The CDC has finally updated their survey of antibody seroprevalence, a measure of the percentage of the population that has N-protein (from infection, not vaccine) antibodies.
They found that as of February 2022, 57.7% of the total U.S. population had antibodies, including 74.6% of children under 18.
Antibodies wane with time, so people infected in early waves may no longer test positive, although they still likely have robust immune responses via other mechanisms. And vaccinated people may never develop N-protein antibodies, because in some cases the vaccine-induced S-protein antibodies can neutralize an infection without N seroconversion occurring.
Bottom line: most Americans and nearly all kids had likely already had COVID by February 2022.
Here are the numbers by state: