Both killed and live influenza vaccines are currently available in the U.S.

  • The “flu shot” has been used to vaccinate against influenza since World War II. It is a trivalent vaccine that confers immunity against Influenza A virus subtypes H3N2 and H1N1 as well as one influenza B virus. The vaccine is updated each year to cover the newly emerging strains. Viruses for the shot are grown in eggs. The vaccine is administered as an intramuscular injection. The killed vaccine elicits a fairly weak immune response (with relatively low titers of IgA and IgG produced), so protection is ephemeral (lasting only about 6 months). Yearly boosters are recommended at the beginning of each flu season. Refer to for more information on who should receive the flu shot.
  • The nasal-spray flu vaccine, or Live Attenuated Influenza Vaccine (LAIV), was licensed for use in 2003. It contains live viruses that are weakened and temperature sensitive (so they can grow in the nose and throat but not the lower respiratory tract). Like the flu shot, it is a trivalent vaccine that protects against influenza A isolates H3N2 and H1N2 and influenza B virus. It is also updated yearly for the emerging viral strains. LAIV is administered as a nasal spray. The live vaccine is advantageous because it stimulates a strong immune response with high production of IgA antibodies. LAIV should still be given each year before epidemic season in order to protect against the new flu strains. For more information on LAIV and who should get the vaccine, check out what the CDC has to say at


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