WASHINGTON (AP) — President Joe Biden stirred some of the first public cracks in transatlantic unity over the invasion of Ukraine with his comments in Warsaw that Russian President Vladimir Putin “cannot remain in power,” tarnishing an otherwise successful four-day trip to Europe.

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He returned home early Sunday morning, where he faces political challenges, a stalled domestic agenda, and low approval ratings.

Democrats are counting on him to regain his footing as they face a potentially punishing midterm election year, and Republicans are poised to take control of Congress.

Answers to 25 common COVID-19 vaccine questions

Vaccinations for COVID-19 began being administered in the U.S. on Dec. 14, 2020. The quick rollout came a little more than a year after the virus was first identified in November 2019. The impressive speed with which vaccines were developed has also left a lot of people with a lot of questions. The questions range from the practical—how will I get vaccinated?—to the scientific—how do these vaccines even work?

Keep reading to discover answers to 25 common COVID-19 vaccine questions.