Font Size

Flu Vaccine (Seasonal and Pandemic Flu)

What Is the Influenza (Flu) Vaccines?

Influenza (flu) vaccines are nasal sprays or injections currently composed either of live flu viruses that have been attenuated (rendered much less able to cause infection) or killed viruses or virus components (both are unable to reproduce) that, when administered to individuals, generate an immune response that will be strong enough to protect that individual from developing influenza disease. The design of the vaccine depends on how it is usually administered; the live attenuated vaccine is usually administered by a nasal spray (intranasal), while the killed virus is usually administered by an intramuscular injection (shot), usually into the deltoid (arm) muscle; there is a vaccine also available for intradermal injection. People cannot get the flu from the injected vaccine because the vaccine contains no live virus. However, nasal sprays use attenuated viruses (meaning that the viruses are live but cannot effectively cause disease) that, in some people (immunosuppressed people), may cause mild flu-like symptoms. (Note that nasal spray vaccines are currently not recommended because of poor immune responses as compared to those from the shots.)

Flu vaccines can be quite different based on the viral type (or strains of the flu virus) used to make the vaccine. For example, seasonal vaccines usually are now made up of a combination of four different influenza viruses (flu strains that differ in some of their surface molecules), although the trivalent (three flu strains) vaccine is still available. Experts choose the viruses in each year's vaccine because the chosen strains represent the most likely viruses to emerge in an upcoming flu season.

Pandemic flu vaccines are created in response to a specific strain of flu virus that is causing widespread disease. They differ from seasonal vaccines in several ways. First, the vaccines are usually made from new flu virus, not detected in previous flu seasons by flu experts and not included in the seasonal flu vaccines. These flu viruses are usually so new that they are not easily recognized by most human immune systems and quickly spread globally. Pandemic flu vaccines contain only a single strain of the pandemic virus (for example, H1N1 virus) instead of the usual three (trivalent) flu types used in a seasonal vaccine mixture. Seasonal vaccines are synthesized and distributed before the start of flu season (designated as October 4 each year until May of the following year) while pandemic vaccines, unfortunately, have to be synthesized and distributed only after the pandemic virus has been identified and started its global spread.

Until 2013, all commercially available flu vaccines were made from viruses cultivated in chicken eggs and then collected, purified, tested for safety and efficacy, and once approved, distributed to care providers. This process usually takes about six months to accomplish, which gives a pandemic flu virus a long time to circulate and infect populations before a vaccine can be developed. In 2013, Flublok was approved for use; this vaccine is a trivalent vaccine made from insect cells that have recombinant DNA that produces viral proteins in an egg-free system (the egg-free system avoids the problem of egg allergy in some patients). Future vaccines may be synthesized differently like Flublok. Current techniques are time-consuming, expensive, and yield vaccines that usually protect against only those viral strains present in the vaccine; the protection does not extend to the wide spectrum of flu virus strains. This limited protection is the reason that new flu vaccines are developed each year

Medically Reviewed by a Doctor on 10/25/2017

Must Read Articles Related to Flu Vaccine

Fever (in Adults)
Fever in Adults A fever is a body temperature of 100.4 F or greater. A fever...learn more >>
Fever in Children
Fever in Children Fever is defined as a rectal temperature over 100.4 F or 38 C. Fever isn't life-threatening unless it is persistently high -- greater than a 107 F rectal temper...learn more >>
Flu in Adults
Flu in Adults Flu (influenza) is an acute infection of t...learn more >>

Patient Comments & Reviews

The eMedicineHealth doctors ask about Flu Vaccine:

Flu Vaccine - Side Effects or Reactions

Did you experience any side effects or reactions after receiving a flu vaccine?

Flu Vaccine - Effectiveness

Was your influenza vaccine effective against the flu?

Flu Prevention

Personal hygiene

  • Wash hands frequently with soap and water.
  • Avoid touching one's eyes, nose, or mouth before washing one's hands.
  • Avoid close contact with people who are ill.
  • Do not share clothes or other personal items with another person during a flu outbreak.
  • Those infected with influenza should stay at home for 24 hours after fevers have resolved.


The best means of preventing the flu is getting an influenza vaccination. The CDC recommends an annual flu vaccine for everyone 6 months of age and older. Two general types of vaccines are available. One is the injectable vaccine (known as the flu shot) made from inactivated virus. The flu shot contains only killed influenza viruses A and B. The other is a live attenuated, or weakened, virus that is squirted into the nose. This is called intranasal vaccine or nasal spray vaccine. The intranasal form is available for certain people who may prefer it to a shot, and it is approved for people from 2 through 49 years of age. It is not recommended for people who are immunosuppressed or have other conditions.

Medical Dictionary