Moderna shares slipped Tuesday morning after the COVID-19 vaccine developer said its potential flu vaccine needs more study in a late-stage clinical trial.

The company said an independent data and safety monitory board found that the potential vaccine “did not meet the statistical threshold necessary to declare early success” in the study.

The board recommended that the trial should continue.

Moderna is developing the vaccine, labeled mRNA-1010, to guard against seasonal flu. It is testing the vaccine in two, late-stage studies in the Southern and Northern hemispheres.

The vaccine developer said Tuesday it expects to launch six vaccines in the next few years.

The company’s COVID-19 vaccine, Spikevax, has been its main revenue generator so far. It brought in more than $18 billion in sales last year.

Moderna also is developing potential vaccines for respiratory syncytial virus, or RSV, and the human immunodeficiency virus that causes AIDS.

Shares of Moderna Inc., based in Cambridge, Massachusetts, fell nearly 5% to $152.50 before markets opened Tuesday.