Novel Self-Amplifying mRNA Vaccine Demonstrates Robust, Durable Immune Response


Vaccines were tested against various strains of the SARS-CoV-2 virus.

A booster dose of a novel, self-amplifying messenger RNA (sa-mRNA) vaccine for COVID-19 induces a “robust, broadly cross-reactive, and durable immune response” in adults lasting for 12 months, according to late-breaking data presented at the ninth European Scientific Working Group on Influenza (ESWI) Influenza Conference held in Valencia, Spain.1

“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,” said Igor Smolenov, chief development officer at Arcturus in a news release.1

In a phase 1/2 randomized, observer-blind clinical trial 36 adults in the United States and Singapore who had been previously immunized with COVID-19 mRNA vaccines as their primary series were randomly assigned 1:1:1 to receive 1 booster of either the ARCT-021, ARCT-154, or ARCT-165 vaccines. All of these vaccines encode the SARS-SoV-2 full-length S glycoprotein of, “respectively, the ancestral strain in native conformation, a prefusion-stabilized B.1 variant including the D614G strain, or the Beta variant.”

Investigators evaluated immunogenicity as neutralizing antibody titers against the SARS-CoV-2 D614G strain, as well as a panel of SARS-CoV-2 variants measured by pseudoviral microneutralization assays on 7 prespecified days after vaccination: day 1, 15, 29, 81, 181, 271, and 366.

Study results showed that all 3 vaccines induced a “robust neutralizing immune response” against the D614G variant at day 29, with geometric mean fold rises (GMFR) from pre-booster levels after each vaccine. ARCT-154—the leading candidate—“induced a broad, cross-neutralizing immune response,” persisting for up to 1 year post-booster, with no further boosting. Trends were similarly observed for other SARS-CoV-2 variants, including Beta, Delta, Omicron BA.1, Omicron BA.2, and Omicron BA.4/5.

Additional exploratory testing was conducted, and confirmed cross-neutralization against emergent Omicron BQ.1.1 and XBB.1.5 sublineages.

Arcturus is partnered with CSL Seqirus for the development of novel mRNA vaccines against SARS-CoV-2 and influenza.

“We are encouraged by the findings of the study, indicating the sa-mRNA platform’s potential to solve the challenge of mRNA vaccine waning immunity over time, and therby provide prolonged protection at lower doses,” said Esther Heijnen, MD, vice president of clinical development, vaccines innovation unit, at CSL.

1. Study shows novel sa-mRNA vaccines offer robust, broad, enduring protection against COVID-19 variants. News release. Arcturus Therapeutics. September 19, 2023. Accessed October 2, 2023.
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