India has recorded the world’s sharpest spike in coronavirus infections this month, with political and financial capitals New Delhi and Mumbai running out of hospital beds, oxygen and medicines.

Scientists are studying what led to an unexpected surge, and particularly whether a variant of the novel coronavirus first detected in India is to blame.

In pictures: Oxygen demand outstrips supply in virus hotspots as India battles Covid surge

The variant, named B.1.617, has been reported in some 17 countries, raising global concern. Here are the basics:

What is the Indian variant?

The B.1.617 variant contains two key mutations to the outer "spike" portion of the virus that attaches to human cells, said senior Indian virologist Shahid Jameel.

The World Health Organisation (WHO) said the predominant lineage of B.1.617 was first identified in India last December, although an earlier version was spotted in October 2020.

The WHO has described it as a "variant of interest", suggesting it may have mutations that would make the virus more transmissible, cause more severe disease or evade vaccine immunity.

Other strains with known risks, such as those first detected in the United Kingdom, Brazil and South Africa, have been categorised as "variants of concern", a higher threat level.

Are variants driving the surge in cases?

It's hard to say.

The WHO says more study is urgently needed. Laboratory-based studies of limited sample size suggest potential increased transmissibility, it concluded.

The picture is complicated because the highly transmissible B.117 variant first detected in the UK is behind spikes in some parts of India.

In New Delhi, UK variant cases almost doubled during the second half of March, according to Sujeet Kumar Singh, director of the National Centre for Disease Control. The Indian variant, though, is widely present in Maharashtra, the country's hardest-hit state, Singh said.

Prominent US disease modeller Chris Murray, from the University of Washington, said the sheer magnitude of infections in India in a short period of time suggests an "escape variant" may be overpowering any prior immunity from natural infections in those populations.

"That makes it most likely that it’s B.1.617," he said. But Murray cautioned that gene sequencing data on the coronavirus in India is sparse, and that many cases are also being driven by the UK and South African variants.

Carlo Federico Perno, Head of Microbiology and Immunology Diagnostics at Rome's Bambino Gesù Hospital, said the Indian variant couldn't alone be the reason for India's huge surge, pointing instead to large social gatherings.

Prime Minister Narendra Modi has been criticised for allowing massive political rallies and religious festivals which have been super-spreader events in recent weeks.

Do vaccines stop it?

One bright spot is that vaccines may be protective. White House chief medical adviser Anthony Fauci said earlier this week that preliminary evidence from lab studies suggests Covaxin, a vaccine developed in India, appears capable of neutralising the variant.

Public Health England said it was working with international partners but that there is currently no evidence that the Indian variant and two related variants cause more severe disease or render the vaccines currently deployed less effective.