The German biotech’s vaccine proved only 47% effective in an initial trial, far lower than those of rivals BioNTech/Pfizer and Moderna, which are also based on mRNA technology and had efficacy rates well above 90%.

Peter Kremsner from the University Hospital in Tuebingen which is running the trial told Reuters the low efficacy is “very likely due to the dose”.

Unlike BioNTech and Moderna, CureVac did not modify the mRNA used in the shot and therefore a higher dosage was not possible because of the side effects to be expected, Kremsner said.

With just 12mg of mRNA per dose, the company has been seeking to position itself as a low-dose alternative to BioNTech and Moderna’s established shots with 30 and 100mg per shot, respectively.

It is also easy to store and cheaper than the other mRNA shots too. The European Union has agreed to pay €12 per dose for supplies of the shot, an EU internal document shows, but has paid more than €15 for the other mRNA shots.

However, the interim trial analysis goes some way to challenging the expectation, which was previously reflected in the company’s share price, that all mRNA vaccines are similar. The news wiped as much as 52% off the company’s value on Thursday.

CureVac said on Wednesday that new variants had proved a headwind, but data from BioNTech and Moderna have, however, so far suggested only somewhat weaker protection against new variants.

Kremsner said he thought it was probably a “long shot” as to whether CureVac’s vaccine might turn out to be more effective when targeting a particular variant.

The disappointing results are a blow since European governments had been hoping to add it to their arsenal after age limits were imposed on the use of the Johnson & Johnson and AstraZeneca vaccines due to a link to an extremely rare but potentially fatal clotting disorder.