On Dec. 21, 2023, CSL and and Arcturus Therapeutics announced the results of a Phase 3 study showing that a booster dose of ARCT-154, a novel, self-amplifying messenger RNA (sa-mRNA) vaccine, elicited a numerically higher immune response (meeting the non-inferiority criteria) against the original Wuhan-Hu-1 virus strain,  and a superior immune response against Omicron BA.4/5 subvariant of SARS-CoV-2 virus compared to a booster dose of the conventional mRNA vaccine Comirnaty^®. ARCT-154 results were achieved with one-sixth the dose of Comirnaty^® (5 μg vs 30 μg).

The study included healthy adults initially immunized with two doses of an mRNA vaccine (Comirnaty^® or Spikevax™); and then a third dose of Comirnaty^® at least three months prior to the booster dose of either ARCT-154 or Comirnaty^® in the study. Both vaccines were well-tolerated, with no causally associated severe or serious adverse events. The study was conducted in partnership with Meiji Seika Pharma, a global health company based in Japan. The study was published in Lancet Infectious Diseases.

In contrast to standard messenger RNA vaccine technology, self-amplifying messenger RNA (sa-mRNA) vaccine technology helps protect against infectious diseases by not only instructing cells in the body to make a specific protein, but also to make copies of these instructions in the cell. The produced protein antigen stimulates the immune response and leaves a blueprint to recognize and fight future infection.