"Traditionally, it's been a region that has been overlooked by non-US based collectors. I imagine most rare and old Napa remains within the US," says Liv-ex Research Analyst Edward Jackson.

"However, US wines are becoming a more prominent part of the secondary market, from nothing to 3.5 percent trade in five years. There's definitely a general trend for collectors to explore other regions of the world now that the focus on collecting fine wines in Europe and Asia is no longer limited to Bordeaux."