How does VUSA track U.S


I’ve got a number of Sterling Vanguard ETFs. Most notably the S+P500 VUSA. This ETF is open during UK trading hours, but clearly tracks the U.S market. How does this work when the U.S is closed for most of the ETFs trading hours…
Not sure I’ve explained my confusion very well…

It’s a good question and I’ve thought about this before, I actually created a spreadsheet tracking it all as I thought there might be a way to profit from it, for example if SPY is running in USA hours and VUSA is tracking it must be behind the USA so you can kind of see the future using the USA version. (or vice versa) but I could not find any patterns. (eg: every day SPY goes up then VUSA goes up day after)

I think the simple answer is that the USA market is running after hours and this is used to calculate the UK market version so cannot be predicted in any way.

edit: found the spreadsheet, SPY vs VUSA for different dates


The VUSA ETF price is determined by what traders are prepared to buy and sell a share of it for. So when the US market is closed the price is determined by the beliefs of market traders about the prices at which the constituents of the S&P 500 will trade when the US market is next open. Beliefs may be guided .by many things including futures prices and out of hours trading