The most common differences between the flu shots that are available for adults are whether they include 3 different strains of the influenza virus (called Trivalent) or 4 different strains of the influenza virus (called Quadrivalent).
Most vaccines are injected into the shoulder muscle, but other vaccines can be sprayed into the nose, or injected into the skin. A new vaccine is available that is designed to be used in those above the age of 65, and contains a higher dose of the vaccine.
Most of the flu vaccines are manufactured with eggs and are not recommended in those that are allergic to eggs. Some new vaccines are manufactured differently and do not contain any egg products.
Currently, the Center for Disease Control and Prevention (CDC) does NOT recommend one vaccine over another. The most important thing to prevent the flu this season is to be vaccinated.