The numbers are startlingly high across India: Nearly 30% of capital city New Delhi's 16 million people have had the coronavirus; in the western city of Pune, more than 50% have had it in some of the most densely packed neighborhoods. In financial center Mumbai's slums, nearly 60% show evidence of infection and recovery.These results from a spate of recent serological surveys show levels of exposure higher than what's been seen anywhere else in the world, and confirm what most experts have long suspected: India's outbreak -- the world's third largest with over3 million confirmed cases -- is much bigger than the official numbers show.
But the data also suggests something more surprising: Enough people may have already developed resistance to the coronavirus in some places to cause a slowdown in spread. Herd immunity may have been achieved in some communities. The idea of herd immunity -- when enough of a population has resistance to a pathogen to stymie further spread -- through natural infection rather than vaccination is controversial, and not just because of the casualties such a strategy would create. Questions still remain over how long antibodies last, and whether re-infection or re-activation of the virus can occur after they fade.  A health-care worker collects a swab sample from a migrant worker for Covid-19 rapid antigen testing at a temporary facility in New Delhi, India, on Tuesday. Photographer: T. Narayan/Bloomberg Unlike in Sweden, which took a laissez-faire approach that some say amounted to an attempt to create herd immunity, the phenomenon seems to have happened in India unwittingly. The country pursued a strict lockdown in May, but the living conditions of its urban poor who often reside in densely packed slums may have proved too ideal an environment for virus spread. Evidence for this hypothesis comes from a recent slowdown in new cases in the surveyed areas. New infections in Mumbai's slums have slowed to a trickle even as residents emerge from their shacks and tenements to try to resume normal life. Delhi's new cases have also fallen from their peak, though there are signs that the city is relying too much on rapid antigen tests that throw up a high level of false negatives. Still, hospitals in the city who were turning patients away for lack of beds in June now report plenty of vacancies. Despite this, the slowdown of infection growth in cities has only been surpassed by an acceleration of spread in its vast rural hinterland, where a lack of health-care will only intensify suffering. Herd immunity or not, India is nowhere near the end of its bruising battle with the coronavirus.--Ari Altstedter | 
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