The American Medical Association, the nation’s largest association of physicians, has criticized the Centers for Disease Control and Prevention's new quarantine and isolation guidance for COVID-19, saying the recommendations "are risking further spread of the virus."

The CDC updated its guidelines on Dec. 27, saying asymptomatic people who test positive for COVID-19 should self-isolate for five days rather than 10.

"The American people should be able to count on the Centers for Disease Control and Prevention (CDC) for timely, accurate, clear guidance to protect themselves, their loved ones, and their communities. Instead, the new recommendations on quarantine and isolation are not only confusing, but are risking further spread of the virus," the American Medical Association's president, Dr. Gerald E. Harmon, said in a statement Wednesday night.

Harmon referenced data cited by the CDC in its rationale for shortening the isolation period, which estimates 31% of people remain infectious five days after a positive COVID-19 test, suggesting that data proves thousands of Americans could return to their lives while still infected.

"With hundreds of thousands of new cases daily and more than a million positive reported cases on January 3, tens of thousands -- potentially hundreds of thousands of people -- could return to work and school infectious if they follow the CDC’s new guidance on ending isolation after five days without a negative test," Harmon said. "Physicians are concerned that these recommendations put our patients at risk and could further overwhelm our health care system."

Harmon said a negative COVID-19 test should be required for ending isolation after a positive test, as reentering society without knowing whether an individual is still positive ultimately risks further transmission of the virus.

Although test availability remains an issue nationwide, Harmon also called on the Biden administration to ramp up production and distribution of tests, adding that "a dearth of tests at the moment does not justify omitting a testing requirement to exit a now shortened isolation."

-ABC News' Arielle Mitropoulos

Officials in Chicago canceled all public school classes on Thursday amid discussions about classroom safety with the city's teachers.

Classes had been canceled on Wednesday after a majority of the Chicago Teachers Union's membership voted in favor of remote learning during a surge in COVID-19 cases. School officials called their action an illegal strike.

"In a time of crisis related to this pandemic, the worst possible thing we can do is abandon the science and data," Mayor Lori E. Lightfoot said on Twitter. "If you care about our students and families as we do, we will not relent. We are standing firm and fighting to get our kids back to in-person learning."

Teachers were locked out of their remote classrooms on Wednesday, according to the union. Union leaders asked members to again try to log in on Thursday, urging them post photos on social media.

The union on Wednesday filed an unfair labor practice charge against the Chicago Board of Education.

"We have rights to safety and we’ve been at the bargaining table for 20 months to secure those rights," Jesse Sharkey, the union's president, said in a statement.

Chicago Public Schools are among the largest in the country, with about 340,000 students in 636 schools.

The Transportation Security Administration reported 3,037 current COVID-19 infections on Wednesday.

The agency's infections have increased by about 16% in two days, according to TSA data.

The agency, which employs about 60,000, said it's had a cumulative 15,191 COVID-19 cases. The agency said 12,154 employees have recovered and 33 have died.

ABC News' Sam Sweeney

The director of the Centers for Disease Control and Prevention has given the final go-ahead for children ages 12 to 15 to get Pfizer's COVID-19 booster.

"It is critical that we protect our children and teens from COVID-19 infection and the complications of severe disease," CDC Director Rochelle Walensky said in a statement endorsing the CDC advisory panel's recommendation to expand booster eligibility.

The CDC recommends that adolescents between the ages of 12 and 17 get a Pfizer booster five months after their second dose.

-ABC News' Cheyenne Haslett