Influenza in Travelers
Abstract & Commentary
Stan Deresinski, MD, FACP, Clinical Professor of Medicine, Stanford; Associate Chief of Infectious Diseases, Santa Clara Valley Medical Center, is Editor for Infectious Disease Alert
Source: Mutsch M, et al. Influenza Virus Infection in Travelers to Tropical and Subtropical Countries. Clin Infect Dis. 2005;40:1282-1287.
Synopsis: Influenza is the most frequently encountered vaccine-preventable infection in travelers to the tropics and subtropics.
Mutsch and colleagues prospectively evaluated the incidence of influenza virus infection among 1450 visitors to tropical and subtropical countries who attended the University of Zurich Travel Clinic. Among these travelers, 289 (19.9%) reported a febrile illness and 211 of these provided paired serum samples. Paired samples were also obtained from 321 matched controls from among the remaining travelers who did not develop a febrile illness. Only 12% had evidence of pre-travel immunity to circulating influenza viruses.
Forty travelers had serological evidence of acute influenza virus infection, with 22 having antibody titer increases of fourfold or greater (probable cases) and the remaining 18 having 2.0- to 3.9-fold increases (possible cases). The infection was asymptomatic in 13 of the 40 subjects. Approximately two-thirds of infections occurred in individuals 20 to 39 years of age. The over-all attack rate was 2.8% (1.2% if only probable cases are considered. Travelers to Africa, Asia, and Latin America were affected; only travel to the Indian subcontinent appeared to constitute an excess risk of influenza virus infection relative to all other destinations. Infections were acquired throughout the year in all seasons.
Comment by Stan Deresinski, MD, FACP
I recently was called to a local hospital Emergency Department to see a previously healthy 38-year-old male who presented with a sore throat, fever, and severe headache that had started 36 hours after return from a one-month trip to rural Vietnam and Thailand. The immediate concern was that he had avian influenza, which, fortunately, proved to not be the case. He did, however, have acute influenza virus infection (PCR repeatedly detected both influenza A and B while only influenza B was recovered on culture). He had received no vaccinations prior to travel. Influenza vaccination could have prevented the entire costly episode.
The results of the study under review indicate that influenza is the most frequently encountered vaccine-preventable infection in travelers to the tropics and subtropics, where influenza cases may occur throughout the year. In temperate regions of the Southern Hemisphere, the influenza season occurs from April through September. In temperate climates of either hemisphere, exposure to influenza may occur when groups from various geographic areas congregate, as on cruise ships.
The US CDC recommends that pre-travel influenza vaccination be recommended for "persons at high risk for complications of influenza" who did not receive the vaccine during the preceding fall or winter if:1,2
- travel is planned to the tropics
- travel is planned with large groups of tourists at any time of year, or
- travel is planned to the Southern Hemisphere during April-September.
The receipt of pretravel vaccine does not eliminate the recommendation for vaccination the following autumn, since the duration of protection may potentially proved inadequate in some cases and, more saliently, because vaccine composition is likely to have changed. Vaccine composition is, to some extent, a wild card in general for travelers, since the available vaccine may not contain appropriate components protective against the virus encountered.
There is no data available regarding benefits of revaccination of individuals before summer travel who were vaccinated during the previous fall or winter.
Two-thirds of cases of influenza detected by Mutsch et al occurred in individuals 20 to 40 years of age, with presumably most being healthy enough to not qualify as being at high risk of complications of influenza and, therefore, not included in the group for whom the CDC recommends pretravel influenza immunization. It seems to me that pretravel influenza vaccination should not be restricted to high risk individuals, but should be considered for many others as well.
References
- Harper SA, et al. Prevention and Control of Influenza: Recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR. 2004;53(RR06):1-40.
- CDC. Influenza. NCID. Health Information for International Travel, 2003-2004.
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