Unfortunately, the U.S./Mexico border is still very porous with people crossing back and forth for supposed "essential" travel. Americans and Mexicans who are residents of either side are crossing, workers, health workers, Mexicans with medical problems who are authorized to cross and those that know the thousands of border crossing guards that use their own criteria to let friends and family cross.
It's almost like a cross-contamination case. Both the U.S. and Mexico have the highest COVID death rates.
Live your lives as you wish, but this will only take longer. 1918 taught us a valuable lesson about pandemics; it takes about three years, people have some strange phobia of wearing a mask in public and social distancing. I believe it is an arrogance factor tempered by politics on both sides of the border. Mexico's president refuses to wear a mask and he could be next.
Mother Nature is doing her thing and viruses do have a purpose in the process. However, we can avoid a lot of it but people just don't like to be told what to do. Something silly about their rights and liberties. Nonsense.