We have learned a lot about the SARS-CoV-2 virus during the four years of the COVID-19 pandemic. Because the virus replicates its RNA and mutates in every infection, its spike proteins keep changing. Thus, by the time a vaccine targeting a specific spike protein becomes available, a new variant of the virus with a new spike protein emerges that might be both more transmissible or cause more severe disease.
For example, new viral variants such as the original Omicron and the many subtypes descended from it were less susceptible to the vaccines developed to protect against the original Wuhan SARS-CoV-2. Moreover, even when the vaccine and virus are a good match, protection from the vaccine wanes substantially with time. For those reasons and because vaccine uptake, especially of...