Colds and flu change over time, which is why we keep catching them. A yearly flu shot (it isn't a vaccine) is based on a prediction of what the future flu will look like, which means that sometimes it works, sometimes it doesn't, but what it is looking to do is try to mimic what is to come. However, the more robust and broad the antibody range is in a person, the better they are able to handle future flus. It is like having diversity, there is less single points of vulnerability, like a distributed network. My thinking isn't the exact opposite of the way things work.
For instance, if a kid gets chicken pox, they itch for a bit and a few days later they are fine. If an adult who has never had chicken pox before gets it, it can be very severe. Chicken pox is a virus.