The fast spread of the Delta variant of the coronavirus in much of the world is thwarting plans in many countries to lift lockdowns and reopen economies, a major setback to efforts to contain the global Covid-19 pandemic. The variant’s spread has heightened a likely feature of an extended global pandemic: the contrast between poorer unvaccinated countries where hospitalizations and death rates are surging and highly vaccinated populations where the link between rising case rates and serious illness has been largely broken.

Delta, which swept through India in May, is estimated to be at least twice as contagious as the original version of the virus. It is now present in 85 countries and is the most common variant in the U.S. Only in South America, where another highly contagious version of the virus is prevalent, does Delta not seem to be making inroads.In parts of Asia, Australia and Europe, governments are reintroducing travel restrictions and delaying the lifting of lockdowns as health authorities find that restrictive measures that kept earlier lineages of the virus in check aren’t curbing Delta.

In the U.K., where the variant accounts for 97% of recent cases, and other places with high rates of vaccination, concern is tempered by evidence that the shots are reducing hospitalizations and deaths. But in largely unvaccinated parts of the world, including some countries of Africa, hospitals are overwhelmed. In Indonesia, caseloads are at the highest level since the start of the pandemic.

Even where the number of cases is low, Delta is generating concerns, including in Japan, which is preparing to host the Summer Olympics in three weeks. A simulation by government researchers this week estimated that the share of the Delta variant among new cases in the greater Tokyo area was about 30% as of the end of June. Delta is expected to account for half of all Tokyo-area cases by mid-July.

In Australia, the Sydney metro area has been locked down for the first time in a year as the virus spread faster than the authorities could trace and isolate people at risk of infection. Early infections suggest it can spread via fleeting interactions, health officials say, which limits the effectiveness of contact tracing. Just 5.8% of the population has been fully vaccinated, according to Our World in Data.

In China, authorities are building huge quarantine facilities for international travelers in two southern cities to deal with the threat posed by the Delta variant, after relying on hotels up to now.

The first will cover 250,000 square meters and provide rooms for 5,000 travelers and another 2,000 beds for logistics personnel, according to a notice by the local government. It is expected to open in September, a local official said Monday.

China has also expanded its definition of close contacts and now requires quarantine for anyone who has been in the same building four days before or after someone who tests positive or develops symptoms, up from two, according to Zhong Nanshan, one of the country’s top epidemiologists, Chinese state media reported Friday.

Based on data gathered from an outbreak in the southern city of Guangdong, Mr. Zhong said the incubation period for Delta was shorter and the length of infection longer, compared with the version identified last year in Wuhan.

The variant is driving a Covid-19 surge in Indonesia, the world’s fourth most populous country, with daily reported infections at their highest levels since the start of the pandemic and average deaths over the past week doubling since early June to more than 400 a day.

With only about 5% of the country fully vaccinated, public-health experts and doctors warn caseloads could continue to rise. The Indonesian government, which had resisted tighter restrictions that could further hurt the country’s economy, said Thursday that it would put in place additional curbs in hard-hit areas, such as requiring all employees in nonessential sectors to work from home and temporarily closing places of worship.

Delta’s emergence in South Korea has delayed a relaxation of social-distancing measures in the area around Seoul after officials had laid out plans to ease the measures from Thursday, which would have allowed six instead of four people to gather privately and extended closing hours for restaurants and cafes until midnight.

Infections rose to a two-month high Wednesday as the number of infected people in their 20s increased by 20% in the past week. The country reported 794 new cases and one additional death Wednesday.

Israel has reimposed an indoor mask requirement, as the number of daily new Covid-19 cases has risen to an average of about 200 in the past week from about 10 a day in early June. As of Monday, children under 18, most of whom haven’t been vaccinated, accounted for more than half of those infected.

In Europe, German authorities are trying to slow the spread of the virus by limiting travel. Passengers arriving from so-called “variant territories”—including the U.K., Portugal, Russia and India among others—must quarantine for 14 days, whether or not they are vaccinated or have tested negative for an infection.