We are learning what our genes do, and we are learning to change our own genes during life. When we learn that a gene causes something bad, we are tempted to eliminate it, and when we learn that a gene causes something good, we are tempted to want everyone to have that gene. But, you can't win.
Case in point: There is a gene mutation found in one percent of whites, which confers HIV resistence. But, the very same gene mutation confers West Nile Virus succeptibility. HIV is a more reliable killer, but it also acts more slowly and there are many ways one can avoid exposure to it. West Nile kills about one in twenty people who are infected with it and get sick (probably closer to one in a hundred people who are infected, without regard to whether they get sick), but kills in days instead of years, and is much harder to guard against because it is mosquito borne.
This time we were lucky. We know the tradeoffs associated with the presence and absence of this mutation, so we will recognize the choices associated with trying to genetically engineer it. But, next time, we may only see one half (or less) of the equation, and be very tempted to go forward with genetic engineering to take advantage of the known feature. It may take decades or more to discover that negative side effects that come with that decision. So, the original choice could turn out to be an uninformed one. It is easy to translate "no known side effects" into "no side effects", but they aren't the same thing.