The Covid-19 pandemic is sweeping through India at a pace that has staggered scientists, Nature reported.
Daily caseloads have exploded since early March: the government reported 273,810 new infections nationally on April 18.
High numbers in India have also helped drive global cases to a daily high of 854,855 in the past week, almost breaking a record set in January.
Just months earlier, antibody data had suggested that many people in cities such as Delhi and Chennai had already been infected, leading some researchers to conclude that the worst of the pandemic was over in the country.
Researchers in India are now trying to pinpoint what is behind the unprecedented spike of the Covid cases, which could be due to an unfortunate confluence of factors, including the emergence of particularly infectious variants, a rise in unrestricted social interactions, and low vaccine coverage, the report said.
Untangling the causes could be helpful to governments trying to suppress or prevent similar surges around the world.
European countries such as France and Germany are also currently experiencing large outbreaks relative to their size, and nations including Brazil and the United States are reporting high infection rates at around 70,000 a day.
But India's daily cases are now some of the highest ever recorded for any country, and are not far off a peak of 300,000 cases seen in the United States on 2 January.
Some researchers say that the speed and scale of the current outbreak suggest a new ingredient: emerging variants of the virus.COVID-19 case numbers started to drop in India last September, after a high of around 100,000 daily infections. But they began to rise again in March and the current peak is more than double the previous one.
"The second wave has made the last one look like a ripple in a bathtub," says Zarir Udwadia, a clinician-researcher in pulmonary medicine at P D Hinduja Hospital & Medical Research Centre in Mumbai.
He describes a "nightmarish" situation at hospitals, where beds and treatments are in extremely short supply.
Shahid Jameel, a virologist at Ashoka University in Sonipat, agrees that the intensity of the current wave is startling.
"I was expecting fresh waves of infection, but I would not have dreamt that it would be this strong," he says.
Studies that tested for SARS-CoV-2 antibodies is an indicator of past infection "in December and January estimated that more than 50% of the population in some areas of India's large cities had already been exposed to the virus, which should have conferred some immunity, says Manoj Murhekar, an epidemiologist at the National Institute of Epidemiology in Chennai, who led the work.
The studies also suggested that, nationally, some 271 million people had been infected with about one-fifth of India's population of 1.4 billion.
Some researchers say that the speed and scale of the current outbreak suggest a new ingredient: emerging variants of the virus, the report mentioned.