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.


RESULTS
Among 1450 travelers recruited who completed questionnaires and provided serum samples before departure, 289 (19.9%) reported febrile illness during or after traveling abroad; of these, 211 (73.0%) provided paired serum samples. Additionally, paired serum samples were collected from 321 frequency-matched afebrile control subjects among the remaining 1161 subjects of the study population. Seroconversion for influenza virus infection was demonstrated in 40 (2.8%) of all travelers; 18 participants (1.2%) had a > or = 4-fold increase in antibody titers. This corresponds to an incidence of 1.0 influenza-associated events per 100 person-months abroad. Among the 211 febrile participants, 27 (12.8%) had seroconversion, 13 (6.2%) with a > or = 4-fold increase; among the 321 afebrile control subjects, 13 (4.0%) had seroconversion, 5 (1.6%) with a > or = 4-fold increase. Twenty-five seroconverters (62.5%; P = .747) acquired influenza outside of the European epidemic season. Sixteen patients (40.0%) sought medical attention either abroad or at home, and 32 (80.0%) were asymptomatic at the time of completion of the survey.


CONCLUSIONS
This survey indicates that influenza is the most frequent vaccine-preventable infection among travelers to subtropical and tropical countries. Infections occur mainly outside the domestic epidemic season, and they have a considerable impact. Pretravel vaccination should be considered for travelers to subtropical and tropical countries.
Mutsch
Margot
M
Division of Epidemiology and Prevention of Communicable Diseases, World Health Organization Collaborating Centre for Travelers' Health, University of Zurich, Zurich, Switzerland. muetsch@ifspm.unizh.ch
Tavernini
Michela
M
Marx
Arthur
A
Gregory
Victoria
V
Lin
Yi Pu
YP
Hay
Alan J
AJ
Tschopp
Alois
A
Steffen
Robert
R
D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D004196:Disease Outbreaks; D005260:Female; D006306:Health Surveys; D006801:Humans; D015994:Incidence; D007251:Influenza, Human / Q000453:epidemiology*; D008297:Male; D008875:Middle Aged; D016017:Odds Ratio; D012307:Risk Factors; D011795:Surveys and Questionnaires; D013557:Switzerland / Q000453:epidemiology / Q000208:ethnology; D013997:Time Factors; D014195:Travel; D014329:Tropical Climate