WASHINGTON (NYTIMES) - Early in the pandemic, when vaccines for the coronavirus were still just a glimmer on the horizon, the term "herd immunity" came to signify the endgame: the point when enough Americans would be protected from the virus so people could be rid of the pathogen and reclaim their lives.

Now, more than half of adults in the United States have been inoculated with at least one dose of a vaccine. But daily vaccination rates are slipping, and there is widespread consensus among scientists and public health experts that the herd immunity threshold is not attainable - at least not in the foreseeable future, and perhaps not ever.

Instead, they are coming to the conclusion that rather than making a long-promised exit, the virus will most likely become a manageable threat that will continue to circulate in the United States for years to come, still causing hospitalisations and deaths but in much smaller numbers.

How much smaller is uncertain and depends in part on how much of the nation, and the world, becomes vaccinated and how the coronavirus evolves. It is already clear, however, that the virus is changing too quickly, new variants are spreading too easily and vaccination is proceeding too slowly for herd immunity to be within reach anytime soon.

Continued immunisations, especially for people at highest risk because of age, exposure or health status, will be crucial to limiting the severity of outbreaks, if not their frequency, experts believe.

"The virus is unlikely to go away," said Professor Rustom Antia, an evolutionary biologist at Emory University in Atlanta. "But we want to do all we can to check that it's likely to become a mild infection."

The shift in outlook presents a new challenge for the public health authorities. The drive for herd immunity - by the summer, some experts once thought possible - captured the imagination of large segments of the public.

To say the goal will not be attained adds another "why bother" to the list of reasons that vaccine sceptics use to avoid being inoculated.

Yet vaccinations remain the key to transforming the virus into a controllable threat, experts said.

Dr Anthony Fauci, the Biden administration's top adviser on Covid-19, acknowledged the shift in experts' thinking.

"People were getting confused and thinking you're never going to get the infections down until you reach this mystical level of herd immunity, whatever that number is," he said.

"That's why we stopped using herd immunity in the classic sense," he added. "I'm saying: Forget that for a second. You vaccinate enough people, the infections are going to go down."

A difficult threshold

Once the coronavirus began to spread across the globe in early 2020, it became increasingly clear that the only way out of the pandemic would be for so many people to gain immunity - whether through natural infection or vaccination - that the virus would run out of people to infect. The concept of reaching herd immunity became the implicit goal in many countries, including the US.

Early on, the target herd immunity threshold was estimated to be about 60 per cent to 70 per cent of the population. Most experts, including Dr Fauci, expected that the US would be able to reach it once vaccines were available.

But as vaccines were developed and distribution ramped up through the winter and into the spring, estimates of the threshold began to rise. That is because the initial calculations were based on the contagiousness of the original version of the virus. The predominant variant now circulating in the United States, called B117 and first identified in Britain, is about 60 per cent more transmissible.

As a result, experts now calculate the herd immunity threshold to be at least 80 per cent. If even more contagious variants develop, or if scientists find that immunised people can still transmit the virus, the calculation will have to be revised upward again.

Polls show that about 30 per cent of the US population is still reluctant to be vaccinated. That number is expected to improve but probably not enough.

"It is theoretically possible that we could get to about 90 per cent vaccination coverage, but not super likely, I would say," said Professor Marc Lipsitch, a public health researcher at the Harvard T.H. Chan School of Public Health.

Although resistance to the vaccines is a main reason the US is unlikely to reach herd immunity, it is not the only one.

Herd immunity is often described as a national target. But that is a hazy concept in a country this large.

"Disease transmission is local," Prof Lipsitch noted.

"If the coverage is 95 per cent in the United States as a whole, but 70 per cent in some small town, the virus doesn't care," he explained. "It will make its way around the small town."

Given the degree of movement among regions, a small virus wave in a region with a low vaccination level can easily spill over into an area where a majority of the population is protected.

At the same time, the connectivity between countries, particularly as travel restrictions ease, emphasises the urgency of protecting not just Americans but everyone in the world, said Dr Natalie E. Dean, a biostatistician at the University of Florida in Gainesville. Any variants that arise in the world will eventually reach the US, she noted.

Many parts of the world lag far behind the US on vaccinations. Less than 2 per cent of the people in India have been fully vaccinated, for example, and less than 1 per cent in South Africa, according to data compiled by The New York Times.

"We will not achieve herd immunity as a country or a state or even as a city until we have enough immunity in the population as a whole," said Professor Lauren Ancel Meyers, the director of the Covid-19 Modelling Consortium at the University of Texas at Austin.

What the future may hold

If the herd immunity threshold is not attainable, what matters most is the rate of hospitalisations and deaths after pandemic restrictions are relaxed, experts believe.

By focusing on vaccinating the most vulnerable, the US has already brought those numbers down sharply. If the vaccination levels of that group continue to rise, the expectation is that over time the coronavirus may become seasonal, like the flu, and affect mostly the young and healthy.

"What we want to do at the very least is get to a point where we have just really sporadic little flare-ups," said Professor Carl Bergstrom, an evolutionary biologist at the University of Washington in Seattle. "That would be a very sensible target in this country, where we have an excellent vaccine and the ability to deliver it."

Over the long term - a generation or two - the goal is to transition the new coronavirus to become more like its cousins that cause common colds. That would mean the first infection is early in childhood, and subsequent infections are mild because of partial protection, even if immunity wanes.

Some unknown proportion of people with mild cases may go on to experience debilitating symptoms for weeks or months - a syndrome called "long Covid" - but they are unlikely to overwhelm the healthcare system.

"The vast majority of the mortality and of the stress on the healthcare system comes from people with a few particular conditions, and especially people who are over 60," Prof Lipsitch said. "If we can protect those people against severe illness and death, then we will have turned Covid from a society disrupter to a regular infectious disease."