Final answer:
The claim that a live vaccinia virus is used in smallpox vaccine preparation is true. The smallpox vaccine uses an attenuated version of the vaccinia virus to stimulate an immune response without causing disease. Live vaccines are effective but carry a small risk of reversion to a pathogenic form via back mutations.
Step-by-step explanation:
The statement that a live vaccinia virus is used in the preparation of the smallpox vaccine is true. The smallpox vaccine normally does not contain the actual variola virus that causes smallpox, but instead contains a different virus known as vaccinia which is similar enough that it provides immunity against smallpox. Vaccination with vaccinia virus is a form of a live virus vaccine, which means that the virus is alive but has been weakened, or attenuated, so that it causes the immune system to react without causing the full-blown disease.
Live vaccines are generally more effective than inactivated or 'killed' vaccines, as they provoke a strong and long-lasting immune response. However, there is a risk with live vaccines that the weakened virus can undergo back mutations, potentially reverting to a form that can cause disease. This risk is considered low but significant, and is a key consideration in the development and administration of vaccines. The success of live viral vaccines in preventing diseases like polio, measles, and chickenpox is well-documented, paving the way for the widespread use of vaccinations to control infectious diseases.