Covid was vanishing last Memorial Day. Cases are five times higher now.

For the third year, Americans are greeting the unofficial start of summer shadowed by the specter of the coronavirus amid rising covid-19 cases and hospitalizations across the country.

The United States is recording more than 100,000 infections a day - at least five times higher than this point last year - as it confronts the most transmissible versions of the virus yet. Immunity built up as a result of the record winter outbreak appears to provide little protection against the latest variants, new research shows. And public health authorities are bracing for Memorial Day gatherings to fuel another bump in cases, potentially seeding a summer surge.

It’s a far cry from a year ago, with predictions of a “hot vax summer” uninhibited by covid concerns. Back then, coronavirus seemed to teeter on the brink of defeat as cases plummeted to their lowest levels since spring 2020 and vaccines became widely available for adults. Even the vaccinated and boosted now grudgingly accept the virus as a formidable foe that’s here to stay as governments abandon measures to contain it.

As the virus morphs and the scientific understanding of how it operates shifts with each variant, Americans are drawing their own lines for what they feel comfortable doing.

“This time last year, I was so hopeful,” said Margaret Thornton, a 35-year-old Philadelphia researcher preparing to spend her summer socializing mostly outdoors because of her weakened immune system. “Now, I don’t know when it’s going to be over, and I don’t think there is necessarily a light at the end of the tunnel. Or rather, if there is a light, is it an opening to get out? Or is it a train?”

Parents of children too young to be vaccinated are making cross-country travel plans. Octogenarians are venturing to bars. And families are celebrating graduations and weddings with throngs of mostly unmasked revelers - mindful they may get sick. Again.

More than half of the U.S. population is living in areas classified as having medium or high covid-19 levels by the Centers for Disease Control and Prevention. The latest cases have yet to overrun hospitals, but that could change as the virus spreads among more vulnerable people. The dominant strains circulating in the United States are the most contagious thus far.

“This one is really revved up, and it’s probably getting up there with something as transmissible as measles,” said Peter Hotez, dean of the National School of Tropical Medicine at Baylor College, describing the BA.2.12.1 subvariant now accounting for more than half of new cases. “Over the Memorial Day holidays, if you are in settings where you are indoors with large numbers of people without masks . . . there is a good likelihood you will suffer a breakthrough infection.”

Experts had hoped that the explosion of the omicron variant this winter, estimated to have infected a quarter of Americans who hadn’t already been infected, and the subsequent spring wave of omicron’s even more transmissible subvariants, would provide a buffer against future surges.

But an emerging body of research suggests those infections will not confer lasting protection as the virus’s latest iterations show remarkable ability to escape immunity. Experts say the recently infected who also received booster shots can count on at least several months of immunity, while the unvaccinated should expect little protection.

“You should not think, ‘Oh, I had omicron, I don’t need any shots’ or ‘I don’t need any more shots,’” said Melanie Ott, director of the Gladstone Institute of Virology and a co-author of a paper recently published in Nature finding limited natural immunity from the omicron variant. “We are going into a surge of the omicron subvariants that are more and more able to infect people who have preexisting immunity.”

Experts say vaccines are still showing durability in protecting people against severe illness. But the initial burst of antibodies from shots or infections fades after several months, said Celine Gounder, an infectious-diseases specialist and senior fellow at Kaiser Health News. That means the virus can develop into an infection before the body’s immune system kicks in.

Burhan Yardimci, his wife and their three young children - who had all contracted coronavirus in February - joined thousands of Turkish Americans on Madison Avenue recently for the return of New York’s annual Turkish Day Parade, canceled the last two years because of the pandemic. The next day, the family stood among another crowd of thousands for the Celebrate Israel Parade.

Yardimci doesn’t take much solace in his recent infection as an extra layer of protection. He thought his booster shot would stop infections, but he knows people who’ve had the virus three times. Because no one in his family became seriously ill, he doesn’t see the need to upend his life when everyone around him appears to be carrying on as normal.

“Hopefully, we’ll never get it again,” said Yardimci, 42.

In the Boston suburbs, Mandy Boyd found herself humbled by coronavirus after getting infected twice in five months: during the massive omicron wave in January, and again in May after attending a 150-person indoor wedding. Neither case was severe.

The experience left the 35-year-old health technology worker reassessing how to protect her 4- and 6-year-old children from infections that would disrupt their schooling or summer camp. She still plans to dine out and go to the gym, but her family will wear masks on their flight to Seattle for an upcoming vacation as well as when they watch a WNBA game while there. She worries about passing on a future variant to her children, even if her short-term immunity protects her from getting sick.

“We’re in a strange spot because it turned into a much more minor virus,” said Boyd of Swampscott, Mass. “From that perspective, I don’t see that the world should stop or schools should close.”

Graduations, proms and weddings have also returned after being canceled in earlier stages of the pandemic when cases were lower than they are now.

