As Covid-19 spread last year, governments around the world touted the prospect of “herd immunity,” a promised land in which the virus would stop spreading exponentially because enough people were immune to it. That now appears to be a fantasy.

The theory was that the pandemic would ebb and then mostly fade once a significant portion of the population, possibly 60 percent to 70 percent, had been vaccinated or had acquired resistance through previous infection. However, new variants such as delta, which are more transmissible and have been shown to evade these protections in some cases, are raising the bar for herd immunity to near impossible levels.

Delta is fueling new outbreaks in countries such as the United States and the United Kingdom, which have already been hit hard by the virus and, presumably, have some natural immunity in addition to vaccination rates of more than 50%. It is also affecting countries that have previously managed to keep the virus out almost entirely, such as Australia and China.

The Infectious Diseases Society of America estimated this month that delta had increased the threshold for herd immunity to well over 80%, and possibly close to 90%. Over the last year, public health officials such as Anthony Fauci have sparked controversy by shifting the goalposts, increasing the number of people who require protection before reaching herd immunity. Meanwhile, vaccine hesitancy and supply issues mean that most countries will fall short of even the original targets.

Nature will not solve the problem either. It is unknown how long the natural immunity gained from surviving Covid-19 will last and whether it will be effective in combating new strains. Future variants, including some that may be able to evade immunity even more efficiently than delta, raise concerns about how – and when – this will end.

There are already signs that some people and places, such as Brazil and other South American countries, are being hit a second time by newer strains.

Without herd immunity, the virus could linger in some form for decades, potentially forcing the world’s most powerful nations to adjust their divergent approaches to opening borders and economies.

Countries such as China, which have pursued strict Covid-Zero policies in an attempt to eradicate every infection, may eventually be forced to consider a softer stance. Others, such as the United States and the United Kingdom, which have reopened despite the virus’s resurgence, risk being infected in waves.

Vaccines haven’t provided the quick fix that some had hoped for. Israel, one of the world’s most vaccinated countries, has already begun administering booster shots, despite evidence that current immunizations aren’t providing the protection hoped for. The United States announced last week that Americans with compromised immune systems will receive a third dose.

Because they are so effective, the most potent vaccines, such as mRNA shots from Pfizer Inc., BioNTech SE, and Moderna Inc., would make it easier to achieve high levels of immunity. Even with these shots, however, breakthrough infections (cases in the immunized) are possible. Other vaccines, including those developed in China by AstraZeneca Plc and Johnson & Johnson, may provide even less protection.

Herd immunity is real, and it protects much of the world from viral threats ranging from measles to polio. Scientists credit it with aiding in the abolition of smallpox. Having herd immunity as a goal likely aided the world’s acceptance of measures such as masking and social distancing. It did, however, create a false narrative.

Some countries discovered the limitations of herd immunity the hard way. Boris Johnson, the Prime Minister of the United Kingdom, had intended to use it as the primary approach to Covid-19, implying that some of his constituents could “take it on the chin” with natural infections before the magnitude of the coronavirus’s force became clear.

Others are now throwing in the towel, with Indonesia leading the way most vocally. Even if everyone in the country was immunized, the world’s fourth-most populous country determined that stopping the virus would be impossible. It is redoubling its efforts to promote mask use and social isolation, while also working to increase the still-low vaccination rate.

Meanwhile, Singapore and Australia are taking a cautious approach to reopening, promising to do so once vaccination levels are sufficiently high. Covid-Zero populations typically have lower levels of natural immunity developed through previous infections.