There are a lot of theories!
. Europe is a linchpin of global travel, meaning waves that begin elsewhere in the world may tend to be channeled through Europe before reaching the United States. Based on common airline routes, it makes sense that COVID would have initially spread from Asia to Europe first. It also makes sense that delta, which originated in India, would have spread to the UK before reaching the United States, as India and the UK have closer travel ties.
that cases would peak in certain European countries before the United States. Travel connectivity could also partly explain why the gamma variant, first documented in Brazil in December 2020,to sweep the globe. Travel-wise, South America is the least-connected continent. In other words, travel routes are like well-worn riverbeds that channel the flow of COVID waves, and Europe is mostly upstream of the U.S.