According to the Centers for Disease Control and Prevention, the single best way to protect yourself against getting the flu is to get the flu vaccine every year.

Here is what you need to know about the influenza vaccine.

According to the CDC, there are two types of vaccines:

The most common is the “flu shot,” which is an inactivated vaccine that contains killed virus and is given with a needle, usually in the arm. The flu shot is approved for use in people older than 6 months, including healthy people and people with chronic medical conditions. There are three kinds of flu shots: a regular shot, a high dose shot for those older than 65 and an intradermal shot for adults.

The second type of vaccine is the nasal spray, which is made with live, weakened flu viruses. The viruses in the nasal spray vaccine do not cause the flu. The CDC has approved its use in healthy people 2 through 49 years of age who are not pregnant.

The viruses in the vaccine can change each year based on international surveillance and scientists’ estimations about which types and strains of viruses will circulate in a given year, says the CDC. Within two weeks following receiving the vaccination, antibodies that provide protection against the influenza viruses in the vaccine develop in the body.