Influenza virus infection in travelers to tropical and subtropical countries.

Background: Influenza outbreaks have been reported among travelers, but attack rates and incidence are unknown.

Methods: A cohort study was conducted. Travelers to subtropical and tropical countries recruited at the University of Zurich Travel Clinic (Switzerland), January 1998 to March 2000, were investigated with pre- and posttravel assessment of hemagglutination inhibition and by questionnaire.