The net worth of American billionaires continues to skyrocket, putting their collective wealth at more than $5 trillion, according to a new study from the Institute for Policy Studies, a progressive research group. That marks a 70 percent gain from the onset of the COVID pandemic. The group is worth $2 trillion more than the bottom 50 percent of the country combined, the study found.

Much of the gains have been driven by resurgent public markets, which had cratered in March 2020. Elon Musk’s fortune has since soared more than any other American; his net worth spiked over 750 percent, to roughly $209 billion. The authors of the study signaled support for a Billionaires Income Tax, which would tax the annual wealth gains of the ultra-rich. President Joe Biden previously endorsed the proposal, though it would face logistical hurdles if implemented.