evidence: - Unfortunately, some people can become infected with an influenza virus that the vaccine is designed to protect against, despite getting vaccinated. Protection provided by flu vaccination can vary widely, based in part on the age and health of the person getting vaccinated. In general, flu vaccines work best among healthy younger adults and older children. Some older people and some people with certain chronic illnesses may develop less immunity after vaccination. Flu vaccination is not a perfect tool, but it is the best way to protect against flu virus infection. [...] Flu vaccines that are given with a needle (flu shots) are currently made in two ways: the vaccine is made either with a) influenza vaccine viruses that have been killed (inactivated) and are therefore not infectious, or b) with proteins from an influenza virus (which is the case for recombinant influenza vaccine). Nasal spray vaccine is made with weakened (attenuated) live influenza viruses and also cannot cause flu illness. The weakened viruses are cold-adapted, which means they are designed to only reproduce at the cooler temperatures found within the nose. The viruses cannot reproduce in the lungs or other areas where warmer temperatures exist. [...] A flu shot: The viruses in a flu shot are killed (inactivated), so you cannot get flu from a flu shot. Some minor side effects that may occur are: