The Delta variant is the fastest, fittest and most formidable version of the coronavirus that causes Covid-19 the world has encountered, and it is upending assumptions about the disease even as nations loosen restrictions and open their economies, according to virologists and epidemiologists.

Vaccine protection remains very strong against severe infections and hospitalisations caused by any version of the coronavirus, and those most at risk are still the unvaccinated, according to interviews with 10 leading Covid-19 experts.

The major worry about the Delta variant, first identified in India, is not that it makes people sicker, but that it spreads far more easily from person to person, increasing infections and hospitalisations among the unvaccinated.

Evidence is also mounting that it is capable of infecting fully vaccinated people at a greater rate than previous versions, and concerns have been raised that they may even spread the virus, these experts said.

"The biggest risk to the world at the moment is...