Even as many Americans celebrate the apparent waning of the pandemic, the thrum of concern over the so-called delta variant grows steadily louder.

The variant, the most contagious version yet of the coronavirus, accounts for more than half of new infections in the United States, federal health officials reported this month. The spread of the variant has prompted a vigorous new vaccination push from the Biden administration, and federal officials are planning to send medical teams to communities facing outbreaks that now seem inevitable.

Infections, hospitalizations and deaths are rising swiftly in some states with low vaccination rates like Arkansas, Missouri, Texas and Nevada, and are beginning to show small upticks in all of the others. The curves have also begun shifting upward in New York City, and the percentage of positive tests in the city has doubled in the past few weeks to just over 1%.

Nationwide, the numbers remain at some of the lowest levels since the beginning of the pandemic, but are once again slowly trending upward, prompting a debate about when booster shots might be needed to protect Americans.

The virus has also set off large outbreaks across the globe, from Japan and Australia to Indonesia and South Africa, forcing many countries to reimpose stringent restrictions on social activity. Even in places like Britain, where wide swaths of the population are immunized, the delta variant has outpaced vaccination efforts, pushing the goal of herd immunity further out of reach and postponing an end to the pandemic.

But scientists say that even if the numbers continue to rise through the fall, Americans are unlikely to revisit the horrors of last winter, or to require booster shots in the foreseeable future.

If Britain’s experience is a harbinger of what’s to come, the overall number of infections may rise as the delta variant spreads through the United States. But hospitalizations and deaths are likely to be much lower than they were following the arrival of previous variants, because the average age of those infected has shifted downward and young people tend to have mild symptoms.

As important, vaccines are effective against the delta variant and already provide a bulwark against its spread.

“I think the United States has vaccinated itself out of a national coordinated surge, even though we do expect cases pretty much everywhere,” said Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health.

“Delta is creating a huge amount of noise, but I don’t think that it’s right to be ringing a huge alarm bell.”

Still, there are likely to be isolated outbreaks in pockets of low vaccination, he and other scientists predicted. The reason is simple: The pattern of the protection against the coronavirus in the United States is wildly uneven.

Broadly speaking, the West and Northeast have relatively high rates of vaccination, while the South has the least. The vaccinated and unvaccinated “two Americas” — as Dr. Anthony Fauci, the administration’s leading adviser on the pandemic, has called them — also are divided along political lines.

Counties that voted for President Joe Biden average higher vaccination levels than those that voted for Donald Trump. Conservatives tend to decline vaccination far more often than Democrats.

“I don’t expect that we will get close to the kind of mayhem we saw earlier,” said Kristian Andersen, a virus expert at the Scripps Research Institute in San Diego. “There are going to be clusters, and they’re going to be in states where you have low vaccination rates.”

In a country that should be able to end its pandemic in short order with widespread vaccination, the delta variant is well designed to take advantage of the cultural divide. The virus seems to combine the worst features of previous variants, Andersen noted.

The variant was first identified in India, where it is credited with causing an overwhelming surge that brought the country’s tally to nearly 30 million infections and at least 400,000 deaths. The variant quickly spread to Britain, where it is now the source of 99% of cases. It has since turned up in 104 countries and all 50 American states.

Data collected by Public Health England indicates that the delta variant is up to 60% more contagious than the Alpha variant, which was itself at least 50% more contagious than the original form of the virus. Delta also seems able to partly dodge the immune system, like the Beta variant first identified in South Africa, although to a lesser degree. And some reports have suggested that delta may cause more severe infections.

But the contagiousness is what makes the delta variant a formidable threat, Hanage said.

“The fact that delta has arrived and done so well, so quickly, in these unvaccinated parts in the middle of the country suggests to me that the lion’s share of its advantage comes from this enhanced transmissibility,” he said.

That means that the strategies that worked against previous versions of the virus may be less effective in curtailing delta’s spread, opening the door to sporadic outbreaks in the United States for the foreseeable future.

Those who have been inoculated against the coronavirus have little to worry about. Reports of infections with the delta variant among fully immunized people in Israel may have alarmed people, but virtually all of the available data indicate that the vaccines are powerfully protective against severe illness, hospitalization and death from all existing variants of the coronavirus.

Even a single dose of vaccines that require two shots seems to prevent the most severe symptoms, although it is a flimsier barrier against symptomatic illness — making it an urgent priority to give people second doses in places like Britain that opted to prioritize first doses.

Like Israel, Britain has seen delta infections in vaccinated people, but they have mostly been among people exposed to large amounts of the virus — health care workers, taxi and bus drivers, for example — and in those who may have mounted weak immune responses because of their age or health conditions, said Dr. Muge Cevik, an infectious disease expert at the University of St. Andrews in Scotland and a scientific adviser to the British government.

