In an investor update on Monday, Arcturus Therapeutics Holdings (NASDAQ:ARCT) reported early-stage clinical trial results for its coronavirus vaccine candidate. The company also said that the Singapore Health Sciences Authority had granted approval to begin a phase 2 study of that candidate, ARCT-021.

Although the results indicate that the one-dose vaccine is effective at blocking the coronavirus, it produces fewer neutralizing antibodies than the vaccine made by Pfizer (NYSE:PFE) and BioNTech (NASDAQ:BNTX), and the vaccine of Moderna (NASDAQ:MRNA). Compounding that, Pfizer/BioNTech's BNT-162b2 and Moderna's mRNA-1273 have been approved by U.S. and European Union healthcare regulators for emergency use.

Like BNT-162b2 and mRNA-1273, Arcturus' vaccine candidate utilizes messenger RNA to instruct the body's cells how to combat the coronavirus.

A syringe being filled with a vaccine from a vial.

Image source: Getty Images.

Still, in its press release disseminating the news, Arcturus emphasized the successes of ARCT-021 in the trials.

"The Phase 1/2 study results, together with recently generated pre-clinical data, indicate that ARCT-021 leads to a potent immune response to SARS-CoV-2, and demonstrates a differentiated biological profile whereby the immune response increases in the weeks following vaccination," said Ooi Eng Eong, a member of its vaccine platform scientific advisory board, according to the company.

The company expects to harvest phase 2 data early next year; it did not get more specific.

Arcturus was considered by many to be an underdog in the race to bring a coronavirus vaccine to market. ARCT-021's results relative to its Pfizer/BioNTech and Moderna rivals are disappointing, and only exacerbate that status.

As a result, the company's stock plummeted on Tuesday: down by nearly 55% on the day, far exceeding the modest decline of the S&P 500 index.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.