Abstract: A method for controlled reduction of the weight average molar mass of a polysaccharide down to a lower and determined weight average molar mass, which method includes a step for mechanical milling of the polysaccharide by a ball mill, until the polysaccharide having the desired weight average molar mass is obtained.

Abstract: The present invention relates to an immunogenic composition for fish comprising nodavirus virus-like particles (VLPs) formed with nodavirus capsid protein assembly, for use as a vaccine. This composition is suitable for administration to fish via the intramuscular or intraperitoneal route, or by bath and/or via the oral route. The invention also relates to the use of such VPLs for the manufacturing of a vaccine for treating or preventing fish against a nodavirus infection. Fish farming baths and concentrates comprising the nodavirus VLP composition, as well as fish farming methods, are also encompassed in the present invention.