Recent models hint that the protective effects of a population that has already been infected by the COVID-19 virus may exist at much lower percentages than previously thought. Instead of requiring that 70 to 80 percent of a population be immune, that threshold may be less than 50 percent, or even as low as ten percent. At the very least, some degree of immunity may slow the spread of successive waves of infection. The new models may also help countries determine which groups should have priority in vaccination because they may be the most vulnerable and the most likely to spread the virus.