Moderna Inc (MRNA.O) executives called on investors to wait for more data amid a sharp selloff in its shares on Friday, after early data showed its mRNA-based flu vaccine was no better than already approved shots in the market.

The company’s shares tumbled 10% after its investor presentation showed antibody levels of its vaccine against all four strains of the influenza virus in an early-stage study were not as robust in older people as Sanofi’s (SASY.PA) Fluzone HD.

“We can’t make a direct comparison. We presented (Fluzone data) only as guidance,” a company executive said on a conference call, responding to concerns that Moderna’s vaccine was inferior to Fluzone HD.

Moderna’s vaccine candidate, mRNA-1010, is based on the messenger RNA technology, which also underpins its COVID-19 shots.

Following the huge success of mRNA-based vaccines in fighting the pandemic globally, drugmakers are rushing to tap the technology and develop shots for other diseases, including influenza.

“On one hand, the antibodies increased to good levels, but on the other hand, the levels aren’t necessarily seen as high or necessarily better than some high-efficacy vaccines such as Flublok or Fluzone HD,” Jefferies analyst Michael Yee said in a client note.

Moderna executives said the antibody response might have been limited by the population’s previous exposure to the flu strains, adding that data on the full immune response, including T-cells, would better measure the effectiveness of the flu vaccines.

“Antibody titers are useful but are not the same as full efficacy,” senior executive Jacqueline Miller said.

The early-stage study, testing the vaccine in 180 subjects, is ongoing and a mid-stage study testing it against an already approved flu vaccine is fully enrolled with 500 participants, the company said, adding that the interim analysis is expected in early 2022.

U.S.-listed shares of BioNTech , which along with partner Pfizer Inc (PFE.N) is also developing an mRNA-based influenza vaccine, were down nearly 8%.