President Joe Biden said Thursday that Covid deaths in the United States will continue to rise due to the spread of the “dangerous” delta variant, calling it a “serious concern.”

“Six hundred thousand-plus Americans have died, and with this delta variant you know there’s going to be others as well. You know it’s going to happen. We’ve got to get young people vaccinated,” Biden said at a community center in Raleigh, North Carolina.

The variant, Biden said, is more easily transmissible and potentially deadly, and is “especially dangerous for young people.”

The president warned that Americans who are still unvaccinated are especially at risk.

“The data couldn’t be clearer: If you’re vaccinated, you’re safe,” Biden said. “You are still at risk of getting seriously ill or dying if you in fact have not been vaccinated, that’s just the fact.”

The delta variant now accounts for at least 20% of all new Covid cases in the U.S., according to Centers for Disease Control and Prevention data. The variant has a doubling rate of about two weeks, setting it on a path to become the dominant strain in the U.S. in a few weeks, White House chief medical advisor Dr. Anthony Fauci said Wednesday.

The agency has designated the more transmissible variant a “variant of concern.”

Experts say the delta variant could also cause more severe disease in those infected, but more data is needed to be sure.