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JAMA. 2009;302(2):140.

MMWR. 2009;58:312

The influenza season in temperate climates extends from October through March in the northern hemisphere and from April through September in the southern hemisphere.^1-3 Recent studies indicate that influenza viruses can circulate throughout the year in the tropics and that influenza is the most frequently acquired vaccine-preventable disease among those traveling to tropical and subtropical countries.^2-5 Influenza outbreaks have been reported among persons who travel from the northern hemisphere to the southern hemisphere and among persons from the northern hemisphere on group tours.^4-7 To reduce the risk for influenza during travel, the Advisory Committee on Immunization Practices (ACIP) recommends that persons from the northern hemisphere who are recommended for annual vaccination or who want to avoid influenza illness but have not yet received the 2008-09 influenza vaccine should consider being vaccinated (1) before travel to the southern hemisphere during influenza season, (2) before travel to the tropics at any time of year, or (3) when traveling as part of a tour group that includes persons from areas where influenza circulates during April-September (e.g., the southern hemisphere).⁸ Vaccine formulations for each hemisphere are updated yearly but might differ according to virus surveillance information from each hemisphere.

Vaccines prepared for use in the northern hemisphere typically are administered to U.S. travelers to the southern hemisphere, even when the vaccine formulation is less than optimal, because influenza vaccines prepared for use in the southern hemisphere are not widely available in the United States. However, this year the influenza virus strains represented in the 2008-09 northern hemisphere influenza vaccine currently available in the United States are identical to the strains represented in influenza vaccines intended for use in 2009 in the southern hemisphere.^8-9

Health-care providers should ask patients about upcoming travel plans, inform them regarding the risk for influenza during travel, and be aware that vaccination of travelers with the currently available northern hemisphere influenza vaccine will provide the most recently updated vaccine formulation for the southern hemisphere.

The expiration dates in the prescribing information indicate that certain lots of northern hemisphere influenza vaccines in the United States can be used as late as June 30, 2009. If possible, influenza vaccine should be administered to travelers a minimum of 2 weeks before departure, but can be administered up to the date of travel. No information is available regarding the benefits of revaccinating persons before summer travel who already were vaccinated during the preceding fall.⁸ In addition, before their trip, persons should learn about health risks in the destination country (information available at http://www.cdc.gov/travel). Members of the public, especially those at higher risk for influenza complications, should consult with their health-care practitioner to discuss the risk for influenza and other travel-related diseases before embarking on travel.^{4, 8}

REFERENCES
1. Strikas RA, Kozarsky PE, Reed C, Kapella BK, Freedman DO. Should health-care providers in the United States have access to influenza vaccines formulated for the southern hemisphere? J Travel Med. 2008;15(6):442-446. FULL TEXT | ISI | PUBMED 2. Camps M, Vilella A, Marcos MA; et al. Incidence of respiratory viruses among travelers with a febrile syndrome returning from tropical and subtropical areas. J Med Virol. 2008;80(4):711-715. FULL TEXT | ISI | PUBMED 3. Chew FT, Doraisingham S, Ling AE, Kumarasinghe G, Lee BW. Seasonal trends of viral respiratory tract infections in the tropics. Epidemiol Infect. 1998;121(1):121-128. FULL TEXT | PUBMED 4. CDC. Health information for international travel 2008. Atlanta: US Department of Health and Human Services, CDC; 2008. Available at http://wwwn.cdc.gov/travel/contentyellowbook.aspx.5. Marti F, Steffen R, Mutsch M. Influenza vaccine: a travelers' vaccine? Expert Rev Vaccines. 2008;7(5):679-687. FULL TEXT | ISI | PUBMED 6. Brotherton JM, Delpech VC, Gilbert GL, Hatzi S, Paraskevopoulos PD, McAnulty JM, Cruise Ship Outbreak Investigation Team. A large outbreak of influenza A and B on a cruise ship causing widespread morbidity. Epidemiol Infect. 2003;130(2):263-271. PUBMED 7. Uyeki TM, Zane SB, Bodnar UR; et al, Alaska/Yukon Territory Respiratory Outbreak Investigation Team. Large summertime influenza A outbreak among tourists in Alaska and the Yukon Territory. Clin Infect Dis. 2003;36(9):1095-1102. FULL TEXT | ISI | PUBMED 8. CDC. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP), 2008. MMWR. 2008;57(RR-7).9. World Health Organization. Recommended composition of influenza virus vaccines for use in the 2009 southern hemisphere influenza season. Wkly Epidemiol Rec. 2008;83(41):366-372. PUBMED

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