the link I had has been outdated (I should have saved that page as a pdf) ... but the information is included in this page on nih regarding the description of the vaccine type.
Self-replicating mRNA constructs (replicons) encode an RNA-dependent RNA polymerase (RDRP) complex required for self-amplification as well as the components found in nonreplicating constructs (
Figure 1B) [
19,
20]. The RDRP complex is often derived from alphaviruses, e.g., Sindbis virus [
19,
20]. Self-replication increases the magnitude and duration of construct expression and, consequently, production of the encoded immunogen. In non-human primates (NHP), low doses of self-replicating mRNA vaccine induced enhanced immunogen production for an extended duration, where production peaked on day 3 and remained detectable for more than 14 days following immunization [
20]. Similarly, immunization with a self-replicating mRNA construct induced more protein synthesis for a longer period of time and a greater immune response in mice, compared with a nonreplicating mRNA vaccine [
21]. An additional advantage of self-replicating mRNA constructs is the ability to incorporate multiple gene sequences into the same replicon, allowing the expression of both the target immunogen and immunomodulatory molecules such as CD40L, CD70, OX40L, and GM-CSF to enhance potency [
22,
23].
this means ... no boosters ... in layman's terms.
mRNA-Based Vaccines