Adeline Rosales, 26, was among the hundreds of California State University Long Beach students in caps and gowns flooding into Angel Stadium in Anaheim on a recent morning. It was her first encounter with some classmates in the College of Health and Human Services who were only familiar as faces on a computer screen during virtual class. She felt comfortable marching alongside them through a tunnel and onto the field knowing the university required vaccines and booster shots. And it was important for her relatives to celebrate the occasion with her because she is the first in the family to graduate college.

But to avoid graduation crowds, she said the family waited several days for their celebratory dinner because they were “a little scared” as infections rose and Los Angeles County moved from a low to medium covid-19 risk level. Rosales lives with her parents, both of whom have preexisting conditions, and six other relatives.

“I don’t want to risk it at this point,” Rosales said. “We’re just trying to be as respectful to my parents as possible.”

For most Americans, coronavirus has faded from the foreground.

More than half say they are not too concerned or not at all concerned with coronavirus, according to a May survey by Monmouth University.

Nearly three-quarters say they hope to vacation this summer and less than a third say coronavirus is a major factor in their plans, according to a recent Washington Post-Schar School poll. The Transportation Security Administration on Thursday reported screening more than half a million additional fliers a day compared with the same day last year.

Experts are paying close attention to the Southeast for a potential covid resurgence because the region did not experience as many cases in the spring as the Northeast, and rising temperatures are driving people indoors.

Florida residents are bracing for the return of another summer surge in sharply different ways.

For Jeff Schulte, a 63-year-old retiree, coronavirus has never really gone away, and he sees no reason to change his behavior for an omnipresent threat. He is not planning on masking, social distancing or getting booster shots this summer.

“For the rest of our lives, it’s here,” he said while smoking a cigarette outside the library in downtown Sarasota. “It’s going to catch every one of us.”

To the north near Tampa, Rick Kilby, 57, donned a KN95 mask as he hawked his book about the Victorian-era belief in the healing properties of Florida springs at Floridania Fest in a Gulfport casino ballroom. Mostly unmasked attendees snaked past his table, conveniently situated near an open door that brought in fresh air.

He’s not worried about getting seriously ill after a second booster shot. But after hearing about five vaccinated friends getting infected in just two days, he does fret about having to cancel his upcoming trip to western Pennsylvania - the only vacation he had planned for the year.

“It’s not like it was two years ago where you are really concerned about going to the hospital and not getting out. Now, it’s more of a concern that this is going to be a real inconvenience to my schedule,” said Kilby, who lives in Orlando. “That’s the wonder of the vaccine. It made it from a life-threatening condition to one that’s really more like having a flu or cold or something.”

Others at the vintage Florida memorabilia festival feared worse consequences.

Patti Kane-Wood, 78, entered the expo wearing a blue surgical mask but felt uncomfortable by how attendees “were squeezed in there like sardines” and didn’t stay long. She has heard about more people getting covid in the last month than in the last two years. While she feels well-protected from getting her second booster dose, she worries about long-term complications after watching friends develop persistent breathing problems following their illnesses. A recent study found vaccines may offer little protection against most long-covid symptoms.

“If I catch covid, even the slightest case of covid, it’s possible I have long covid and have issues for the rest of my life,” Kane-Wood said. “I’m very afraid because people are very relaxed now and understandably so, but it’s not a time to let our guard down.”

Parents of young children are entering Memorial Day weekend frustrated that children younger than 5 remain the only group ineligible for vaccines. The prospect of regulators clearing shots by the end of June is becoming increasingly likely after Pfizer-BioNTech reported data showing their three-dose regimen proved 80 percent effective in preventing symptomatic infections in children 6 months to 4 years old.

In the meantime, parents are navigating how to protect their unvaccinated children when cases are rising and others are dropping their guards.

In Portland, Oregon, Jessica Poole said she is not taking her 5-year-old daughter, Lucía, and 3-year-old son, Max, to indoor play facilities, where Lucía would catch illnesses even before covid. She asks Lucía to wear a mask while she’s at prekindergarten. And the family isn’t planning any travel, because Max is too young to get vaccinated.

But Poole, 37, is not trying to avoid the virus at all costs.

“Whatever strain is going around now, you can’t be too terrified of it,” Poole said outside a CrossFit gym where she planned to work out without a mask on. “We need to live a normal life now.”

At a southeast Portland pub, George Cummings, 85, took a leap of faith as he joined his friends from a local mountaineering and climbing club for drinks. He knows he’s at a higher risk because of his age and wears a mask at the grocery store. He said he has not received a second booster shot because his doctor had not told him they were available.

He went maskless as he drank lemonade, ate a cheeseburger and mingled with a group of two dozen in the crowded bar.

“I’m not sure I’m 100 percent comfortable with my decision, but the alternative was not to go to the event,” said Cummings, who lives alone and had suspended his social life for the better part of two years.

“It’s almost a question of, do you want to live - and that includes some sort of social life for a human being - or am I going to hide in my basement?”

The Washington Post’s Jack Wright in New York, Yvonne Condes in Anaheim, Calif., and Doug Moser in Boston contributed to this report.