The coronavirus was possibly in the U.S. as early as mid-December 2019. "Antibodies were detected in 39 samples on the West Coast in Washington, Oregon and California, all of them collected between Dec. 13 and 16." But before we get too far down the 'I already had it bro' track, bear in mind that at present "no more than 23 percent of the U.S. has antibodies for the virus," so the upper bound of possible immunity is still relatively low.