That could include wealthier urban communities, in which people isolated during the first wave but had started mingling by the second. вЂњThe virus may be getting into populations that were previously able to protect themselves,вЂќ she says. The antibody data did not reflect the uneven spread of the virus, agrees Gagandeep Kang, a virologist at the Christian Medical College in Vellore, India. Antibody studies may not have been representative of the entire population and potentially overestimated exposure in other groups, he says. One explanation might be that the first wave primarily hit the urban poor. But the latest eruption of COVID-19 is forcing them to rethink. These figures made some researchers optimistic that the next stage of the pandemic would be less severe, says Ramanan Laxminarayan an epidemiologist in Princeton University, New Jersey, who is based in New Delhi. The studies also suggested that, nationally, some 271 million people had been infectedВ - about one-fifth of IndiaвЂ™s population of 1.4 billion. Studies that tested for SARS-CoV-2 antibodies-an indicator of past infection-in December and January estimated that more than 50% of the population in some areas of IndiaвЂ™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. вЂњI was expecting fresh waves of infection, but I would not have dreamt that it would be this strong,вЂќ he says. Shahid Jameel, a virologist at Ashoka University in Sonipat, agrees that the intensity of the current wave is startling. He describes a вЂњnightmarishвЂќ situation at hospitals, where beds and treatments are in extremely short supply. вЂњThe second wave has made the last one look like a ripple in a bathtub,вЂќ says Zarir Udwadia, a clinician-researcher in pulmonary medicine at P D Hinduja Hospital & Medical Research Centre in Mumbai, who spoke to NatureВ during a break from working in the intensive-care unit. But they began to rise again in March and the current peak is more than double the previous one. вЂRipple in a bathtubвЂ™Д¬OVID-19 case numbers started to drop in India last September, after a high of around 100,000 daily infections. But IndiaвЂ™s daily totals 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. Untangling the causes could be helpful to governments trying to suppress or prevent similar surges around the world.Д®uropean 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.

Researchers in India are now trying to pinpoint what is behind the unprecedented surge, 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. 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.

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. Daily case numbers have exploded since early March: the government reported 273,810 new infections nationally on 18 April. The pandemic is sweeping through India at a pace that has staggered scientists.

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