CDC recommends use of injectable influenza vaccines (including inactivated influenza vaccines and recombinant influenza vaccines) during 2017-2018.
The "flu shot"—an inactivated vaccine (containing killed virus) that 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.
The regular seasonal flu shot is "intramuscular" which means it is injected into muscle (usually in the upper arm). It has been used for decades and is approved for use in people 6 months of age and older, including healthy people, people with chronic medical conditions and pregnant women. Regular flu shots make up the bulk of the vaccine supply produced for the United States.
The high-dose vaccine is for people 65 and older which also is intramuscular. This vaccine was first made available during the 2010–2011 season.
The intradermal vaccine is for people 18 to 64 years of age which is injected with a needle into the "dermis" or skin. This vaccine was made available for the first time for the 2011–2012 season.
About two weeks after vaccination, antibodies that provide protection against influenza virus infection develop in the body. Flu vaccine is the best way to prevent infection, but everyone must be re-vaccinated each year because the flu viruses change each year.