Modified live vaccines (MLV) have been available for decades and are widely used in the prevention of many diseases in humans and animals. However, they do have some drawbacks that should be considered when making decisions about their use.
First, MLV's can cause the vaccinated individual to become infected with the virus they are attempting to prevent. This is known as “vaccine virus shedding”, and it can occur for up to a few weeks after the vaccine is administered. This means that the vaccinated individual can transmit the virus to other individuals, even if they do not show any signs of illness themselves. This is a particular concern when MLV's are used for diseases that are highly contagious, such as measles or mumps.
Second, MLV's can cause an individual’s immune system to become overstimulated, resulting in a condition known as vaccine-associated enhanced disease. In this situation, the vaccinated individual can experience more severe reactions to the disease than unvaccinated individuals. This is particularly concerning in cases of diseases where the virus is particularly dangerous.
Finally, MLV's can be difficult to store and transport, which can lead to problems with availability and quality control. These problems can result in a decrease in vaccine effectiveness, making it less likely to provide the desired level of protection against disease.
Overall, while MLV's can be effective in the prevention of certain diseases, they have some drawbacks that must be taken into consideration when deciding whether or not to use them. It is important to weigh the potential benefits and risks of using MLV's before making a decision.