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 the 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 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, senior Indian virologist Shahid Jameel said.
The World Health Organisation (WHO) said the predominant lineage of B.1.617 was first identified in India in December 2020, although an earlier version was spotted in October.
On 10 May, the WHO classified it as a "variant of concern," which also includes variants first detected in Britain, Brazil and South Africa. Some initial studies showed the Indian variant spreads more easily.
"There is increased transmissibility demonstrated by some preliminary studies," WHO Covid-19 technical lead Maria Van Kerkhove said, adding it needed more information about the Indian variant to understand how much of it is circulating.
Are variants driving the surge in cases?
It's hard to say.
Laboratory-based studies of limited sample size suggest potential increased transmissibility, according to the WHO.
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 National Centre for Disease Control director Sujeet Kumar Singh. The Indian variant, though, is widely present in Maharashtra, the country's hardest-hit state, he 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 suggested an "escape variant" may be overpowering prior immunity from natural infections in those populations.
"That makes it most likely that it's B.1.617," he said.
Murray cautioned that gene sequencing data on the coronavirus in India was sparse and said many cases were also being driven by the UK and South African variants.
Rome's Bambino Gesu Hospital microbiology and immunology diagnostics head Carlo Federico Perno said the Indian variant could not alone be the reason for India's huge surge, pointing instead to large social gatherings.
Do vaccines stop it?
One bright spot is that vaccines may be protective.
White House chief medical adviser Anthony Fauci said that preliminary evidence from lab studies suggest Covaxin, a vaccine developed in India, appeared capable of neutralising the variant.
Public Health England said it was working with international partners but there was no evidence yet that the Indian variant and two related variants caused more severe disease or rendered the vaccines less effective.
"We don't have anything to suggest that our diagnostics, our therapeutics and our vaccines don't work. This is important," Van Kerkhove said at WHO.