WILMINGTON, Del. (AP) — The White House's top medical adviser says the COVID-19 omicron variant is “just raging around the world."

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And President Joe Biden is planning on Tuesday to give what his press secretary says is “a stark warning of what the winter will look like” for unvaccinated Americans.

Dr. Anthony Fauci says “the real problem” for the U.S. hospital system is that “we have so many people in this country who are eligible to be vaccinated who have not yet been vaccinated.”

The prospect of a winter chilled by a wave of coronavirus infections is a severe reversal from the optimism projected by Biden months ago when he suggested the country would be back to normal by this Christmas.

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.