Differential clearance rate of proteins encoded on a self-amplifying mRNA COVID-19 vaccine in muscle and lymph nodes

ARCT-154, a recently approved self-amplifying mRNA (saRNA) vaccine for SARS-CoV-2, has shown superior induction and prolonged maintenance of neutralizing antibodies compared to the conventional mRNA vaccine BNT162b2. However, the scientific evidence explaining this superiority remained elusive. Hence, we explored the temporal changes in spike protein and replicase components following a single dose of ARCT-154 vaccination in mice. The encoded spike protein reached its highest level approximately 3 days after vaccination and quickly disappeared from the rectus femoris muscle, the injection site. Although the spike protein levels also peaked at an early time point in the lymph nodes, it remained detectable 28 days after the vaccination and then disappeared by 44 days after the vaccination. Expression of nsP1, nsP2 and nsP4 was observed in the injected muscle and/or the lymph nodes for up to 15 days post-vaccination. Data were analyzed using unpaired two-tailed Mann-Whitney U-tests. These data suggest that prolonged expression of spike proteins in lymph nodes may, if not entirely, be responsible for the induction of higher and prolonged levels of neutralizing antibodies by the saRNA vaccine.