It's now predictable that the South will see surges in the summer and Northern states will see surges in the winter -- especially around the holidays, Birx said.

In new book, Birx describes how data failures, bureaucracy and politics hindered Covid-19 response

Birx said she closely follows data out of South Africa, which has recently seen a rise in new Covid-19 cases.

"Each of these surges are about four to six months apart. That tells me that natural immunity wanes enough in the general population after four to six months -- that a significant surge is going to occur again," Birx told "Face the Nation."

"This is what we have to be prepared for in this country. We should be preparing right now for a potential surge in the summer across the Southern United States because we saw it in 2020 and we saw it in 2021."

Public health officials need to make clear to the public that protection against the infection wanes over time, and precautions should be taken with vulnerable or compromised people, said Birx.

She said Covid-19 home testing kits and booster shots are critical tools to help Americans handle surges.

Birx's warning comes as US cases are again rising with the spread of another Omicron strain, the BA.2 subvariant. The seven-day average of US cases was almost 54,000 Saturday, up from about 49,000 a week earlier and almost 31,000 a month ago.

Nearly 60% of adults and 75% of children have antibodies indicating that they've been infected with Covid-19, according to new data from the US Centers for Disease Control and Prevention.

It is unclear what that means for protection against future infections, health experts say, and for that reason, the CDC says it is still important to stay up to date on Covid-19 vaccinations and boosters.

Dr. Anthony Fauci, President Joe Biden's chief medical adviser and the director of the National Institute of Allergy and Infectious Diseases, caused a stir last week when he said, "We are certainly, right now, in this country, out of the pandemic phase." He clarified that later to explain he didn't mean that the pandemic is over, but that the country is in a transition phase.

"We're not over the pandemic. Don't let anybody get the misinterpretation that the pandemic is over, but what we are in is a different phase of the pandemic," Fauci said. "A phase that's a transition phase, hopefully headed toward more of a control where you can actually get back to some form of normality without total disruption of society, economically, socially, school-wise, etc."

CNN's Theresa Waldrop, Naomi Thomas and Brenda Goodman contributed to this report.