Photo Gallery: A Flu Shot in Time Saves...

Each year's supply of flu vaccines is based on the most virulent strains of flu that circulated earlier that year, but this year, the recipe is the same, according to Beth Capobianco of VNA Community Healthcare.

"That doesn't happen that often," Capobianco said, of the vaccine that protects against three strains of the flu and the H1N1 virus. "We recommend people get it again because it will boost their immunity against H1N1."

The Centers for Disease Control and Prevention webiste reports that the World Health Organization recommends which strains of the virus should be included in the vaccine based on how well the prior year's vaccine will protect against new strains, how many more strains have been identified and how they are spreading. 

Capobianco said the vaccine takes approximately three weeks to take effect and that people should get the shot in time for peak flu season, which is January and February.