PROVIDENCE, Rhode Island — Wintry weather combined with the pandemic to frustrate air travelers whose return flights home from the holidays were canceled or delayed in the first days of the new year.

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More than 2,500 U.S. flights and more than 4,100 worldwide were grounded Sunday, according to tracking service FlightAware.

That followed Saturday’s mass cancelations of more than 2,700 U.S. flights, and more than 4,700 worldwide.

Saturday’s single-day U.S. toll was the highest since just before Christmas, when airlines began blaming staffing shortages on increasing COVID-19 infections among crews.

A winter storm that hit the Midwest on Saturday made Chicago the worst place in the country for travelers throughout the weekend as the region’s airports continued to recover Sunday morning.

About a quarter of all flights at O’Hare Airport were canceled Sunday.

American Airlines said most of Sunday’s canceled flights had been canceled ahead of time to avoid last-minute disruptions at the airport.

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.