As the Delta variant became the dominant strain of coronavirus across the United States, all three Covid-19 vaccines available to Americans lost some of their protective power, with vaccine efficacy among a large group of veterans dropping between 35 per cent and 85 per cent, according to a new study. Researchers who scoured the records of nearly 800,000 US veterans found that in early March, just as the Delta variant was gaining a toehold across American communities, the three vaccines were roughly equal in their ability to prevent infections. But over the next six months, that changed dramat…

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