President Joe Biden said he is weighing a requirement for federal workers to get vaccinated as the highly contagious delta variant spreads throughout the U.S.

A COVID-19 vaccine mandate for federal workers is “under consideration,” Biden told reporters on Tuesday after visiting the Office of the Director of National Intelligence in Virginia.

The White House is considering whether to require workers to confirm that they’ve been vaccinated, or submit to regular testing and follow strict protocols like wearing masks, even in areas where there isn’t significant community spread, according to a person familiar with the matter.

The president’s comments reflect the heightened concern among American public health officials about the delta variant. He made the remarks after the U.S. Centers for Disease Control and Prevention tightened mask guidance, advising that fully vaccinated individuals cover their faces indoors in places where the virus is rapidly spreading.

“The more we learn, the more we learn about this virus and the delta variation, the more we have to be worried and concerned,” Biden said.

The White House on Tuesday told staff that they must again wear masks at the complex.

COVID-19 fatalities in the U.S. surged 48% over the past week to a daily average of 239 and hospitalizations are also on the rise. The CDC says the delta variant now makes up 83% of all sequenced COVID-19 cases in the U.S., up from 50% at the beginning of the month. Areas of the country with limited vaccination coverage are allowing spread of the highly transmissible variant.

CDC Director Rochelle Walenksy said recent data suggests that some vaccinated people who get infected with the delta variant can spread it to others. Yet Biden, Walensky and other health officials stressed that getting a COVID-19 vaccine offers the best protection against death, hospitalization and illness.

Just over 69% of American adults have had at least one vaccine shot, still short of the 70% goal Biden set for the country to meet by July 4.

“There’s only one thing we know for sure: If those other 100 million people got vaccinated, we’d be in a very different world,” Biden said Tuesday. “So get vaccinated. If you haven’t, you’re not nearly as smart as I said you were.”

In a written statement, Biden called the revised CDC guidelines “another step on our journey to defeating this virus.” He said he would follow it when he travels to areas with high rates of infection. Biden also said that on Thursday he would “lay out the next steps” for getting more Americans vaccinated, but did not specify them.