Germany's CureVac announced Wednesday that final trial results showed its coronavirus vaccine had an efficacy rate of just 48 percent, far lower than those developed by mRNA rivals BioNTech and Moderna.
The outcome had been expected after poor interim results were released earlier this month.
The company in part blamed "the unprecedented context of 15 strains circulating" among the trial volunteers, as well as varying responses across different age groups.
The Covid vaccines developed by German rival BioNTech/Pfizer and US firm Moderna were both approved some 18 months ago after showing around 95 percent efficacy.
Their trials at the time only had to contend with the original strain of the virus.
CureVac said its jab, known as CVnCoV, did slightly better among people aged 18 to 60 than among older ages, with efficacy climbing to 53 percent.
Among that same 18-60 age group, the vaccine offered 100 percent protection against hospitalisation and death.
"In this final analysis, CVnCoV demonstrates a strong public health value" for people aged 18 to 60, "which we believe will be an important contribution to help manage the Covid-19 pandemic and the dynamic variant spread," chief executive officer Franz-Werner Haas said in a statement.
CureVac's late-stage Phase 2b/3 trial involved some 40,000 people in 10 countries in Europe and Latin America. Of those, 228 contracted the coronavirus.
CureVac, founded in 2000 by mRNA pioneer Ingmar Hoerr, said in May that independent analysis "found no safety concerns" with its two-dose vaccine.
The company said it had shared its data with the European Medicines Agency (EMA) and would give further details at a press conference on Thursday.
Despite being a laggard in the vaccine race, CureVac believes it has advantages over its mRNA competitors.
CureVac's product can be stored at standard refrigerator temperature, unlike the first-generation Pfizer and Moderna vaccines which require super-cold freezers.
CureVac's vaccine also requires a lower dosage, allowing for faster and cheaper mass production.
The European Union has secured up to 405 million doses of the CureVac vaccine.
The company is already working on a second-generation vaccine for which it has teamed up with pharma giant GSK.