WASHINGTON (BLOOMBERG) - The fast-spreading Delta variant is clouding Americans' hopes for a carefree summer - and casting a shadow of doubt over plans to get back to business as usual in the fall.

The shift in sentiment marks a reversal from the spring, when it looked like the United States' immunisation campaign would turn the tide definitively against the coronavirus.

But lingering vaccine hesitancy in some areas has converged with the arrival of the more contagious Delta strain and darkened the mood across much of the country.

"The Delta variant recreates this anxiety that many of us had prior to being vaccinated," said Brown University's Health Services, Policy and Practice Associate Professor Megan Ranney.

"Are we safe? Are our kids safe? Is it OK for me to go to a restaurant? The things that we had started to accept were normal again," said Prof Ranney.

While US health officials say Delta is on its way to becoming the country's dominant strain, an analysis by genomic testing firm Helix suggests that it is already there, accounting for about 40 per cent of new infections.

As hospitalisation rises in some states, President Joe Biden's administration is sending response teams to less-vaccinated areas to try to combat its spread.

First seen in India, the mutant is estimated to be 55 per cent more transmissible than the Alpha variant that surfaced in Britain. While the severity of disease the newer strain causes is not fully clear, some data suggests it leads to a higher risk of hospitalisation than Alpha.

Vaccines from Moderna, Pfizer-BioNTech, Johnson & Johnson and AstraZeneca give protection against Delta, research shows. Yet for millions of unvaccinated Americans, largely concentrated in the south and central regions of the country, the variant presents a serious threat, Prof Ranney said.

Many unvaccinated people live in low-income areas and are at greater risk of developing severe disease due to underlying conditions, she added, so Delta's spread could lead to even greater disparities in health outcomes.

There is also the looming potential that if the virus continues to spread and evolve, it could take on a form capable of evading vaccines. A related strain called Delta Plus has emerged, but researchers have said there is no evidence yet to suggest it adds to the danger.

"Weapons we have work against this virus," said Dr Mark Pandori, director of the Nevada State Public Health Laboratory.

But "we could let this virus stay in the community, and it will become something that our weapons don't work on. And that's what we should be afraid of", he added.

Already, the variant has disrupted some nations' plans for a return to normal life.

British Prime Minister Boris Johnson pushed back the lifting of many restrictions by a month to mid-July, and is now saying some precautions will remain even longer. Regions of Australia have responded to its spread with lockdowns.

In the US, where just about half the population is fully vaccinated, Delta has gotten Americans' attention: 84 per cent have heard of it, and 72 per cent are at least somewhat concerned, an Axios-Ipsos poll showed this week.

Yet, few people are taking more precautions, according to the poll. And guidelines from health officials are mixed: Los Angeles County recommended wearing masks in indoor public spaces, regardless of vaccination status, but the Centres for Disease Control and Prevention (CDC) still say fully vaccinated people can shed masks in most settings.

"There's a degree of flexibility," Dr Anthony Fauci, Mr Biden's medical adviser on Covid-19, said in a press briefing last Thursday. "People at the local level, depending upon the on-ground situation, will make recommendations or not according to the local situation."

In areas where the strain is feeding cases, there are some signs of increased caution. In Arkansas, one of the least-vaccinated states, daily vaccinations now appear to be trending up, according state epidemiologist Jennifer Dillaha. On a good day, the state might see 5,000 shots administered; last Tuesday, it was more than 10,000, she said.

"People are beginning to anticipate the rest of the summer with more apprehension," she added.

Some Arkansans are being more careful about going out in public, and even some who are fully vaccinated are wearing a mask, she said. State officials are already looking to the autumn, asking about how the variant could affect schools when they come back into session.

In Missouri, where Delta has also fuelled cases, the St Joseph School District moved summer programmes at two schools online after students had to quarantine due to Covid-19 exposure, Superintendent Doug Van Zyl said. Now, as those programmes have ended, the school district is looking at its plans for the autumn, he added.

Continuing to use vaccines and masks will be key to allowing workers to return to offices, said Dr David Holtgrave, dean of the School of Public Health at the University at Albany.

"The more we use those tools now, with urgency and on a large scale, the more we'll be able to think about returning to office and school this fall in a way that looks like a return to normal," he said. "The less we do that now, the more disruption there will be."

But the most pressing question may be how well vaccines will continue to hold up. As Delta spread in Colorado, health officials there began seeing higher attack rates and potential increases in cases among vaccinated people in long-term care facilities, said Dr Ginger Stringer, epidemiology response programme manager at the Colorado Department of Public Health & Environment.

The department asked for help from the CDC to understand what Delta's implications were for vaccine effectiveness in general, and for older people and those with chronic diseases.

Health officials would like to know how likely it is that a fully vaccinated person who contracts Delta will transmit it, said Dr Dillaha, the Arkansas epidemiologist.

The uncertainty around that question is part of why Los Angeles County recommended masking for all residents, Dr Barbara Ferrer, director of the county's department of public health, said at a media briefing.

Those gaps in knowledge add to the difficulty of making public-health decisions for a population fatigued by pandemic limits. Policymakers are facing "a horrible dilemma", said Dr Jonathan Javitt, a senior fellow at the Potomac Institute for Policy Studies.

"People have finally gotten out. They've finally tasted a bit of freedom," added Dr Javitt, who is also chief executive officer of NRX Pharmaceuticals, which is developing a therapy for severe Covid-19 patients. After experiencing months of restrictions, "there's only so much the average person is willing to take", he said.