Two sources of immunity from infectious diseases are widely recognized: vaccination and immunity acquired naturally in survivors of previous infections. The latter may be involved in the roller-coaster up-and-down behavior seen in the progression of daily new COVID cases after the current pandemic began in early 2020. Cases increased as the virus spread throughout the nation, in part due to insufficient attention to limiting exposures, but what could account for subsequent decreases? We (and others) ascribe the long downward slide from the 2020-21 winter peak to the coincident national vaccination program. Here we set out to investigate possible contributions from infection-acquired immunity.
