The possibility of obtaining herd immunity by the time summer is over and a new school year begins may be an "ambitious goal," Vivek Murthy, President Joe Biden's nominee for U.S. surgeon general, told George Stephanopoulos on "This Week" Sunday.

"I think what -- I think we can see improvement," Murthy said. "I think we can see reductions in cases and hospitalizations and deaths. I think we can see many more people immunized."

The "important thing" will be getting as many people vaccinated as possible, Murthy added.

"The more people we vaccinate, the better we will do, the fewer outbreaks we will see, the sooner we can get back to our way of life," he said.

Murthy also emphasized the importance of dispelling misinformation as well as continuing to take the necessary precautions, such as wearing masks and avoiding indoor gatherings.

"Then I think we can be on a path to not only turning the pandemic around, but, most importantly, getting our schools open, our workplaces back up and running, and regaining our way of life," he said.

The U.S. has surpassed 25 million cases of COVID-19, according to a Johns Hopkins University tally.

The national total confirmed case count currently sits at 25,003,695.

Last week, the U.S. surpassed 400,000 deaths from the virus. The death toll is currently at 417,463 in the U.S. and at 2,122,766 worldwide.

The number of global confirmed cases is nearing 100 million, currently sitting at 98,853,428.

-ABC News’ Joshua Hoyos

There have now been more than 20 million vaccine doses given out in the U.S., the CDC reported on Saturday.

The milestone was reached 23 days after the goal set by former President Donald Trump administration's Operation Warp Speed to reach that number of doses by the end of 2020.

President Joe Biden's administration has pledged to distribute 100 million doses in the first 100 days, as well as ramp up production of materials to administer vaccines amid an increase in demand and shortages of supply at the state level.

-ABC News' Ahmad Hemingway contributed to this report.

The University of Michigan has paused all athletics, including men's and women's basketball, for at least 14 days following "several" positive tests by people within the Michigan Athletic Department. The positive tests were also for the so-called U.K. variant, a more-contagious version of the virus. The decision to pause athletics was made in part due to the fact the tests were for the B.1.1.7 variant.

The decision was made not by the school but by the Michigan Department of Health and Human Services. The athletic programs will not be able to play or practice during the period.

"Canceling competitions is never something we want to do, but with so many unknowns about this variant of COVID-19, we must do everything we can to minimize the spread among student-athletes, coaches, staff, and to the student-athletes at other schools," Athletic Director Warde Manuel said in a statement.

The Wolverines men's basketball team, which is currently ranked No. 7 in the latest Associated Press poll, last played Friday against Purdue. Michigan won, 70-53, to advance to 13-1. They currently lead the Big Ten Conference at 8-1. They were supposed to play on Wednesday at Penn State.

The women's basketball team, currently 10-1 overall and ranked No. 11, was supposed to play Purdue on Sunday. The powerhouse men's ice hockey program, ranked in the top 10 nationally, will be affected as well.

The school said no determination has been made on how games scheduled to be played through Feb. 7 will be impacted.

California's top epidemiologist has recommended pausing the administration of COVID-19 vaccines from Moderna's lot 041L20A due to "possible allergic reactions" that are under investigation.

"A higher-than-usual number of possible allergic reactions were reported with a specific lot of Moderna vaccine administered at one community vaccination clinic," Dr. Erica Pan, state epidemiologist for the California Department of Public Health, said in a statement late Sunday night. "Fewer than 10 individuals required medical attention over the span of 24 hours. Out of an extreme abundance of caution and also recognizing the extremely limited supply of vaccine, we are recommending that providers use other available vaccine inventory and pause the administration of vaccines from Moderna Lot 041L20A until the investigation by the CDC, FDA, Moderna and the state is complete."

The California Department of Health said all of the affected individuals appeared to be experiencing "a possible severe allergic reaction during the standard observation period," which the U.S. Centers for Disease Control and Prevention says some people have experienced when receiving a COVID-19 vaccine.

More than 330,000 doses from that specific lot have been distributed to 287 providers across the state, with the shipments arriving between Jan. 5 and 12, according to the California Department of Public Health, which said it has not been notified of any other cluster or individual events related to this lot.

When asked for comment, a spokesperson for Moderna told ABC News: "Moderna acknowledges receiving a report from the California Department of Public Health (CDPH) that a number of individuals at one vaccination center in San Diego were treated for possible allergic reactions after vaccination from one lot of Moderna’s COVID-19 Vaccine. The company is fully cooperating with CDPH in investigating these reported adverse events. Consistent with the statement from CDPH, at this point Moderna is unaware of comparable adverse events from other vaccination centers which may have administered vaccines from the same lot."

ABC News' Sony Salzman contributed to this report.