In a rare press briefing held at the Centers for Disease Control and Prevention in Atlanta, top federal health officials pleaded with the American public not to give up on wearing masks and social distancing to fight the coronavirus.
 
The message came two days after President Donald Trump said Americans are "tired" of the virus.

"I recognize that we are all getting tired, the impact that COVID-19 has had on our lives," said Jay Butler, the agency's deputy director for infectious diseases. Wearing masks "continues to be as important as it's ever been," he added.

Butler called out what he said was a "distressing trend" of increasing cases caused by people moving indoors and attending small family gatherings.

CDC Director Robert Redfield addressed new agency guidance that urges caution if a person has even brief encounters --15 minutes over a 24-hour period of time -- with a person with COVID-19. This diverges from previous guidance that suggested that two people had to be in contact for at least 15 minutes continuously.

"It's based on data that one didn't have four months ago," Redfield said.

Redfield added that the CDC also is looking at whether testing can be used to shorten the length of a person's quarantine following exposure.

"CDC is a science-based, data-driven service organization," Redfield said. "We're not an opinion organization. So if we get data that supports a change in our recommendation, then those recommendations will be changed."

On the vaccine front, Health and Human Services Secretary Alex Azar said the administration expects to have enough vaccines for "vulnerable individuals" by the end of the year; for seniors, first responders and health care workers by the end of January; and the broader public by end of March or early April.

Redfield also said he was "optimistic" there will be a limited supply of at least one COVID-19 vaccine available for distribution before the end of the year. "But we're not quite there yet," he said. "That is why it's so important that all of us remain diligent in our efforts to defeat this virus."

ABC News' Anne Flaherty contributed to this report.

Illinois saw 69 COVID-19-related deaths on Wednesday, its highest number since June 17, health officials said. Moreover, the state saw 4,342 new cases of COVID-19.

The state now has recorded 355,217 cases and 9,345 deaths, according to the state Department of Health. Health officials also reported that the positivity rate has increased for 15 consecutive days.

In response to the rising cases, Gov. JB Pritzker announced new restrictions Wednesday, Chicago ABC station WLS reported.

Starting this Friday, indoor dining at bars and restaurants will no longer be allowed. In addition, bars, restaurants and casinos must close by 11 p.m. All gatherings will be limited to 25 people or 25% capacity, whichever is less, according to WLS.

These restrictions will remain in place for at least the next 14 days, according to Pritzker.

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.