Varicella at sea: a two-year study on cruise ships.

BACKGROUND Being highly contagious by person-to-person transmission, varicella can easily spread within the multinational population of a cruise ship and into communities ashore. The aim of the study was to report the prevalence of varicella infections in a fleet of cruise ships during a two-year period and to discuss measures to prevent and contain shipboard outbreaks. MATERIAL AND METHODS All probable varicella cases among passengers and crew on 34 cruise ships were registered for 2 years by the medical facilities onboard. Patients remained isolated until 6 days after rash onset. Susceptible contacts were identified and offered post-exposure prophylaxis. Crew nationality, number of vaccinated contacts, and direct vaccination costs were registered. RESULTS During two years 187 varicella cases (36 passengers, 151 crew) were registered and 2,685 varicella vaccinations were administered at an estimated direct vaccination cost of US $ 283,832. Of the 34 ships, only 3 reported no cases of varicella. There were 8 clusters ('outbreaks') of ≥ 5 varicella cases presenting less than 42 days apart, comprising a total of 89 patients. While > 130 nations were represented among the crew, the 151 crew cases came from 26 countries, and 88 (58%) of them came from 5 sub-tropical/tropical countries. CONCLUSIONS All cruise vessels must expect to encounter varicella cases or outbreaks onboard every few years. Every varicella case can start an outbreak and thus trigger several time-consuming and expensive containment measures, including isolation and mass vaccination of susceptible contacts. Mandatory pre-contract evidence of varicella immunity from all seafarers or from subgroups according to position or nationality might be worth considering. Seafarers known to be immune to varicella should always carry valid documentation while traveling.