(CNN) As a renewed sense of optimism sweeps the US this summer with lower reported Covid-19 cases, the US surgeon general has issued a warning for those not vaccinated: Don't let your guard down quite yet.
In December, British researchers discovered that a new variant was sweeping through their country. When it arrived in other countries, the variant, now known as Alpha, tended to become more common in its new homes as well. By April, it had become the dominant variant in the United States, and it has remained so ever since.
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