Coronavirus updates: 'Close contact' definition updated by CDC

Boston University announced that beginning Thursday, it will require all students to display a green "daily attestation badge" before entering community spaces across campus to prove that they are up-to-date on testing requirements. 

"To emphasize the importance of these rules, beginning on Thursday, October 22, 2020, we will require a green daily attestation badge in order to enter our dining halls ... and several other public spaces on our campus," Kenneth Elmore, associate provost and dean of students, said in a letter to students. "We hope this will be a reminder to everyone of the importance of daily symptom attestation and testing for keeping our campus safe." 

The university had already been using badges that appear on students' mobile devices for faculty and administrators to check students' compliance with the university's COVID-19 guidelines.

The announcement comes after a "very worrisome increase" in the university's daily COVID-19 case numbers. Over the last seven days, the university has reported its largest increase of new cases since students moved into campus in August.

The move also comes as new cases surge in the Northeast and colleges especially have been struggling to control the spread of the virus.

ABC News' Arielle Mitropoulos contributed to this report.

A new study out Wednesday from the CDC found that multiple brief exposures within 6 feet of someone infected can heighten the risk of COVID-19 transmission. 

Previously, the CDC had advised that a good rule of thumb is that you can contract the virus if you spend at least 15 minutes within 6 feet of a person with COVID-19. 

In the new study, however, researchers found that a corrections officer working in a Vermont prison appears to have contracted the virus during "multiple brief encounters" with six incarcerated people who had COVID-19 but didn't know it yet. The six people were still awaiting the results of their tests.

The encounters were very brief (one minute or less), but the corrections officer overall had 22 encounters adding up to about 17 nonconsecutive minutes of possible exposure. 

This scenario shows that in addition to 15 consecutive minutes within 6 feet, it might also be possible to contract the virus if you have multiple, shorter exposures that collectively add up to more than 15 minutes.

ABC News’ Sony Salzman contributed to this report.

Dr. Moncef Slaoui, chief adviser for the Trump administration's Operation Warp Speed effort to accelerate a vaccine rollout, told ABC News Wednesday that every American could be immunized June 2021. 

"It's a plan, it's not a certainty," Slaoui told ABC News' Bob Woodruff. "But the plan should make it such that by June everybody could have been immunized in the United States. We will have enough vaccine doses. I really hope most people will take the vaccines." 

He added there are two vaccines in Phase 3 trials that are on hold, "and they are imminently going to have the hold lifted," referring to the fact that AstraZeneca and Johnson & Johnson have paused their trials. While this "does have an impact" on speed, Slaoui added, "that's fine, because the number one priority is safety of course." 

"If that means the trial has to stop for a month ... that's what we'll do," he said.

Spain has become the first nation in the European Union to hit the 1 million mark for coronavirus cases.

The country has reported a total of 1,005,295 cases and 34,366 fatalities, according to data from Johns Hopkins University.