Reinfection and Breakthrough Infection of SARS-CoV-2: An Emerging Challenge That Is Threatening Our World

The pandemic of coronavirus disease 2019 has threatened humans for more than one and a half years. In particular, viral mutation like delta strain has led to third- or fourth-wave transmission among the countries in Asia, Europe, and North America. Although large-scale vaccination has been carried out in many countries, the incidence of reinfection and vaccine-past breakthrough infection is becoming an emerging challenge to humans worldwide. The related mechanisms underlying the reinfection and breakthrough infection remain unknown. In this review, we summarized the challenge and related reasons for severe acute respiratory syndrome coronavirus 2 reinfection and breakthrough infection. Simultaneously, we addressed some critical contents of the study in future.