Shares of Arcturus Therapeutics Holdings ( ARCT -3.02% ) were plunging 53.7% as of 11:16 a.m. EST on Tuesday. The huge decline came after the biotech reported data from an early-stage clinical study of COVID-19 vaccine candidate ARCT-021.
Investors had high hopes that a single dose of ARCT-021 could be highly effective at preventing infection by the coronavirus that causes COVID-19. But Arcturus' latest results poured cold water on those expectations.
Arcturus reported that its vaccine produced neutralizing antibodies after only one dose. But the levels of the neutralizing antibodies were lower than those produced by Pfizer's and Moderna's two-dose COVID-19 vaccines.
Raymond James analyst Steven Seedhouse stated in a note to the firm's clients that the efficacy for ARCT-021 with a single dose could be as low as 62%. That's a disappointing level compared to Pfizer's efficacy of 95% and Moderna's 94%.
It's possible that a two-dose regimen of ARCT-021 could be much more effective. But it wouldn't give Arcturus a competitive advantage over rivals like a highly effective single-dose vaccine would. There's also at least a possibility that further clinical studies will show that a single dose of ARCT-021 does generate a higher efficacy than the early-stage results appear to indicate. For now, though, much of the excitement about Arcturus' prospects have understandably fizzled.
Arcturus received approval from regulators in Singapore to advance ARCT-021 to a phase 2 clinical study. The company anticipates reporting interim results from this study in early 2021 and beginning a global phase 3 study in the second quarter of next year. The biotech stock probably won't have any significant chances of rebounding unless the results from these studies are much better than the early-stage data.