Re: How does a dead or weakended virus turn into a vaccine?

Safe and effective vaccines can be produced from either dead or weakened viruses.

Viruses are “killed” by using chemical agents such as formaldehyde. The killing process is known as virus inactivation and the vaccines which result are known as inactivated virus vaccines. After the virus is inactivated, it is no longer able to replicate and produce disease in the body. However, after injection it is still able to stimulate an immune response. This immune response, primarily the production of circulating antibodies, are able to prevent illnesses when the host is later exposed to the infectious virus. Inactivated virus vaccines can be produced from the whole virus or the virus can be disrupted with only pieces of the virus are used in the vaccine. Vaccines produced from the whole virus are called inactivated whole virus vaccine, while vaccines using pieces of disrupted viruses are known as inactivated subunit vaccines. The current vaccines used to prevent influenza are available in both the inactivated whole virus and subunit vaccine formulations.

“Weakened” viruses are produced by taking a virus infectious for humans and passing the virus a number of times through another species, such as monkeys, chickens, guinea pigs, or mice. Passage can be done either in the animals or through the use of animal cell cultures. As this virus is passed from animal to animal, variants develop which can lose their disease producing ability in humans. The virus is still “alive”, but when inoculated into humans will replicate only enough to produce an immune response and not enough to produce an illness. This immune response is again able to prevent illness when the host is exposed to infectious virus at a later time. These types of vaccines are called live attenuated virus vaccines. Examples of these attenuated vaccines would be measles, mumps, rubella (German measles), polio and varicella (chickenpox) vaccines.

Animal cell cultures are usually used to produce both live attenauated and inactivated virus vaccines. However, influenza virus vaccines are still produced using embryonated hen's eggs.

Live attenauted virus vaccines tend to produce a longer lasting immunity. This longer immunity may be done to the more natural presentation of the virus. However, inactivated vaccine tend to be easy to produce, since virus still infectious for humans can be used and the developer does not have to try to get it to be attenuated or "weakened".

When new vaccines are developed they are first tested under controlled conditions to show that they are both safe and effective. Studies are usually conducted first in animal models and then moved to human testing. The vaccine manufacturers then submit their test data to the U.S. Food and Drug Administration (FDA) for review. The FDA decides whether the data are good enough to support use of the vaccine in the general population. If the FDA believes that the data show the vaccine is safe and effective, it will issue the manufacturer an approval license. The administration of the new vaccine can then begin.