Arcturus Reports its sa-mRNA COVID-19 Vaccine Protects Against Variants


These novel vaccines are utilizing a self-amplifying messenger RNA (sa-mRNA) vaccine platform. Study data was reported at the ESWI Influenza Conference in Spain.

3 Key Takeaways

  • Arcturus Therapeutics' sa-mRNA COVID-19 booster vaccine demonstrates long-lasting protection.
  • The vaccines showed broad-spectrum protection against multiple COVID-19 variants.
  • The sa-mRNA booster vaccines reported mild to moderate adverse events that resolved quickly, with low rates of related or severe adverse events.

Arcturus Therapeutics reported a booster dose of its investigational sa-mRNA COVID-19 vaccine induced robust neutralizing immune response against the COVID-19 variant, D614G. The company said there was a durable immune response in adults that remained elevated through 12 months after vaccination.

“Current mRNA technologies provide effective initial immunogenicity against COVID-19, but the results of this study show that our sa-mRNA vaccine platform can offer improvements in duration and breadth of protection against new and emerging variants,” Igor Smolenov, chief development officer, Arcturus, said in a statement.

Trial Results

This study was a phase 1/2 randomized, observer-blind trial conducted in both the US and Singapore, and included 36 adult participants who were previously immunized with approved COVID-19 mRNA vaccines as a primary series.

Participants were randomized 1:1:1 to receive one booster dose on day 1 of either the ARCT-021, ARCT-154, or ARCT-165 vaccines, all of which encode the SARS-CoV-2 full-length S glycoprotein of, respectively, the ancestral strain in native conformation, a prefusion-stabilized B1 variant including the D614G mutation, or the Beta variant. Immunogenicity was assessed as neutralizing antibody titers against the SARS-CoV-2 D614G strain, and a panel of variants measured by pseudoviral microneutralization assay on Days 1, 15, 29, 91, 181, 271, and 366.

All 3 vaccines induced robust neutralizing immune response against the D614G variant at Day 29 with geometric mean fold rises (GMFR) from pre-booster levels of 20.0, 36.7 and 23.5 after ARCT-021, ARCT-154, and ARCT-165, respectively. ARCT-154, a leading candidate, induced a broad, cross-neutralizing immune response, which persisted up to one-year post-booster with no further boosting. Similar trends were observed for other variants including Beta, Delta, Omicron BA1, Omicron BA2, and Omicron BA4/5. Additional exploratory testing confirmed cross-neutralization against emergent BQ11 and XBB15 Omicron sub-lineages with GMFRs of 12.8 and 3.4, respectively, at Day 29 post-booster.

According to Arcturus, solicited adverse events (AE) were assessed up to 7 days, unsolicited AEs up to 28 days, and serious AEs up to 366 days after vaccination. Adverse events were mild or moderate and resolved quickly, and rates of related or severe AEs were low.


Arcturus is in an exclusive partnership with CSL Seqirus, CSL’s vaccine subsidiary, in developing novel mRNA vaccines against COVID-19 and influenza.

“We are proud of the role Arcturus has played, collaborating with CSL, in advancing sa-mRNA vaccine development,” Smolenov said.

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