London CNN Business  — 

Shares in CureVac plummeted on Thursday after the German biotech company said that interim trial results showed its experimental Covid-19 vaccine was only 47% effective in preventing symptomatic disease.

The disappointing results sent shares down 45% in afternoon trading in Germany, wiping nearly $8 billion off the company’s market value. Shares listed in the United States dropped sharply at the start of trading.

Efficacy may have been reduced because almost all the volunteers in the 10-country study were exposed to more transmissible new Covid variants, as opposed to other vaccines already on the market that were tested against older strains of the virus, CureVac said.

“At second interim analysis, statistical success criteria not met. Favorable safety profile confirmed,” the company said in a statement Wednesday.

The vaccine, called CVnCoV, uses messenger RNA or mRNA — just as the highly successful Pfizer (PFE) and Moderna (MRNA) vaccines do. It’s delivered in lipid or fat particles as those two vaccines are. But both Moderna (MRNA) and Pfizer (PFE) modified the mRNA, while the CureVac vaccines uses unmodified mRNA. It is not clear whether that is what made the difference.

“In the unprecedented context of at least 13 variants circulating within the study population subset assessed at this interim analysis, CVnCoV demonstrated an interim vaccine efficacy of 47% against Covid-19 disease of any severity and did not meet prespecified statistical success criteria,” the company said.

CureVac’s shares had soared on hopes for the vaccine when the company went public last year. Dietmar Hopp, the billionaire co-founder of software giant SAP (SAP) owns nearly half of the company, which has also received the backing of the Bill and Melinda Gates Foundation, GlaxoSmithKline (GLAXF) and the German government.

In its statement, the company said the Phase 2b/3 study of 40,000 volunteers would continue. It had planned to seek authorization in Europe first. CureVac based the analysis on the first 147 cases of Covid-19 among the volunteers. “More than half of the cases (57%) were caused by variants of concern,” the company noted.

“While we were hoping for a stronger interim outcome, we recognize that demonstrating high efficacy in this unprecedented broad diversity of variants is challenging. As we are continuing toward the final analysis with a minimum of 80 additional cases, the overall vaccine efficacy may change,” CEO Franz-Werner Haas said in the statement.

“In addition, the variant-rich environment underlines the importance of developing next-generation vaccines as new virus variants continue to emerge.” CureVac is working with GlaxoSmithKline on a second-generation coronavirus vaccine.