The Delta variant is changing calculations of governments around the world, raising doubts about how quickly they can leave the coronavirus pandemic behind, widening the gulf between highly vaccinated places and the rest, and infiltrating countries that had previously kept the virus at bay.

The variant’s ability to spread more easily than previous dominant versions of the virus has driven rapidly rising Covid-19 caseloads even in highly vaccinated countries such as the U.K. and Israel. There, the vaccines have suppressed serious illness and deaths. But, in many parts of the rest of the world, the pandemic that has already sickened at least 200 million people and killed more than four million is intensifying.

The good news that vaccines are highly effective against Delta at preventing severe illness and death in those fully inoculated is a vindication of governments’ bold bets on the shots as the surest way back to normalcy.

Immunity against the coronavirus is waning in people who were fully vaccinated with the shot made by BioNTech SE and Pfizer Inc. in January because of the rapidly spreading Delta variant, said BioNTech’s chief executive, Ugur Sahin.

He said, however, most of them will remain protected against severe disease and might not yet need a third dose.

For advanced economies, the highly transmissible variant means many more people will need to be vaccinated to keep new outbreaks in check, and the hopes that Covid-19 would rapidly fade into the background like seasonal influenza are receding.

Already, governments in Europe and some U.S. states are taking steps to boost sputtering vaccination campaigns to try to stay ahead of the variant. Others are reimposing mask mandates and asking some businesses to close again. Travel restrictions, quarantining when instructed, and working from home are commonplace.

T he prognosis for the developing world is gloomier. Already, Delta has driven record infection waves across Asia and Africa, including in countries that had previously managed the virus relatively well.

“The horrible reality I don’t think people have really got to grips with yet is this is a new, nasty disease, which is going to be here forever, probably,” said Martin Hibberd, professor of emerging infectious diseases at the London School of Hygiene and Tropical Medicine. “It’s an additional burden that all of us will just have to cope with.”

The Delta variant of coronavirus was first spotted in India late last year and has since been detected in more than 100 countries and territories, according to the GISAID Initiative, an online database where scientists share genetic analyses of viral samples. Around 80% of samples submitted to GISAID in mid-July were Delta, twice the proportion of a month earlier, highlighting how quickly the variant has displaced older versions of the virus.

The Centers for Disease Control and Prevention says Delta accounts for 83% of samples subject to genetic sequencing in the U.S. In the U.K., that figure is 99%. Delta accounts for more than half the viral genomes analyzed in France, Germany, Sweden and Portugal, according to the CDC’s European equivalent. The variant accounts for more than 70% of GISAID samples received from Asia, and more than 60% of those submitted from Africa.

Earlier in the pandemic, before Delta arrived, scientists estimated that maybe two-thirds of a population would need immunity, from vaccination or a combination of vaccination and prior infection, to prevent the virus that causes Covid-19 from spreading easily enough to spark large new outbreaks. Reach that goal and Covid-19 would retreat into a state more akin to influenza, a seasonal hazard that rarely stretches healthcare systems.

But the speed of Delta’s advance across the world is a product of its enhanced transmissibility. Estimates vary, but scientists believe the Delta variant is perhaps two to three times more transmissible than the original virus that first emerged from China in 2019. In the absence of any mitigating public-health measures, every 10 people infected with the original virus would go on to infect 25 people, on average. With Delta, 10 infections would likely result in between 60 to 70 new infections, a big increase.

That puts the threshold for so-called herd immunity, or what some scientists prefer to call community protection or population immunity, further out of reach. Where previously governments and public health officials had hoped herd immunity might have been achievable with between 60% and 70% of the population vaccinated, Delta means that threshold is likely between 80% to 90%, said Mark Woolhouse, professor of infectious disease epidemiology at the University of Edinburgh.

Some estimates put the threshold higher still, at more than 95%, once vaccines’ less than 100% effectiveness at reducing viral transmission has been taken into account. On top of that, there is uncertainty over how long the protection afforded by vaccines lasts or if new variants may emerge that can sidestep such defenses.

