Arcturus' stock sinks after self-amplifying COVID-19 mRNA vaccine posts 55% efficacy in phase 3

How good does a COVID-19 vaccine need to be to muscle in on the market at this late stage of the game? That is the question facing Arcturus as, armed with evidence of 55% efficacy as a primary series, it gears up for booster trials of its self-amplifying COVID-19 mRNA vaccine in the West.

Arcturus presented data from a phase 3 clinical trial of the vaccine candidate, ARCT-154, on Wednesday. The study, which was sponsored by Arcturus’ partner Vinbiocare Biotechnology, randomized more than 16,000 unvaccinated participants in Vietnam to receive two 5-mcg doses of ARCT-154 or placebo to test the vaccine’s ability to prevent COVID-19.

The trial met its primary endpoint. In a study that took place as Vietnam was transitioning from delta to omicron, ARCT-154 achieved 55% efficacy against symptomatic COVID-19. Efficacy against severe disease and death was 95%, with nine COVID-19-related deaths in the control arm and one in the vaccine group. The split of severe COVID-19 cases between the two arms was 41 to two.

While it is unclear what share of the cases were omicron, the results are broadly in line with knowledge of how vaccines against ancestral SARS-CoV-2 hold up against variants. As has been seen with other jabs in the real world, ARCT-154 failed to match the protection against symptomatic infection seen in the first wave of trials in 2020 but still managed to largely keep people out of the hospital. 

Vinbiocare has submitted the data to the Vietnam Ministry of Health as part of a request for emergency use authorization. However, around 80% of people in Vietnam have already received a primary course of a COVID-19 jab, leaving the company with a fairly small number of unvaccinated people to compete for.  

The bigger, although still tough to crack, opportunity lies in the booster space. Arcturus is going after that opportunity in the U.S. and Europe, with a pivotal study of a 5-mcg booster in the works following talks with regulators on both sides of the Atlantic. Booster uptake in high income countries is still below 50%, dragged down by the 30% rate in the U.S., but uptake has plateaued in recent months. 

Moderna and Pfizer are in pole position to hoover up the remaining opportunity for initial boosters and to meet the demands of countries that decide to give further rounds of booster shots. Faced with those entrenched incumbents, a company will need a differentiated vaccine to make a dent—and investors are doubting Arcturus’ jab fits the bill. Shares in Arcturus fell 20% to just above $20 in premarket trading.