Perspective Overview of Changing Population Immunity to COVID-19 in the Context of Infection, Vaccination, and Emerging SARS-CoV-2 Variants

The changing state of protective immunity to COVID-19 in the global population in the six years since COVID-19's origin in 2019 is examined in the context of the (i) circulation of SARS-CoV-2 in the population, (ii) widespread use of different types of COVID-19 vaccines beginning in December 2020 and continuing to the present time, and (iii) ongoing evolution of SARS-CoV-2 to produce mutant viruses with greater infectivity, replication rate, evasion of immunity, and transmissibility. The outlook, and possible vaccine strategies, for the future control of COVID-19 are also examined.