To date, a few verified repeat SARS-CoV-2 infections have been documented around the world. "Should reinfection [with SARS-CoV-2] prove commonplace, and barring a highly effective vaccine delivered to most of the world's population, SARS-CoV-2 will likely become endemic," write Jeffrey Shaman and Marta Galanti in this Perspective. For many viruses, a number of processes - particularly insufficient adaptive immune response, waning immunity, and immune escape - can allow subsequent reinfection. In the case of SARS-CoV-2, many questions remain about the nature of these immune responses and trajectories, though, say the authors, insight from other respiratory viruses points to the possibility of reinfection with SARS-CoV-2. If this does happen, the pattern of endemicity that results will depend on the typical time scale at which individuals experience reinfection, seasonal differences in transmissibility, vaccine availability and efficacy, and social, immune, and innate factors that modulate virus transmissibility, say the authors. In addition, the cyclic persistence of SARS-CoV-2 in human populations may be affected by ongoing opportunities for interaction with other respiratory pathogens, say the authors; it is possible infection with a different virus could provide some short-lived protection to SARS-CoV-2. Greater monitoring of the clinical and population-scale interactions of SARS-CoV-2 with other respiratory viruses, particularly influenza viruses, is needed, they write. At the population scale, a possible overlap between influenza and SARS-CoV-2 outbreaks poses a serious threat to public health systems. Conversely, the nonpharmaceutical interventions adopted to mitigate SARS-CoV-2 transmission (personal protective equipment, social distancing, increased hygiene, limited indoor gatherings) may reduce the magnitude of seasonal influenza outbreaks, note Shaman and Galanti. Based on modeling of post-pandemic scenarios for SARS-CoV-2 to date, a duration of immunity similar to that of the other betacoronaviruses (~40 weeks) could lead to yearly outbreaks of SARS-CoV-2, the authors note, whereas a longer immunity profile, coupled with a small degree of protective cross-immunity from other betacoronaviruses, could lead to apparent elimination of the virus followed by resurgence after a few years. "Other scenarios are, of course, possible, because there are many processes at play and much that remains unresolved," say the authors.