Prevention paradox

The prevention paradox is a situation where the most people that a prevention program helps are those that are already at low risk for the problem. This can happen because the people at higher risk for the problem are harder to reach or help. For example, imagine you are trying to prevent people from getting sick with a certain virus. You could give a vaccine to everyone, but most of the people who get sick with the virus are people who did not get the vaccine. So, if you give the vaccine only to people who are most at risk for getting the virus, then more people will stay healthy and you'll end up helping more people overall.