In general, updates to the composition of Southern Hemisphere flu vaccines should not be interpreted to mean that the composition of Northern Hemisphere flu vaccines is mismatched to circulating flu viruses. The flu season occurs at different times in each hemisphere. Typically flu season lasts from October to May in the Northern Hemisphere and from April to September in the Southern Hemisphere. Decisions on the composition of flu vaccines must be made months prior to the flu season in each hemisphere so that flu vaccines can be produced and made available in time for the season. Since flu viruses are always changing, different flu viruses can circulate in different geographic areas of the world at different times of the year. As a result, separate recommendations are made for the flu vaccines produced in the Northern and Southern Hemispheres based on the data available at the time vaccine composition recommendations are made. The flu vaccines for each hemisphere are optimized to protect against the flu viruses that research indicates will circulate during the upcoming flu season in each hemisphere.