The S&P 500 fell 33.9% from 19 February to 23 March 2020 during the COVID-19 crash.
The decline followed the spread of COVID-19 beyond China, rising expectations of economic shutdowns and a sudden reassessment of corporate earnings and financial risk. The S&P 500 entered a bear market in March, meaning a fall of at least 20% from a recent closing high.
The Federal Reserve cut interest rates to near zero and introduced extensive emergency measures, while governments announced large fiscal-support programs. These interventions, together with changing economic expectations and vaccine developments later in the year, helped fuel a powerful rebound.
The S&P 500 is frequently mistaken for the Dow because both are widely quoted U.S. benchmarks. The S&P 500 represents about 500 large companies and is weighted by market capitalization, whereas the Dow contains 30 companies and uses a price-weighted method.