https://www.mercurynews.com/2021/12/03/omicron-vs-delta-comparing-covids-most-worrisome-variants/ > Since the COVID-19 virus first appeared in December 2019, thousands of variants have emerged — but, until this month, Delta reigned supreme. > The arrival of Omicron — with the first U.S. case detected in San Francisco on Wednesday and more in New York, Minnesota and Colorado a day later — is a reminder that Delta isn’t the endgame.