The U.S. should start seeing a "significant decrease" in COVID-19 mortality by the end of January, as the most at-risk populations are vaccinated, Moncef Slaoui, the top scientific adviser to Operation Warp Speed, told CNN's "State of the Union" Sunday.

The big picture: Slaoui noted that "between 40–50% of all deaths" have been elderly people in nursing homes, who along with health care workers should be fully immunized "by the end of the month of December or by the middle of the month of January."

• "I'm hopeful by the end of all the month of January we should already see quite a significant decrease in the mortality and severe morbidity associated in the elderly population," he added.

By the numbers: More than 270,000 people have died from the virus in the U.S., according to data from the COVID Tracking Project. 101,190 people are currently hospitalized with it.

Worth noting: Slaoui called Biden's plan to ask Americans to wear a mask for his first 100 days in office "a good idea."

• "It's never too late. ... We have a light at the end of the tunnel, but we will not all have the vaccine in our arms before May or June, so we need to be very cautious and vigilant," Slaoui said.