Dr. Anthony S. Fauci, the Biden administration’s top adviser on the pandemic, predicted on Sunday an “uptick” in coronavirus infections similar to the current increase in Europe, despite the current decline in cases, hospitalizations and deaths in the United States.

It is “no time at all to declare victory, because this virus has fooled us before and we really must be prepared for the possibility that we might get another variant,” Dr. Fauci said on ABC’s “This Week.” “And we don’t want to be caught flat-footed on that.”

While anticipating a new rise, Dr. Fauci said that at this time he does not expect a surge. Still, the BA.2 subvariant has been shown to be a more contagious version of the Omicron variant, though it, too, causes less-severe illness in most people.

Epidemiologists have said the current lull in U.S. cases mirrors the dip that Europe enjoyed before many officials in the region relaxed restrictions like indoor masking.

Even as U.S. case numbers continue to fall to levels reminiscent of last summer, BA.2 variant infections are making up a greater share of them. That proportion ranges from about 20 percent in the Midwest to 39 percent in New York and New Jersey, according to data from the Centers for Disease Control and Prevention.

With cases remaining low, Dr. Fauci said he doesn’t currently see the nation ramping up restrictions, “but you always have to have the flexibility.” He also cautioned that restrictions could come back into play if hospitalizations begin to rise.

About 65 percent of Americans are vaccinated, Dr. Fauci noted, and about half of the people eligible for a booster have gotten the shot. He said it remains critical for Congress to continue funding the tests, treatments and booster shots that may soon be in greater need.

Federal officials have already said that a stalled aid package for coronavirus countermeasures means that monoclonal antibody shipments to states will slow by about 30 percent this week and that orders for more antiviral pills are on hold.

Dr. Vivek Murthy, the U.S. surgeon general, echoed Dr. Fauci’s call for congressional funding for those anti-viral treatments and antibody infusions so they are available if needed in greater quantities.

He declined to predict an immediate surge in an interview on “Fox News Sunday,” saying the cases in Europe tend to presage those in the United States, and that Americans can expect to see rises and falls. He urged the nation to focus on “preparation, not panic.”

“We’re in that mile 18 of a marathon,” Dr. Murthy said. “We can’t quit, because Covid’s not quitting.”