There are currently more than 1,200 variant lineages of the COVID-19 coronavirus identified in the world. Viruses are constantly mutating, so any mutation from the original strain identified in Wuhan, China, is considered a variant. The more times COVID-19 is transmitted from person to person, the higher the risk of mutations. But not all variants are of equal concern to public health experts, who focus on the ones that could make COVID-19 more transmissible, more severe or that could interfere with vaccine efficacy or testing accuracy. Of those 1,200 lineages, the CDC reports five variants of concern circulating in the United States. B.1.1.7 — the most prevalent in the U.S., at 53,819 cases — was first reported in Kent, in southeastern England, in December 2020. It spread rapidly across the UK and then on to more than 100 countries. P.1, the second most common variant in the U.S. at 2,598 cases, was first reported on Jan. 10, 2021, in Japanese travelers returning to Tokyo from Manaus, Brazil.