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Every fall, millions of U.S. children get flu vaccinations at their pediatricians' offices. The CDC recommends an annual flu vaccine for all Americans who are at least 6 months old.

You may have questions about the vaccination. Why can't last year's flu shot protect your child this year? Do you need to get her a separate vaccine for protection against the H1N1 strain? Should you request the vaccination in nasal spray rather than injection form?

For guidance, WebMD spoke with internist Lisa Grohskopf, MD, a medical officer in the CDC's Influenza Division.

Q: Why is it important for all children aged 6 months and up to get vaccinated?

A: Children, particularly those under age 5, are subject to potentially very severe complications from influenza disease; some of those children are hospitalized. Within that group, children under 2 are especially prone to complications. For everyone aged 6 months and up, the annual flu vaccine is the best way that we have available to protect against complications.

Q: Is it really necessary to vaccinate my child this year if she received a flu shot last year?

A: The flu vaccine has four different vaccine virus ... strains in it. In a typical season, at least one of those strains will change.

Also, we know from a number of studies that the antibody response to the vaccine tends to decline over time, so that’s why it’s important to get a flu vaccine every year.

Q: How effective is the vaccine at preventing flu?

A: It depends on how well the vaccine matches the flu strains that are circulating. The vaccine strains have to be chosen well in advance of the flu season starting, and in years when there's a good match, it's likely to work better.

It also depends upon a person’s age and health status.

Q: Are there any reasons why a child who's old enough should not get the flu vaccine?

A: The main contraindication for the flu vaccine is a severe allergic reaction to anything that is in the flu vaccine. One possibility is egg [allergy], because all of the flu vaccines that are available in this country are manufactured through a process that uses chicken eggs, the vaccine is most likely going to have a very, very trace quantity of egg protein left in it.