Low Efficacy of Arcturus’ COVID-19 Vaccine Disappoints Investors at Phase III

Low Efficacy of Arcturus’ COVID-19 Vaccine Disappoints Investors at Phase III
Close up of Vials and Syringes with Covid-19 vaccine are displayed on a tray during vaccination.

Arcturus Therapeutics, a San Diego biotech with ‘self-amplifying’ RNA technology, announced its COVID-19 vaccine has reached its primary endpoint in a phase III trial this week.

The trial included more than 16,000 people and took place in Vietnam with help from Vietnamese pharma company Vinbiocare. While the vaccine prevented the majority of severe disease, it only had an efficacy of 55% at preventing symptomatic disease.

After the announcement, Arcturus’ stock fell by 20% on the Nasdaq, suggesting investors were unimpressed with the vaccine’s performance.

Arcturus has a number of products in development, but its COVID-19 vaccine makes use of its STARR technology platform, which combines self-replicating RNA with a specialized nanoparticle delivery system called LUNAR. The advantage of this system compared with other RNA vaccines is that a lower dose is required and, theoretically at least, this kind of vaccine has been predicted to trigger a better and more long-lasting immune response than other vaccines.

The phase III part of the trial involved more than 16,000 participants who were randomly assigned to receive placebo or a two-dose vaccination schedule (5 mcg/dose) of Arcturus’ vaccine candidate ARCT-154.

While the vaccine was highly effective at preventing severe or fatal disease with an efficacy of 95.3%, it was only 55% effective at preventing the onset of symptomatic COVID-19. But the company stated it was pleased with the results. “This represents a key milestone for the company and provides significant clinical validation of our STARR platform,” said Joseph Payne, President and CEO of Arcturus, in a press statement.

Side effects were minimal to moderate and no cases of myocarditis or pericarditis, which have been observed in low numbers for other mRNA vaccines, were observed. However, as myocarditis or pericarditis events were extremely rare for the other RNA vaccines the numbers in this study were not high enough to rule them out as an occasional adverse event.

Arcturus is filing the product for market approval in Vietnam based on the results of this study, but whether it will have success elsewhere remains to be seen. Timing comes into this, as with several vaccines on the market already and dropping or plateauing vaccination rates, a new COVID-19 vaccine needs to show that it has a unique selling point and ideally can outperform those already available on the market to be successful at this point. The impact of new variants such as Omicron on vaccine efficacy is also uncertain.

Like a number of other late comers to the COVID-19 vaccination stage, such as Novavax and Valneva, Arcturus is currently planning to try and get into the booster market with its vaccine. It is planning a booster study including 2,400 participants for use in Western markets to begin soon.

As well as its COVID-19 vaccine candidate, Arcturus is also working on a flu vaccine, a therapy for the liver disease ornithine transcarbamylase deficiency and a treatment for cystic fibrosis.