Israel and Australia reimposed Covid-19 restrictions on Friday after earlier success fending off the virus as cases surged of the highly contagious Delta variant.
The variant, first reported in India, also threatens Africa, Central Asia and South-east Asia.
Much of Sydney entered lockdown, a shock for people who had returned to relative normality after months with very few local cases.
Israel reimposed indoor mask-wearing less than two weeks after lifting the measure.
While mass vaccination has helped to bring down infections in many countries, the rise of the Delta variant has stoked a resurgence of outbreaks.
On Friday, Maharashtra state in India tightened restrictions and warned of a "more severe third wave" after the country recorded a third death caused by a new variant, named Delta Plus by the health ministry.
In Australia about a million people in four eastern and central Sydney districts were ordered to stay home for at least a week.
Sixty-five infections have been linked to a driver infected about two weeks ago when he took a flight crew from Sydney Airport to a quarantine hotel.
It was a dramatic development for a city that had been largely enjoying a resumption of normal life.
"To be honest, I probably think that it should have happened a couple of days ago," Bondi local Alana Trepper said.
Israel, which has one of the world's most successful vaccination campaigns, said infections had surged since a requirement to wear masks in enclosed public places 10 days ago was dropped.
After four days of more than 100 new cases a day – including 227 on Thursday – the health ministry reversed the decision.
The head of Israel's pandemic response taskforce, Nachman Ash, said the rise in cases had not been matched by a rise in admissions to hospital or deaths.
Russia, Uzbekistan and Fiji have all recorded a rise in cases caused by the Delta variant.
Indonesia is battling a surge in cases including the deaths of more than a dozen fully inoculated doctors, a medical association in the southeast Asia nation of 270 million people said on Friday.
In a sign of growing uncertainty over the level of threat, residents of the Portuguese capital Lisbon faced tighter restrictions.
"The measures change all the time! It's hard to keep track," said Isabel Goncalves, a shop worker in the city.
In Spain, an end-of-year student trip to the holiday island of Mallorca has sparked a major cluster of Alpha variant infections, another highly infectious strain which first appeared in Britain.
More than 2,000 people in the Madrid region have been told to self-isolate following the outbreak.
But in Reykjavik, Health Minister Svandis Svavarsdottir announced Iceland would become the first nation in Europe to remove all virus restrictions.
"We're reinstating the society we're used to living in," she said.
Iceland was able to take the decision after 383,582 doses were administered among its population of 365,000 people .
Elsewhere, faltering or sluggish inoculation programmes and slow regulatory approval of new vaccines are a drag on efforts to reopen economies.
Thousands of South African opposition activists rallied in Pretoria to demand regulators approve more vaccines and speed up the pace of inoculation.
"Our agenda is simple. Give our people vaccines, we want to open our economy," said Julius Malema, leader of the leftist Economic Freedom Fighters.
While Africa has so far been spared the worst of the pandemic, infections are surging at an alarming rate in at least 12 countries.
Africa Centres for Disease Control and Prevention director John Nkengasong described the third wave hitting the continent as "extremely brutal" and "very devastating".
The Delta variant has been reported in 14 African countries, with unprecedented deaths and hospital admissions pushing health services to the brink, the agency said.
According to the WHO, about one per cent of the continent's population is fully vaccinated, the lowest proportion globally.
WHO Africa director Matshidiso Moeti said cases were "outpacing vaccinations".
"Africa urgently needs a million more vaccines. We need a sprint".