A new "double mutant" variant of coronavirus has been detected in India as cases surge again in the world's second most populous nation.
Cases in India had mysteriously been plummeting since September and life was returning to normal last month.
Infections had dropped to around 9000 a day from a peak of around 100,000 new daily cases at the nation's peak in September.
However, cases began spiking last month and more than 47,000 new infections were detected in the past 24 hours, along with 275 deaths – the highest one-day death toll in more than four months.
Officials are reluctant to say whether new variants are behind the increase, as 10,787 samples from 18 Indian states also showed up 771 cases of known variants: 736 of the UK, 34 of the South African and one Brazilian.
However, authorities have made a new discovery in the nation's hardest-hit western state of Maharashtra, which is home to India's financial capital of Mumbai.
They have found what they describe as a "double mutant" in more than 200 samples.
Epidemiologists said the term "double mutant" refers to an entirely new variant that has the characteristics of two already identified variants.
"Double mutant is not a scientific term. It is just another mutant which seems to be unique to India," said Ramanan Laxminarayan, founder of the Centre for Disease Dynamics, Economics & Policy in New Delhi.
"Is there a reason to be worried about this particular variant? Not as yet, because we have no evidence that these variants are more transmissible or more lethal than what we already have," he said.
The virus has mutated many times since the pandemic broke out. Most mutations are harmless, but scientists have been investigating which ones might make the virus spread more quickly or make people more ill.
Before this new wave of infections took hold, India's harsh restrictions had gradually been eased to boost the economy.
India has so far recorded more than 11.7 million cases – the third-most infected nation behind the United States and Brazil – with over 160,000 deaths, one of the lowest mortality rates among the worst-hit countries.
Authorities thought they had seen the worst of the pandemic and in January launched the huge inoculation drive on the back of being the world's biggest vaccine maker.
But a jump in infections and a slower-than-expected vaccination rollout is setting off alarm bells.
"We need to recognise that we are now facing an increasing number of cases in many parts of the country and vaccination has to be one of the key aspects of responding to that strategically," public health expert Anant Bhan told AFP.
India kicked off its vaccination drive with healthcare and frontline workers, before expanding it to include over-60s and over-45s with serious illnesses. From April 1, everyone over 45 will also be eligible.
More than 50 million shots have been administered and about three million a day are being added, but at this rate the target will not be met.