Some vaccinations last, but others do not. For instance, the Advisory Committee on Immunization Practices recommends that tetanus and diphtheria vaccines be given every 10 years to assure immunity to both tetanus and diphtheria. So, the process of protecting yourself and your community from disease relies on more than one step. The vaccine designed for last year's influenza is often not effective against this year's flu. For such little things, viruses can be very challenging, even to the smartest scientists.
Viruses engage in very sophisticated exchanges among themselves and undergo constant change.
Just as changing one letter in your name would make it unrecognizable to your computer, tiny changes in the genetic makeup of influenza virus may make the virus unrecognizable to your immune system, even following vaccination. So, scientists must develop new influenza vaccines for every flu season. In addition to changes in the flu virus itself, there are several types of flu. So, when we hear of H. influenza type a or type b, we know that this refers to the type of capsule in which the virus resides. Public health officials and vaccine manufacturers from around the world study influenza viruses continuously.