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In an end of the year address on Tuesday, President-elect Joe Biden warned Americans that even as the first COVID-19 vaccinations begin in the United States, the "darkest days" in the pandemic are yet to come.

Biden spoke in Wilmington, Delaware and urged Americans to "remain vigilant" as experts say "things are going to get worse before they get better" in the pandemic, "notwithstanding the fact that" two coronavirus vaccines have now been approved.

"I'm going to tell it to you straight," Biden said. "I'm going to tell you the truth. And here's the simple truth: our darkest days in the battle against COVID are ahead of us, not behind us. So we need to prepare ourselves, to steel our spines."

Biden, who received a shot of a COVID-19 vaccine on Monday, urged Americans to continue avoiding large gatherings, noting his own family won't be holding its usual Christmas gathering of between 20 and 25 people.

"We all have to care enough for each other that we have to stay apart just a little bit longer," Biden said.

After Congress passed a COVID-19 relief package Monday night, the president-elect also praised lawmakers for approving the bill while calling it "just the first step" in addressing the crisis.

"Congress did its job this week," Biden said. "And I can, and I must, ask them to do it again next year." Brendan Morrow