Use of northern hemisphere influenza vaccines by travelers to the southern hemisphere.
The influenza season in temperate climates extends from October through March in the northern hemisphere and from April through September in the southern hemisphere. 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. 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. 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). Vaccine formulations for each hemisphere are updated yearly but might differ according to virus surveillance information from each hemisphere.