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