The U.S. could still see another coronavirus surge — even as vaccinations against Covid-19 rapidly rise across the nation — as states relax restrictions and more Americans travel for spring break, the head of the Centers for Disease Control and Prevention warned on Monday.

“With the coming warmer weather, I know it’s tempting to want to relax and let our guard down,” CDC Director Dr. Rochelle Walenksy said at a press briefing on Monday. “Particularly after a hard winter that sadly saw the highest level of cases and deaths during the pandemic so far.”

The Transportation Security Administration, or TSA, screened more than 1.34 million people on Sunday, 86,000 more people than the same day a year ago just after the World Health Organization declared Covid-19 a pandemic.

TSA screenings topped 1 million every day since Thursday, hitting the best stretch in a year. While air travel is far short of 2019 levels, more Americans are returning to the skies despite guidance from the CDC warning against nonessential trips, even for those who are fully vaccinated.

Even though many colleges in the U.S. scaled back their spring breaks in an effort to curb partying and infections, top Biden administration officials are still worried about travelers who are “enjoying spring break festivities, maskless,” Walensky said.

“I’m pleading with you, for the sake of our nation’s health,” Walenksy said at a press briefing Monday. “Cases climbed last spring, they climbed again in the summer, they will climb now if we stop taking precautions when we continue to get more and more people vaccinated.”

Even with declining infections and a rapidly expanding vaccine rollout, the U.S. continues to report a dangerously high baseline of daily cases that could reverse if Americans let their guard down, top Biden health officials have warned. Some 37.5 million people, about 11% of the American population, are fully vaccinated in the U.S. so far, according to the CDC.

The U.S. has come a long way since early January when it reached a peak of just over 250,000 daily new cases, based on a weekly average. The nation is now reporting a daily average of 53,670 new infections over the last week, a 10% decline compared with a week ago, according to a CNBC analysis of data compiled by Johns Hopkins University.

— CNBC’s Leslie Josephs and the Associated Press contributed to this report.