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Such assertions, if proved, could complicate efforts to make a long-lasting vaccine and to achieve herd immunity
As the United States marks its sixth month since the arrival of the coronavirus, Cunningham’s story is among a growing number of reports of people getting covid-19, the disease caused by the novel coronavirus, recovering and then falling sick again — assertions, that if proved, could complicate efforts to make a long-lasting vaccine, or to achieve herd immunity where most of the population has become immune to the virus.