Severe acute respiratory syndrome coronavirus 2 virus (SARSCoV-
2) infection, which causes coronavirus disease 2019
(COVID-19), has led to many cases and deaths worldwide.
Therefore, a number of vaccine candidates have been developed
to control the COVID-19 pandemic. Of these, to date,
21 vaccines have received emergency approval for human
use in at least one country. However, the recent global emergence
of SARS-CoV-2 variants has compromised the efficacy
of the currently available vaccines. To protect against these
variants, the use of vaccines that modulate T cell-mediated
immune responses or innate immune cell memory function,
termed trained immunity, is needed. The major advantage of
a vaccine that uses bacteria or viral systems for the delivery of
COVID-19 antigens is the ability to induce both T cell-mediated
and humoral immune responses. In addition, such
vaccine systems can also exert off-target effects via the vector
itself, mediated partly through trained immunity; compared
to other vaccine platforms, suggesting that this approach can
provide better protection against even vaccine escape variants.
This review presents the current status of the development of
COVID-19 vaccines based on recombinant viral and bacterial
delivery systems. We also discuss the current status of the
use of licensed live vaccines for other infections, including
BCG, oral polio and MMR vaccines, to prevent COVID-19
infections.