his 60th birthday with hundreds of people. A spokeswoman for Obama said that the party was being held outside and not in an area of high transmission, and that the former president would abide by all CDC guidelines.

Across the country, the questions are piling up again: Can I eat inside at a restaurant or bar? What about a sporting event? Should children be wearing masks when they go to school in September? Will a vaccine for children be available by then and will it be required? What — exactly — are people supposed to be scared of? And what should they do about it?

There is no single answer. The risk is different for different people, depending on whether they are vaccinated and the level of virus in their community. At the same time, the pandemic is fast-moving and ever-changing, which is part of the CDC's challenge.

"They are in a bit of a no-win situation — this is very challenging to message on," said Jen Kates, a senior vice president of the Kaiser Family Foundation. "What's happening is this is real-time public health messaging in a pandemic around data that is just emerging. That is just the reality, and that doesn't necessarily provide comfort or always the kind of answers that people understand."

At the White House on Monday, Jen Psaki, the press secretary, confronted the messaging challenge head on. She cited statistics showing that the vaccines prevent most illness and death, insisted that the country would not go back to large-scale shutdowns and noted that cases like that of Graham remain exceedingly rare.

But clarity has been elusive as the virus — along with the scientific understanding about how best to combat it — has continued to morph, sometimes day by day.

Even as the CDC has recommended mask-wearing for everyone in areas of rapid spread, the White House has been making a different and somewhat contradictory push by requiring unvaccinated federal workers — but not those who are vaccinated — to wear masks at work.

Experts agree that some of the confusion could have been avoided if the CDC and Walensky had been more transparent and simply released the data underlying the latest mask recommendations, which later leaked in the news media.

In addition, del Rio said, the agency mangled its most important message — that Americans need to get vaccinated — by focusing so much on "breakthrough infections" in vaccinated people, which are still very rare.

"It has distracted from the point that we need to make, which is that breakthrough infections are very uncommon and your biggest risk continues to be being unvaccinated," he said. "I'm concerned that because people are so focused on breakthroughs, we are losing the forest for the trees."

Public health experts often say that in any infectious disease outbreak, messaging is not part of the response: It is the response. The CDC has an entire arm of its website devoted to training for health communicators, with links to podcasts, sample social media posts and a 120-page "Health Communication Playbook."

"This isn't just about science," Kates said. "It's about human behavior and decisions that actual people have to make, and those decisions affect the course of the pandemic."

Public health communications are also inherently political, because the message comes from political leaders. That, too, has posed a challenge for the White House. When Biden became president, he vowed that he and his White House aides would not meddle in CDC decisions, as was the case during the Trump administration.

But Dr. Tom Frieden, a former CDC director, said in a recent interview with NPR that the Biden White House may have gone "too far in the other direction," leaving Walensky, who has no prior government experience, and the CDC to manage their own messaging without input from White House communications experts.

"You sometimes hear people say, 'Let's get the politics out of public health,'" Frieden said. "Well, public health is the activity of communities getting safer and healthier. And there are many political decisions that happen during that process. What's problematic is when partisanship gets in the way."