MEXICO CITY—More than five hundred rich Mexicans, together with several of the country’s financial and organization elite, invested their yearly winter season vacation in the luxe resort city of Vail, Colo. In excess of two carefree weeks, they skied, shopped, hosted gatherings in their condos, and dined on sushi and steak at fashionable eating places.

By the time they returned to Mexico in early March, at least fifty of them experienced Covid-19, the ailment caused by the new coronavirus, together with the head of Mexico’s stock trade and the chief govt of the enterprise that…