Futures for the S&P 500 moved from a loss of 0.4 percent just before Trump spoke from the Oval Office at 9 p.m. EST to a loss of 3.3 percent an hour later.
The major futures markets are now pointing to a decline of 5 percent or about 1150 Dow points.
The declines in the futures markets follow steep losses in regular trading Wednesday as investors become increasingly worried that responses from government and central banks will be insufficient to prevent the outbreak from severely impacting the global economy.
|I:DJI||DOW JONES AVERAGES||33290.08||-533.37||-1.58%|
|I:COMP||NASDAQ COMPOSITE INDEX||14030.375849||-130.97||-0.92%|
The Dow’s drop of 1,464 points dragged it 20 percent below the record set last month and put the index in a bear market. A Thursday selloff will also firmly send the S&P 500 and Nasdaq into a bear market.
Trump said he will enact a 30-day ban on all travel to and from Europe, excluding the United Kingdom, in order to slow the virus' spread. He urged Americans to listen to directives from their local officials, such as bans on mass gatherings and social distancing protocols.
In addition, Trump announced actions designed to ease the economic cost of the outbreak including unspecified aid for workers impacted by the virus, a deferment of tax payments for some individuals and businesses and low-interest loans for small businesses.
In Europe, London's FTSE dropped 5.8 percent, Germany's DAX fell 5.9 percent and France's CAC was down 6 percent.
In Asian markets on Thursday, Japan's Nikkei was down 4.4 percent, Hong Kong's Hang Seng dropped 3.7 percent and China's Shanghai Composite declined 1.5 percent.
The Associated Press contributed to this article.