The ARCT-154 vaccine was administered at a significantly lower dose of 5μg as against Comirnaty at 30μg. Credit: Viacheslav Lopatin / Shutterstock.

CSL and Arcturus Therapeutics have published follow-up analysis from a Phase III clinical trial indicating that ARCT-154, a self-amplifying messenger RNA (sa-mRNA) Covid-19 vaccine, offers a longer duration of immunity as a booster dose.

The study compared ARCT-154’s immunogenicity against that of the conventional mRNA Covid-19 vaccine Comirnaty one, three and six months after a booster dose.

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The randomised, double-blind trial assessed the vaccine’s ability to induce immunity against both the original Wuhan strain and the Omicron BA.4/5 variant.

ARCT-154 was administered at a significantly lower dose of 5μg as against Comirnaty at 30μg.

Six months after vaccination, the analysis showed that ARCT-154 induced a longer immune response and had an advantage in antibody persistence over Comirnaty.

Participants in both groups initially had similar geometric mean titers (GMTs) of surrogate virus-neutralising antibodies against the Wuhan-Hu-1 strain.

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One month after receiving the booster, the ARCT-154 arm showed a higher immune response with a GMT of 5,390 versus the Comirnaty group’s GMT of 3,738, resulting in a GMT ratio of 1.44.

By day 91, titers were equal to or greater than those on day 29 in 205 of the 369 ARCT-154 recipients, but only in 108 of the 356 Comirnaty recipients.

The different rates of antibody waning meant that by day 181, the GMTs of ARCT-154 recipients were 4,119, maintaining a GMT ratio of 2.21 between the vaccine groups.

A similar pattern of superior immunogenicity and a slower decline in neutralising antibodies following the ARCT-154 vaccine was also observed for the Omicron BA.4/5 variant.

CSL Vaccines Innovation Unit senior vice-president Jonathan Edelman said: “These results further support sa-mRNA’s differentiating attribute to provide prolonged protection against Covid-19 at lower doses.

“Protecting the global public from viral respiratory diseases remains a top priority for us, and we look forward to continuing to collect and share data at the twelve-month post-booster mark.”

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