Moderna Inc shares tumbled 11% after early data showed that the company's mRNA-based flu vaccine might not be strong enough to give it an edge over already approved flu vaccines in the market, especially for older people.

The Cambridge, Massachusetts-based vaccine maker said in an investor presentation that antibody levels of its vaccine against all four strains of the influenza virus in an early stage study were not as robust in older adults as Sanofi's Fluzone HD.

"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.

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

Following the huge success of mRNA-based vaccines in fighting the COVID-19 pandemic globally, drugmakers are rushing to tap the technology and develop shots for other diseases, including influenza.

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.

This data, though limited, puts Moderna ahead of other drugmakers, including rival Pfizer Inc, that are also testing their flu vaccine based on messenger RNA.

Moderna also said it was developing a pan-respiratory annual booster vaccine, which would target multiple viruses including COVID-19, flu and Respiratory syncytial virus.

The new vaccine, mRNA-1345, has demonstrated positive early-stage results, it said.

Vaccine developer Novavax is also testing a combined flu and COVID-19 vaccine, which has shown promise in preclinical studies.