On Sunday, hundreds of flights were canceled in the United States for the third day in a row as crews were grounded owing to rising COVID-19 infections caused by the highly transmissible Omicron strain, causing tens of thousands of holiday visitors to modify their plans.

According to a count by flight-tracking website FlightAware.com, commercial airlines canceled more than 720 flights within, into, and out of the United States on Sunday.

This was down from over 1,000 on Christmas Day but similar to Christmas Eve, indicating that more cancellations were expected. In addition, almost 1,400 flights were canceled or delayed.

The fast spread of the Omicron variety coincided with the Christmas holidays, which are often a busy travel season. Infections have risen dramatically in several regions of the country, with the New York State Department of Health saying on Friday that it has seen a “startling” four-fold spike in COVID-19 hospital admissions among children under the age of 18 during the week beginning December 5.

While new study shows that Omicron causes milder sickness and fewer hospitalizations than prior versions, health experts remain cautious and believe there is still much to learn about the variation.

Dr. Mary Bassett, the interim state health commissioner in New York, said the state’s “dramatic surge” in COVID hospitalizations in the previous three weeks demonstrated the need for immediate action.

She recommended that all children above the age of five be completely vaccinated, while children under the age of five be protected by ensuring that others around them are protected through vaccination, boosters, mask-wearing, avoiding crowds, and testing.

It claimed that no 5- to 11-year-olds brought to the hospital with COVID had been properly vaccinated in the previous week. It didn’t say how many people were admitted to the hospital or how serious the instances were.