The highly-contagious Delta variant is causing a surge in new Covid-19 cases even in countries with high vaccination rates and experts warn that inoculation campaigns are in a race against time to contain it.
For the moment the pandemic is still slowing down with the World Health Organization (WHO) reporting the lowest number of new cases worldwide since February and decreasing deaths attributed to the coronavirus.
But concerns are growing about the fast-spreading variant, prompting new restrictions in countries that had previously managed to control their epidemics.
Cases are on the rise in Russia, Australia, Israel and across parts of Africa, in part due to Delta.
Other countries fear they could be next.
- More contagious -
The Delta variant of Sars-CoV-2 was first detected in India where it began circulating around April.
It is now present to varying degrees in at least 85 countries according to the WHO.
In Europe, Delta initially gained a foothold in the United Kingdom, where it quickly outpaced the previous variant of concern Alpha, and now comprises 95 percent of all sequenced cases in England.
Delta is thought to be some 40 to 60 percent more contagious than Alpha, which itself is more contagious than the strain responsible for the first wave of Covid-19.
The pattern has repeated elsewhere.
In the United States last week 35 percent of positive tests that were sequenced were identified as the Delta variant, up from about 10 percent on June 5 -- numbers similar to what is being observed in Israel.
The European Centre for Disease Control (ECDC) has estimated Delta could account for 70 percent of new infections in the EU by the beginning of August and 90 percent by the end of that month.
Researchers looking at data from the greater Paris region estimate in a soon-to-be-published report that Delta could be as much as 50 to 80 percent more contagious than other strains.
- Vaccination key -
Top US infectious disease scientist Anthony Fauci called the variant the "greatest threat" to efforts to control the virus and called for an acceleration in vaccinations, American media reported on Tuesday.
While several studies have shown that vaccines are slightly less effective against Delta, they are still highly effective.
But only after the second dose.
Recent data from the UK government shows that full immunisation can offer about 96 percent protection against hospitalisation and 79 percent protection against symptomatic infection by the Delta variant.
Protection after only one dose, however, is much weaker -- 35 percent, according to the same data.
"One dose is not enough," says a public health notice from the ECDC, "full vaccination is needed to protect the most vulnerable."
- Return to restrictions? -
Delta is so contagious that experts say you would need to inoculate more than 80 percent of a population in order to contain it -- a target that would be challenging even for nations with significant vaccination programmes.
In Africa, the WHO estimates that only one percent of the population is fully vaccinated -- the lowest ratio globally.
The Delta variant has been reported in 14 African countries, accounting for most new cases in the Democratic Republic of Congo and Uganda, the WHO has said, calling for a vaccine "sprint" across the continent.
A further complication is that Delta seems to largely bypass immunity that might be conferred by a previous infection, said Samuel Alizon, a biologist specialised in infectious disease modelling.
"We can't really rely on natural immunity anymore," he told AFP.
With large numbers of younger people remaining unvaccinated, tough measures may have to be reintroduced to stop the spread, even in nations where large-scale vaccine rollouts had held out the promise of reopening.
In Europe, the ECDC warned that any further relaxation of containment measures would risk a surge in new cases across age groups.
This could lead to a rise in "hospitalisations, and deaths, potentially reaching the same levels of the autumn of 2020 if no additional measure are taken," it added.
Some countries, like Israel and Australia, have already announced a return to certain restrictions.
"Our goal is to end it," said Israel's prime minister Naftali Bennett, "to take a bucket of water and pour it on the fire when the fire is still small."