The head of the Centers for Disease Control and Prevention on Monday warned that an uptick in travel and spring break trips should be seen as “warning signs” of a potential backslide in progress against the coronavirus.

Over 1.3 million people passed through airport security on Friday. Sunday saw a similar number. CDC Director Rochelle Walensky said it’s the most travelers seen in a single day since before the World Health Organization declared the coronavirus a pandemic roughly a year ago.

She added that she’s seen footage of people “enjoying spring break festivities maskless.”

Cartoons on the Coronavirus

“This is all in the context of still 50,000 cases per day,” Walensky said.

She also noted coronavirus resurgences seen in Europe as an example of what could happen in the U.S. if mitigation measures are ignored.

“I’m pleading with you for the sake of our nation’s health,” Walensky said. “These should be warning signs for all of us. 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.”

While the CDC has issued guidance for activities fully vaccinated people can resume, it still recommends they avoid unnecessary travel.

But with travel ticking up anyway, White House senior adviser for COVID-19 response Andy Slavitt said that it is not the role of the U.S. government to issue so-called “vaccine passports.”

“Americans are asking the question: ‘How will I be able to demonstrate reliably that I’ve been vaccinated?’ And we have a couple of core beliefs on that. One is that it’s not the role of the government to hold that data and to do that,” Slavitt said.

He added that the data should be kept secure and private and that nonprofit organizations and private sector companies are working on “exactly that type of thing.”

Walensky also added that the CDC still predicts that the variant first found in the U.K. will become the dominant strain in the U.S. later this month or in early April. She said the strain is responsible for 25% of cases in Florida and California.