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

What is the 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.

Both the mutations are not new. They have been detected separately in other variants before, but were found for the first time in one strain together.

One of the mutations, known as E484Q, is similar to the E484K mutation in the variants identified in the UK and South Africa. It's reported to be able to decrease the effectiveness of antibodies.

The other mutation, known as L425R, was also found in a variant detected in California. It's reportedly to make the variant more transmissible by increasing the binding power of the virus' spike proteins with human cells.

The World Health Organization (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 categorized 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, B.117 variant cases almost doubled during the second half of March, according to Sujeet Kumar Singh, director of the National Center for Disease Control. The Indian variant, though, is widely present in Maharashtra, the country's hardest-hit state, Singh said.

Prominent U.S. disease modeler 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 variants first detected in the UK and South Africa.

Carlo Federico Perno, head of microbiology and immunology diagnostics at Rome's Bambino Gesù Hospital, said the B.1.617 variant couldn't alone be the reason for India's huge surge, pointing instead to large social gatherings.

Prime Minister Narendra Modi has been criticized 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 neutralizing the variant.

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