Every year, the Centers for Disease Control and Prevention (CDC) determines what strains of influenza virus should be included in the annual seasonal flu vaccine for the United States. This decision is based on which strains have been circulating recently and are expected to be common during the flu season in the following autumn and winter.

The World Health Organization (WHO) recommended that for 2011-12, the pandemic H1N1 virus should be included in the seasonal flu vaccine. The CDC has decided to include it in the United States’ seasonal flu vaccine.

The news serves as a reminder that the pandemic H1N1 virus is still circulating and is expected to continue to circulate. Health officials expect H1N1 to be a continued risk next fall and winter.

Why is there a new flu vaccine?
Each year, health officials determine which three strains of influenza virus are most likely to circulate in the next flu season. Those strains are included in the seasonal flu vaccine.

What will be in the 2011-12 seasonal flu vaccine?
The CDC has decided, on the WHO's recommendation, to use the same influenza strains as last season: the pandemic strain of H1N1, H3N2 influenza A and influenza B.

When will the new vaccine be ready?
In North America, the flu season usually starts in November or December. The influenza vaccine takes several months to manufacture; it usually arrives the fall. This year, because the flu vaccine was identical to last year's, the vaccine arrived early in September.

I’ve already had a seasonal flu shot this year. Do I need to get another one in the fall?
Yes. We recommend people get a seasonal flu shot every year. If you’ve had pandemic H1N1 influenza or the vaccine--or last year's seasonal flu shot--you probably already have some immunity. But it’s still a good idea to get a flu shot, especially if you’re over 65 or at high risk for flu complications. Having an additional dose of H1N1 will not hurt you; in fact, it could provide added protection.