A top coronavirus adviser to President-elect Joe Biden delivered a stern holiday message to Americans on Thursday – “no Christmas parties” – and warned they face a COVID-19 siege for weeks to come despite the latest moves toward U.S. government approval of a vaccine.

“The next three to six weeks at minimum … are our COVID weeks,” Dr. Michael Osterholm, a member of Biden’s coronavirus advisory board, told CNN. “It won’t end after that, but that is the period right now where we could have a surge upon a surge upon a surge.”

Osterholm stressed that it would be several months before the nation sees widespread availability of vaccines, the first of which cleared a key U.S. regulatory hurdle on Thursday.