This year the predominant influenza strain is an H3N2 influenza A virus, which is co-circulating with influenza B strain. The 2009 H1N1 influenza A virus so far is not very prominent. All these strains are included in this year's flu vaccine.

Should flu vaccines be repeated annually?

Yes. The protection lasts only one season because the type of viruses change every year and the vaccine is made to try to match those viruses. Influenza viruses continually mutate or change, making people susceptible to the flu throughout their lives. It is important to remember, the flu vaccine is not a guarantee against getting the flu. It reduces your chances of getting the flu and lessens the severity of the illness should you become sick. Preliminary estimates from the Centers for Disease Control and Prevention indicate that this year's vaccine can reduce the chance of having influenza that results in needing medical attention by at least 62 percent.

Which flu season was the most severe?

The 1918 worldwide flu epidemic infected over 500 million people, and killed 50 million. In the United States, more than 500,000 died. The 1918 flu epidemic killed more people in less time than any disease in modern history. It ranks with the Black Death of the 14th Century as one of the few events in recorded history to reduce the population of the earth by as much as one percent.

Although such a severe influenza season has not been seen since, every annual epidemic of influenza results in hundreds of thousands of hospitalizations and hundreds of deaths in the United States. Therefore, we urge you to get your flu vaccine to reduce your chances of becoming ill, and to start early antiviral treatment should you get the flu.