Rat droppings from New York City. Poop from dog parks in Wisconsin. Human waste from a Missouri hospital. These are some of the materials that are readying us for the next chapter of the coronavirus saga.

More than four years into the pandemic, the virus has loosened its hold on most people’s bodies and minds. But a new variant better able to dodge our immune defenses may yet appear, derailing a hard-won return to normalcy.

Scientists around the country are watching for the first signs.

“We’re not in the acute phases of a pandemic anymore, and I think it’s understandable and probably a good thing” that most people, including scientists, have returned to their prepandemic lives, said Jesse Bloom, an evolutionary biologist at the Fred Hutchinson Cancer Center in Seattle.

“That said, the virus is still evolving, it’s still infecting large numbers of people,” he added. “We need to keep tracking this.”

Dr. Bloom and other researchers are trying to understand how the coronavirus behaves and evolves as populations amass immunity. Other teams are probing the body’s response to the infection, including the complex syndrome called long Covid.

And some scientists have taken on an increasingly difficult task: estimating vaccine effectiveness in a crowded respiratory milieu.