Answer:
Here are four traits that would make it hard for a vaccine to be used:
1. Rapid mutation rate. If a pathogen mutates rapidly, it will be able to evade the immune system's defenses, including the antibodies produced by a vaccine. This is a particular problem with viruses, which can mutate very quickly.
2. Ability to evade the immune system. Some pathogens are able to evade the immune system by hiding inside cells or by changing their surface proteins so that they are no longer recognized by the immune system. This makes it difficult for the immune system to mount an effective response to the infection.
3. Ability to spread easily. If a pathogen is easily spread from person to person, it will be more difficult to prevent infection through vaccination. This is a particular problem with respiratory viruses, which can be spread through coughing and sneezing.
4. Lack of animal reservoirs. If a pathogen does not have animal reservoirs, it will be more difficult to develop a vaccine against it. This is because vaccines are typically developed using weakened or killed versions of the pathogen. If there are no animal reservoirs, there will be no source of the pathogen to use for vaccine development.
It is important to note that these are just a few of the traits that can make it difficult to develop a vaccine against a pathogen. There are many other factors that can contribute to the difficulty of vaccine development, such as the cost of vaccine development, the availability of funding, and the political will to support vaccine development.