The Food and Drug Administration has approved the vaccines drug companies will start producing for the upcoming flu season.

Experts study virus samples and patterns collected worldwide each year to identify strains likely to cause the most illness during the upcoming season. The closer the match between the circulating strains and the strains in the vaccine, the better the protection against the flu.

Last year two shots were needed because the swine flu strain developed after the seasonal vaccines were already produced. Unless something like that happens again, this year a single shot will protect against three strains of flu -- including swine flu.