One flu shot protects a person for an entire influenza season. Different strains of the flu virus emerge in the United States each year, so individuals must receive new vaccines annually. It takes approximately 14 days for the flu shot to reach its full strength, according to Emory Healthcare.
The flu vaccine is up to 90 percent effective in people age 65 and younger. While the vaccine is not safe for everyone, such as individuals who are extremely allergic to chicken eggs, it is safe for pregnant women. Side effects that can occur from receiving the flu shot include a low-grade fever and redness at the injection site, notes Emory Healthcare. Side effects are typically mild and only last a couple days.