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Top 10 Most VIX-Correlated ETFs and CEFs Over the Past Week and Month
A majority of these funds are municipal funds from BlackRock and Nuveen (NKO, NXK, BKK, NGB, RNJ), with the rest being comprised of other bond/income trusts and short funds.
Every last one of these funds is a ProShares short fund. Given that no other issuer has short/inverse funds in this top 10, I think this makes it quite clear that ProShares ETFs likely have some direct holding with high negative correlation to the VIX.