More than a million new infections in four days, rampant oxygen shortages and the Indian capital, Delhi, awash with sirens: this is what happens when wildly infectious new variants hit a population that was no longer socially distancing, or never could.

That mutations of the virus that causes Covid-19 are to blame for the catastrophe spreading across India is the best guess of epidemiologists and other scientists confronting one of the most aggressive waves of the pandemic witnessed anywhere in the world.

India is recording more than 295,000 cases per day and more than 2,000 deaths, with experts predicting fatalities will grow over the next weeks as some of those who have become infected in recent days succumb to the disease.

What is shocking and baffling public health experts is the phenomenal speed with which the virus is spreading, coming after six months in which Covid-19 infections across the vast, crowded country seemed simply to vanish, even as cold weather was driving up cases elsewhere in the world.

Seroprevalence samples taken in December and January indicated that about one in five Indians – or about 267 million people – had probably been infected by then, leaving more than 1 billion still susceptible. And yet cases kept falling.

Was it a form of herd immunity? Were some Indians protected by earlier vaccinations for others viruses? Doctors still are unsure what drove the decline. “That’s something we don’t really have answer for, we just don’t,” says Dr Shahid Jameel, a virologist and director of the Trivedi school of biosciences at Ashoka University.

But even in a country with spotty health surveillance, he is confident the fall in caseloads was genuine. “Testing had gone down, but testing had not gone down to such an extent that we wouldn’t be picking the virus up,” he says. “The curve was declining faster than the number of tests being taken, so it was a real decline.”

Few were seriously troubled by the relatively small numbers of new cases that continued to be reported in Kerala and Maharashtra, two of the states with the best public-health surveillance in the country, indicating the virus was still out there, circulating.

Such was the sense of security that when vaccinations were opened in January to health workers, take-up was significantly slower than expected. And what was the rush? Virus cases fell to 10,000 per day at one stage. Senior Indian government figures were lining up to praise Narendra Modi, the prime minister, as a “vaccine guru” who had steered the country out of the health crisis. Public rallies had resumed and cricket stadiums were full.

A doctor at the Rajiv Gandhi Super speciality hospital in Delhi, Ajeet Jain, spoke to the Guardian in March of his frustration at the relaxing of quarantine measures in big cities.

“People are not accepting that the disease is there,” Jain said. “The practice of sanitisation has gone down, the social distancing is almost zero, mask-wearing is still going on but rest of the things, physical distancing, is almost over.”

Within days of his remarks, Covid-19 came roaring back. Sequencing of the virus in India is limited, but tests have shown that a soup of variants is responsible for new outbreaks in different parts of the country.

“We are getting a lot of the UK variant, we are getting this B1617 variant, also called the double mutation, and we are also seeing a lot of mutations that are isolated but appear to have the capacity to evade antibody responses,” Jameel said.

“We can say they are all more infectious based on their behaviour. Though in India we have not been able to correlate the mutant variants with the surge, based on what we’ve seen earlier [in the UK and elsewhere], it’s the logical explanation.”

It is still unclear if any of the dominating variants circulating in India are more deadly. But the catastrophe playing out across the country suggests some are at least far more infectious than those responsible for the surge last year.

“In the first wave, one knew someone who knew someone who had Covid,” Jameel says. “Right now, I can tell you since yesterday, at least five people I knew closely have died. These are people I knew, people I’ve grown up with, who have taught me, who I played with.”

This content first appear on the guardian

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