Although the
coronavirus disease 2019 (COVID-19) epidemic is still ongoing, vaccination rates are rising slowly and related treatments and drugs are being developed. At the same time, there is increasing evidence of preexisting immunity against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in humans, mainly consisting of preexisting
antibodies and immune cells (including T cells and B cells). The presence of these
antibodies is mainly due to the seasonal prevalence of four common coronavirus types, especially OC43 and HKU1. The accumulated relevant evidence has suggested that the target of
antibodies is mainly the S2 subunit of S
protein, followed by evolutionary conservative regions such as the nucleocapsid (N)
protein. Additionally, preexisting memory T and B cells are also present in the population. Preexisting
antibodies can help the body protect against
SARS-CoV-2 infection, reduce the severity of
COVID-19, and rapidly increase the immune response post-
infection. These multiple effects can directly affect
disease progression and even the likelihood of death in certain individuals. Besides the positive effects, preexisting immunity may also have negative consequences, such as antibody-dependent enhancement (ADE) and original antigenic sin (OAS), the prevalence of which needs to be further established. In the future, more research should be focused on evaluating the role of preexisting immunity in
COVID-19 outcomes, adopting appropriate policies and strategies for fighting the pandemic, and
vaccine development that considers preexisting immunity.