[glormph]

AFAIK it depends on how long lived the antibody response is. Memory cells can last different amounts of time for different infections, or (like for certain calicivirus infections?) hardly develop at all. Have no nice paper on hand to back this up though. But if it's a short lived response (as in years, not decades), it's good to be freshly exposed and like another commenter said, kids get colds all the time. And likely parents of young children too (I know I did). Maybe that helps?