The Centers for Disease Control and Prevention issued updated guidance Wednesday on what is considered “close contact” regarding coronavirus transmission.

The new Centers for Disease Control and Prevention (CDC) guidance redefined a “close contact” as interacting with a person who has coronavirus for 15 minutes or longer over the course of 24 hours. The guidance was influenced by a study, which showed that a person tested positive for the virus after having a series of short interactions over a 24-hour period with an individual who had coronavirus, Politico reported.

“Individuals who had a series of shorter contacts but over time added up to more than 15 minutes became infected,” CDC Director Dr. Robert Redfield said at a press conference Wednesday, according to Politico.

The 24-hour period of contact can be up to two days prior to an infected person’s positive test, the guidance said. The redefinition of what a “close contact” is may upend school and business reopening plans, according to Politico.

Wednesday’s guidance change comes weeks after the CDC reversed its guidance on aerosol spread of coronavirus. On Oct. 5, the CDC said the virus may “linger in the air for hours.”

“Some infections can be spread by exposure to virus in small droplets and particles that can linger in the air for minutes to hours,” the updated guidance said.

Meanwhile, parts of the U.S. and European Union are beginning to see an increasing amount of reported cases and deaths, according to data compiled by The Financial Times. The renewed coronavirus surge in Europe is likely a second wave,” CNBC reported.

Former Food and Drug Administration chief Dr. Scott Gottlieb predicted Tuesday that the U.S. is about “a week away from seeing a rapid acceleration in cases,” according to CNBC. CDC deputy director for infectious diseases Jay Butler, characterized the recent spike in cases as a “distressing trend” at the Wednesday briefing, Politico reported.

Trump declared a national emergency in March shortly after the World Health Organization declared a pandemic as coronavirus spread rapidly around the world.