Moderna said on Tuesday it had not enrolled enough cases in a late-stage trial of its experimental flu vaccine to determine if the shot was successful or not, sending the company’s shares down 8% in premarket trading.

The company said, ahead of its annual vaccines conference, that it will continue testing the flu shot for efficacy.

Data from the second trial comes two months after its flu vaccine generated a strong immune response against influenza A strains, but was inferior to an approved vaccine when compared to the less-prevalent influenza B strain in the first late-stage study.

Moderna also forecast sales from its respiratory vaccines to be between $8 billion and $15 billion in 2027. It is testing vaccines against respiratory syncytial virus (RSV), influenza and a next-generation Covid-19 shot in late-stage studies.

The company said it expects to launch six major vaccines in the next few years.