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The seasonal flu is caused by viruses that are constantly changing. Each year the flu vaccine contains the strains of virus that are present when the vaccines are manufactured. As a result, the vaccine provides protection against only the strains that it contains. The newer strains of virus can still cause flu even though you have had the flu vaccine. The newer strains of virus may not be very different, so in a given year the vaccine can still protect you and even if you get flu again, the symptoms would be mild.

Therefore, instead of getting the flu vaccine multiple times, it is recommended to get the vaccine every year.

Next year, the H1N1 virus strain may become part of the seasonal flu vaccine and we would not need separate vaccines for the seasonal flu and the H1N1 flu next year as we are required to do this year.