“Put 10% of the cash in short-term government bonds and 90% in a very low-cost S&P 500 index fund. (I suggest Vanguard’s.),” Warren Buffett wrote in a letter to shareholders.

The Vanguard S&P 500 ETF (NYSEArca: VOO) comes with a 0.05% expense ratio. The average Vanguard U.S. equity index ETF has a 0.1% expense ratio.

Additionally, the Vanguard Total Stock Market Index fund is now the largest mutual fund in the world, surpassing PIMCO in the amount of bond fund assets it manages.

BlackRock’s ETF business has also seen robust gains due to the shift in investor interest. The iShares ETFs have experienced $53 billion in new inflows this year through Aug. 18, the largest amount of any fund company. The fund provider’s passive index-based ETFs show an average expense ratio of 0.35%.

For more information on the ETF flows, visit our ETF performance reports category.

Max Chen contributed to this article.