Abstract
BACKGROUND: During spring 2014, two large influenza outbreaks occurred among cruise ship passengers and crew on trans-hemispheric itineraries. METHODS: Passenger and crew information for both ships was obtained from components of the ship medical records. Data included demographics, diagnosis of influenza-like illness (ILI) or acute respiratory illness (ARI), illness onset date, passenger cabin number, crew occupation, influenza vaccination history, and rapid influenza diagnostic test (RIDT) result, if performed. RESULTS: In total, 3.7% of passengers and 3.1% of crew on Ship A had medically attended acute respiratory illness (MAARI). On Ship B, 6.2% of passengers and 4.7% of crew had MAARI. In both outbreaks, passengers reported illness prior to the ship's departure. Influenza activity was low in the places of origin of the majority of passengers and both ships' ports of call. The median age of affected passengers on both ships was 70 years. Diagnostic testing revealed three different co-circulating influenza viruses [influenza A(H1N1)pdm09, influenza A(H3N2), and influenza B] on Ship A and one circulating influenza virus (influenza B) on Ship B. Both ships voluntarily reported the outbreaks to the Centers for Disease Control and Prevention (CDC) and implemented outbreak response plans including isolation of sick individuals and antiviral treatment and prophylaxis. CONCLUSIONS: Influenza activity can become widespread during cruise ship outbreaks and can occur outside of traditional influenza seasons. Comprehensive outbreak prevention and control plans, including prompt antiviral treatment and prophylaxis, may mitigate the impact of influenza outbreaks on cruise ships.