CSL and Arcturus Therapeutics announced the publication in Lancet Infectious Diseases of a Phase 3 study showing that a booster dose of ARCT-154, a novel, self-amplifying messenger RNA (sa-mRNA) vaccine, expedited 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. 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 phase 3 study results with ARCT-154 were used to support the approval of ARCT-154 for primary immunization and as a booster dose in Japan in November of this year. The ARCT-154 study is ongoing and will continue to collect safety data and assess durability of the immune response in participants at 3-, 6- and 12-months post-vaccination.

The randomized, double-blind, active-controlled study, conducted at 11 sites in Japan, was designed to compare the immunogenicity and tolerability of the sa-mRNA vaccine ARCT-154 with authorized mRNA COVID-19 vaccine Comirnaty. Investigators compared immune responses to ARCT-154 and Comirnaty booster doses in healthy Japanese adults 18 years of age or older initially immunized with two dose of mRNA COVID-19 vaccine (Comirnaty or Spikevax) and then a third dose ofComirnaty at least three months prior to receiving a booster dose of one of the study vaccines.