The Centers for Disease Control and Prevention on Wednesday issued new guidance that expanded the definition of a "close contact" with a person infected with COVID-19, creating implications for contact tracing in schools and businesses and the number of people who will have to quarantine.

The CDC had previously said a close contact is someone who has spent at least 15 consecutive minutes or more within six feet of someone who has coronavirus. But under the new definition published on its website, the agency revised the period of time as a cumulative 15 minutes or more over the span of 24 hours.

In other words, brief but repeated interactions adding up to 15 minutes can lead to infections.

The guidance also clarified the contagious period as starting at two days before a person tests positive.

In an extended footnote, the CDC said that other factors to consider include proximity, the duration of exposure, whether the infected individual has symptoms or was generating aerosols through talking or singing, and environmental conditions like crowding and whether exposure was indoors or outdoors.

The complex set of changes were first reported by the Washington Post and Stat. It comes on the same day the CDC published a study of a case in Vermont where the virus was contracted by a 20-year-old prison employee who interacted with infected individuals 22 times for a total of 17 minutes during an eight-hour shift. The interactions were caught on video camera footage.

The new guidance will undoubtedly have an affect on schools, workplaces and congregate settings, which have rules on quarantining. It will also require contact tracers to reach out to a larger pool of people when they are investigating a new infection.

In New York City, a daily health screening required by the Department of Education for in-person school attendance asks whether students have had a close contact with someone testing positive for COVID-19 in the past 14 days.

Under the city's current definition, however, a close contact means being within six feet for at least 10 minutes.

On Wednesday, CDC officials and Secretary of Health and Human Services Alex Azar held a rare in-person press conference at the agency's headquarters in Atlanta during which they addressed the surging infections across the country.

They did not, however, go over the new guidance on close contacts.

To date, more than 8 million Americans have contracted coronavirus. At least 220,000 people have died. All told, 31 states have been classified as under a "red zone" by the federal government, signifying more than 100 new COVID-19 cases per 100,000 residents. Hospitals in several states are under strain.

“Unfortunately, we are seeing a distressing trend here in the United States,” said Jay Butler, the CDC’s deputy director for infectious diseases. He cited the cooler temperatures as well as "smaller, more intimate gatherings of family, friends and neighbors" that have moved indoors.

The officials also spoke about the government's preparation with states on delivering a vaccine. All states have been asked to identify vaccine distribution centers by November 1st.

Butler said he expected there would be one if not two approved vaccines by the end of the year. Elderly and vulnerable populations as well as front-line healthcare workers will be prioritized for vaccinations.

Experts have expressed concerns about the vaccine development process under the Trump administration, which has repeatedly interfered with government health officials.

But Azar insisted the White House was adhering to the highest ethical standards. He pointed to three clinical trials, two involving vaccines and another for an antibody treatment, that were recently halted due to safety concerns.

"The system is working," he said. "We are playing by the book."