Currently, influenza vaccines that are made available to the public are either one of two types: an egg-based vaccine featuring inactivated influenza virus or a live, attenuated vaccine featuring a weakened version of the virus. Neither can cause infection with the flu.
The traditional seasonal influenza vaccine is an example of an inactivated vaccine, in which influenza virus grown in eggs is “killed” by a variety of methods. Influenza vaccines have been prepared in eggs for many years, but the process is lengthy and requires hundreds of millions of eggs. When urgent needs arise, it is difficult to boost the vaccine supply quickly.
These vaccines contain live influenza virus produced in eggs or cell culture that has been weakened, so it cannot cause disease. In humans, these weakened influenza viruses are able to grow in the relatively cool upper respiratory tract but not in warmer parts of the body, such as the lungs.
Last Updated January 14, 2011