In countries with low vaccination rates, however, the delta variant has found fertile ground. In Africa, where only about 1% of the population is fully immunized, the variant’s prevalence has been doubling roughly every three weeks. The number of cases across the continent rose by 25% and deaths by 15% in the week ending June 27, compared with the previous week.

The situation is much less dire in the United States, where nearly 60% of adults are fully vaccinated. Even Mississippi, the state with the lowest vaccination rate, has protected 43% of adults. Nationwide, COVID-19 has dropped from being the leading cause of death in January to now the seventh, averaging 330 deaths per day.

But cases are rising rapidly in counties where less than 30% of residents have been fully vaccinated. And the trend is likely to accelerate as the weather cools and people head indoors, where the virus thrives.

If the prevalence in those communities spikes high enough, even vaccinated people there will be at risk of infection, though not of serious illness. Moreover, the variant may find opportunities to keep circulating.

One recent study linked 47 cases of infection with the delta variant to an indoor gymnasium, among them three people who had received one dose of the Pfizer-BioNTech or Moderna vaccines and four people who were fully immunized.

“When you have populations of unvaccinated individuals, then the vaccines really can’t do their jobs,” said Stacia Wyman, an expert in computational genomics at the University of California, Berkeley. “And that’s where delta is really a concern.”

Britain’s experience with the delta variant has highlighted the importance not just of vaccination, but the strategy underlying it. The country ordered inoculations strictly by age, starting with the oldest and carving out few exceptions for younger essential workers, outside of the medical profession.

That meant the most vulnerable were protected first, while the most socially active part of the population — younger people — was until recently largely unprotected. Younger people were instrumental in the spread of the virus.

In England, everyone in their late teens and 20s became eligible for shots only in mid-June, two months later than in the United States, and many are still waiting for second doses. Those second doses have become all the more crucial as delta spreads, as the variant overwhelms the first doses in some cases.

In one study published in the journal Nature last week, only about 10% of blood samples from people who received one dose of either the AstraZeneca or the Pfizer-BioNTech vaccines were able to neutralize the delta variant, compared with 95% of those who got both doses. (Other studies suggest that a single dose is at least enough to prevent serious illness and death, however.)

More than 90% of people older than 55 are fully vaccinated in Britain. That has not entirely blunted the toll on hospitals following the spread of the delta variant: Patient admissions have begun climbing as quickly as cases in recent days, a reminder that some infections still inevitably lead to severe illness. But the proportion of cases leading to hospitalizations is lower than it was in previous waves.

“The actual transmission pattern is really strongly concentrated in the unvaccinated population, which in the U.K. is almost all young people,” said Jeffrey Barrett, who directs the coronavirus sequencing initiative at the Wellcome Sanger Institute. “You get cases, but they don’t usually get very sick.”

In the United States, some states are already seeing a rise in hospitalizations. Even if those numbers remain small compared with last winter’s, they will strain hospitals in states like Oregon, already at maximum capacity as a result of other factors including a heat wave.

“We don’t really have a huge margin for error,” said Brian O’Roak, a geneticist at Oregon Health and Science University in Portland. “If we do see a sharp rise in hospitalizations, we’re going to be back where we were during the last surge.”

In previous waves, there was a neat, linear relationship between the number of infections, hospitalizations and deaths in the United States. Fortunately, those patterns do not hold for the delta variant, because a large proportion of people at the highest risk now have been inoculated.

The country also opened vaccinations up to all adults, and even to 12- to 17-year-olds, which may break chains of transmission more effectively than in Britain.

The AstraZeneca vaccine dispensed in Britain appears to be less effective at preventing infections with delta than the mRNA vaccines in wider use in the United States. That, too, could give the United States an advantage over the variant.

Because of delta’s partial ability to undermine the immune system, the rate of breakthrough infections — cases that occur despite vaccination — with the variant appears to be greater than with previous forms of the virus, except for Beta.

Many experts have worried that even mild infections raise the risk of so-called long COVID, the constellation of symptoms that can persist months after an active infection is resolved. That raised a horrifying prospect: a surge in long-term illness throughout unvaccinated regions.

But many scientists now believe that breakthrough infections are unlikely to cause the syndrome. When a vaccinated person is infected, the virus may go through a few rounds of replication, but “the immune response is so quick and so robust that it basically stops the infection in its tracks,” said Angela Rasmussen, a virus expert at the Vaccine and Infectious Disease Organization at the University of Saskatchewan in Canada.

The coronavirus pandemic began as a patchwork in the United States, and the delta variant seems likely to restore the pattern, many experts believe. And the virus is unlikely to be the last serious threat. Already the gamma variant, identified in Brazil, has found a foothold in Washington state, and a more recent variant, Lambda, is on the march in South America.

“People are positive-minded, but this is just the beginning,” said Ravindra Gupta, a virus expert at the University of Cambridge. “This is going to be a slow burn.”

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