Dr. Anthony Fauci and White House coronavirus response coordinator Dr. Deborah Birx warned Tuesday that the U.S. hasn't yet seen the full impact of Thanksgiving gatherings.

“We have not yet seen the full blunt and the effect of the traveling and the congregating,” Fauci said at The Wall Street Journal's CEO Council summit .“That should be sometime probably next week, or a week and a half.”

"Then we're going to enter into the Christmas season, again, with more traveling and with more congregating at family and social gatherings. So we're in for a very challenging period," Fauci continued. "And the only way we're going to counter that is by a consistent uniform implementation and adherence to public health measures."

Fauci also said he’s accepted President-elect Joe Biden’s offer to become his chief medical adviser.

“I've already told the president-elect that I would gladly accept that responsibility. It really is very similar to what I'm doing now,” Fauci said.

ABC News' Anne Flaherty contributed to this report.

The University of Michigan said this Saturday's football game against Ohio State is now canceled due to "an increasing number of positive COVID-19 cases and student-athletes in quarantine over the past week."

"The number of positive tests has continued to trend in an upward direction over the last seven days," said Michigan's Athletic Director, Warde Manuel. "We have not been cleared to participate in practice at this time. Unfortunately, we will not be able to field a team due to COVID-19 positives and the associated quarantining required of close-contact individuals."

ABC News' Josh Hoyos contributed to this report.

Nearly 1.5 million children in the U.S. have tested positive for COVID-19 since the pandemic began, the American Academy of Pediatrics and the Children’s Hospital Association said in its weekly report.

During the week ending Dec. 3, there were 123,688 new cases of children with COVID-19, bringing the total number of U.S. children diagnosed to 1,460,905.

ABC News' Eric Strauss contributed to this report.

Scientists from the U.S. Food and Drug Administration on Tuesday appeared set to confirm that the Pfizer/BioNTech COVID-19 vaccine is safe and effective against the virus, according to documents released ahead of a crucial meeting on the vaccine this week.

According to a document prepared by FDA staff, government scientists plan to tell the advisory committee that the data is adequate to determine the vaccine is both safe and effective.

While that doesn't mean the FDA is prepared to authorize it just yet, the briefing materials suggest agency staff aren't warning of any last-minute issues.

"Safety data from approximately 38,000 participants [greater than or equal to] 16 years of age randomized 1:1 to vaccine or placebo with a median of 2 months of follow up after the second dose suggest a favorable safety profile, with no specific safety concerns identified that would preclude issuance of an EUA," the FDA wrote.

The report notes the most common reaction was a skin reaction at the injection site, followed by fatigue or headaches.

The FDA released data on the Pfizer/BioNTech vaccine trials and the agency's scientific analysis ahead of an advisory committee meeting on Thursday, when independent experts are set to discuss and vote on whether to recommend the vaccine be authorized for emergency use.

ABC News' Stephanie Ebbs and Anne Flaherty contributed to this report.

Navajo Nation begins a three-week emergency lockdown on Monday, during which residents are required to remain at home 24 hours a day, seven days a week.

Essential businesses, including grocery stores, gas stations, laundromats, hay vendors and restaurants providing drive-thru or curbside service, may remain open on weekdays between 7 a.m. and 7 p.m. Off-reservation travel is not permitted and on weekends, a full lockdown will be in place.

"The Navajo Nation’s health care system is in a state of major crisis," Myron Lizer, vice president of Navajo Nation, said in a statement. "We cannot be careless and we have to stay the course."

The rules include exceptions for essential workers and emergency situations. Residents are permitted to leave their homes for essential food, medicine and supplies, as well as outside exercise within the vicinity of their homes and wood gathering.

The lockdown rules will be in effect until Dec. 28.