WASHINGTON (NYTIMES) - 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, hospitalisations 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 upwards in New York City, and the percentage of positive tests in the city has doubled in the past few weeks to just over 1 per cent.

Nationwide, the numbers remain at some of the lowest levels since the beginning of the pandemic, but are once again slowly trending upwards, 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 immunised, 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 hospitalisations 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 downwards 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 per cent 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 per cent more contagious than the Alpha variant, which was itself at least 50 per cent 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 immunised people in Israel may have alarmed people, but virtually all of the available data indicate that the vaccines are powerfully protective against severe illness, hospitalisation and death from all existing variants of the coronavirus.

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

The situation is much less dire in the United States, where nearly 60 per cent of adults are fully vaccinated. Even Mississippi, the state with the lowest vaccination rate, has protected 43 per cent 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 per cent 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.

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.

"The actual transmission pattern is really strongly concentrated in the unvaccinated population, which in the UK 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 hospitalisations. 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 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."