NEW YORK (Reuters Health) - Although the influenza (flu) season is just ending in the northern hemisphere, it is now beginning in the southern hemisphere. Northerners traveling to the southern hemisphere -- or to the tropics, where influenza virus circulates year round -- should be immunized to avoid influenza illness, according to advice from the Centers for Disease Control and Prevention, Atlanta.

In the northern hemisphere, influenza season runs from October to March, while in the southern hemisphere, the season covers the remaining months, April to September.

Typically, the influenza vaccine used in the northern hemisphere is given to US residents traveling to the southern hemisphere, even though the match in virus strains may not be perfect.

However, this season, as it turns out, the strains used in the northern hemisphere vaccine are identical to those included in the southern hemisphere vaccine, health officials note in the CDC's Morbidity and Mortality Weekly Report for April 2.

The Advisory Committee on Immunization Practices recommends influenza vaccination for travelers to the southern hemisphere or tropics who belong to a group for which annual vaccination is recommended or who do not belong to a high-risk group, but want to avoid influenza illness and have not yet been vaccinated.

In addition to travelers to these areas, vaccination is advised for people who are traveling with an individual who has been in these areas, even if they have not.

SOURCE: Morbidity and Mortality Weekly Report, April 2, 2009.

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