“Because the herd immunity bar has been raised so high, it is not a question of when we will get there, but if we will get there,” said Prof. Woolhouse. Bringing the virus under control may require interventions like masks and social distancing to be in place long term, he said.

In advanced economies, even with substantial pools of vaccinated people, Delta is forcing public health authorities to rethink their approaches. Following a sixfold increase in cases, the Netherlands closed its nightclubs this month after only opening them in late June. Mask wearing and measures to enforce social distancing remain in place in France, Germany and Italy.

Plans for various shades of “vaccine passports” to participate in social activities are under discussion or being rolled out in France, Italy, Israel and the U.K., part of a broad effort both to find alternatives to lockdowns to reduce transmission and encourage greater uptake of vaccination among the hesitant.

In the U.S., the Delta variant has halted what had been a sharp decline in caseloads. The seven-day average for new daily Covid-19 cases has exceeded 49,000, more than double the number from only 10 days earlier, according to data from Johns Hopkins University. Los Angeles County, the U.S.’s most populous, has reintroduced an indoor mask requirement following a pickup in infections.

Overcoming vaccine resistance has proved challenging, especially in areas like the Deep South, where states have the lowest vaccination rates. The U.S. missed President Biden’s goal of getting 70% of the adult population to take at least one dose by July 4; by Friday, the number stood at 68.8%, federal data show.

In Africa, where fewer than 1.5% of the continent’s 1.3 billion people are fully vaccinated, a record surge in infections in recent weeks has overwhelmed hospitals and driven deaths to a point where mortuaries in countries such as Zambia and Namibia ran out of space. Last week, South Africa recorded 10,000 excess deaths.

Governments that were focused on healing their economies from the continent’s deepest contraction on record last year have had to impose tight new restrictions. South Africa, Uganda and Namibia all went back into lockdown.

The head of the Africa Centres for Disease Control and Prevention said Thursday that even under a best-case scenario, he expects only 25% to 30% of Africans to be vaccinated by the end of the year.

Data from South Africa highlights another hurdle on the road to durable, population-level immunity and the special challenge Delta poses for the world.

Epidemiologists as recently as May predicted that a third surge of Covid-19 would be less severe than two earlier bouts that had already infected a large proportion of South Africa’s 60 million citizens. A recent nationwide study of donated blood determined that some 47% of South Africans already had Covid-19 antibodies before the current wave took off, while the country’s largest health insurer estimated that 62% had already recovered from the disease.

B ut as it experiences a record surge in infections, scientists have realized that the antibodies generated by infection from the Beta variant that was until recently dominant in South Africa aren’t good at neutralizing Delta. Coupled with Delta’s higher transmissibility, epidemiologists say the country is just as vulnerable as at the start of the pandemic.

“We are basically back where we started,” said Juliet Pulliam, who directs South Africa’s Centre of Excellence in Epidemiological Modeling and Analysis.

In Asia, the variant is now causing trouble in countries whose sure-footed responses in earlier phases of the pandemic were held out as lessons for the world but prompted some to move slower in securing vaccines.

South Korea has implemented its strictest distancing measures yet as it experiences its largest outbreak since the start of the pandemic. The new rules ban gatherings of more than two people after 6 p.m., limit weddings and funerals to family members, ban spectators at sports events, and state that most facilities must close by 10 p.m.

Japan saw infections begin to rise in late June, prompting Prime Minister Yoshihide Suga to declare a state of emergency in Tokyo starting July 12. The emergency measures, set to extend until Aug. 22, will last throughout the Tokyo Olympics, which will be held mostly without spectators.

Southeast Asia is particularly vulnerable because of low vaccination rates. Most countries there have fully vaccinated less than 15% of their populations. Vietnam, which is currently facing its most severe outbreak, has vaccinated less than 1%. Even Singapore, with the highest vaccination rate in the region at about 50%, is fighting off a Delta-driven surge.

Delta has been most devastating in Indonesia, where cases began to surge in early June. Hospitals were quickly overwhelmed and death rates rose to record highs. More than 1,000 Covid-related deaths have been recorded daily over the past week, and the per-capita death rate is the highest in the region at 283 per million people. The government has imposed its strictest measures to date in the most-affected areas.