From the NY Times:
Experts are pondering ways to induce more companies to make flu vaccine for the American market. The issue is not that manufacturers are worried about lawsuits over liability, as President Bush has suggested. Litigation is seldom, if ever, cited in authoritative analyses of vaccine shortages. The main problem is that influenza vaccine needs to be reformulated every year, and companies suffer huge losses if they overestimate the amount that will be needed because they end up having to destroy millions of doses. The administration needs to find a way to expand and stabilize the vaccine manufacturing base. The lesson of the Chiron debacle is that a diversity of supply is critical.