All I know is I got my flu shot last September in California and it definitely protected me from getting very sick. There were only 2 days where I felt I needed to rest. My husband (no flu shot) had fever, fatigued every day, excessive coughing up etc, for three weeks.
Also I would say SargassoPirate gave great advice to future cruisers. Let me add, if you choose to use the steps try not to hold the handrails and if you do make sure you use the hand gel which is located all over the ship.