What are the different types of flu vaccines?

Flu vaccines are routinely available for seasonal influenza. Pandemic vaccines may also be developed for specific strains of the flu virus that cause widespread disease, such as occurred with the H1N1 virus in 2009.

Each year, composition of the influenza viruses change, making the vaccine used in previous years ineffective. Each year, a new vaccine must be prepared that will be effective against the types of influenza virus that are expected to circulate in the upcoming influenza season. These are known as seasonal flu vaccines. The reason for the differences in circulating strains of the flu virus is that the virus can mutate (or change its structure) rapidly, leading to new subtypes of the virus. The key is to be able to predict which influenza viruses are going to cause infection and to prepare a vaccine against those viruses. Usually, scientists can predict accurately which types of influenza virus will cause infections and prepare an appropriate vaccine. Typically, the viruses used to prepare flu vaccine are grown in eggs, but a newer, egg-free version of the vaccine has been developed. In 2017, updated guidelines from the Influenza Vaccine and Egg Allergy Practice Parameter Workgroup commissioned by the Joint Task Force on Practice Parameters (JTFPP) stated that the risk is so small that even asking patients about egg allergy is no longer necessary. Health care professionals can now safely administer flu vaccines to people who are allergic to eggs. Flu viruses may also be made using recombinant technology that does not involve growing the actual flu virus in eggs.

The vaccine is generally effective against the influenza virus within two weeks of administration. The vaccine is only effective against the strains of the virus that match the vaccine. These strains vary from flu season to flu season each year. This is the reason that revaccination is required annually with the vaccine that matches the strains of influenza that are currently prevalent.

The injection (“flu shot”) vaccine

Flu vaccine is an inactivated vaccine, meaning that it contains killed influenza virus, or a recombinant vaccine, meaning that it was made without using virus particles at all. Health care providers inject the vaccine into muscles or skin, stimulating the immune system to produce an immune response (antibodies) to the influenza virus.

Medical professionals administer the “flu shot” vaccine as a single dose of liquid injected through the skin into muscle (intramuscular or IM). Typically, health care professionals inject the flu vaccine into the deltoid muscle at the side of the arm, using alcohol rubbed over the skin for sterilization. Health care professionals administer the vaccine annually, each fall. Side effects of the flu vaccine are uncommon.

Two types of vaccines are available: a trivalent vaccine that targets three strains of flu virus, as well as a quadrivalent vaccine that targets four strains. Both the trivalent and quadrivalent vaccines are available as an intramuscular injection. For people who are 18-64 years old, a jet injector (that uses a strong stream of fluid to penetrate the skin rather than a needle) can be used for administration of certain vaccine types. Special vaccines preparations are available for people over 65 years of age that produce a stronger immune response.

The U.S. Centers for Disease Control and Prevention (CDC) recommends the use of injectable influenza vaccines (either trivalent or quadrivalent, including inactivated influenza vaccines and recombinant influenza vaccines) or the nasal spray vaccine for the influenza season of 2019-20.

The nasal-spray vaccine

The nasal-spray flu vaccine (sometimes called LAIV for live attenuated influenza vaccine, brand name FluMist) was first licensed in 2003. It is directed against the same strains of virus as the flu shot but differs in that it contains weakened live influenza viruses instead of killed viruses and is administered by nasal spray instead of injection. The vaccine is termed an attenuated vaccine because the vaccine viruses are weakened so that they themselves do not cause severe flu symptoms. The nasal spray flu vaccine for 2019-20 is approved for use in non-pregnant individuals 2-49 years old. It should not be used for people with certain underlying medical conditions. All LAIV are quadrivalent (four-component).

The American Academy of Pediatrics (AAP) recommends that all children 6 months and older receive the injectable (shot) vaccine if possible. The AAP recommends the injectable flu vaccine for children because it has provided the most consistent protection against all strains of the flu virus in recent years. The nasal spray vaccine may be used this year to vaccinate children who would not otherwise receive the flu shot (such as with a shortage of the injectable vaccine or refusal to receive an injection), as long as they are 2 years of age or older and healthy without an underlying medical condition.

The live viruses in the nasal-spray vaccine are weakened so that they do not cause severe symptoms. However, mild symptoms can occur as a side effect of the vaccination. Side effects of the nasal-spray flu vaccine can included runny nose, headache, sore throat, and cough. Children who receive the vaccine may also develop mild fever and muscle aches.