Body armor is totally unnecessary unless maybe you’re an on-duty cop or similar. I’ve witnessed several mass shootings and I can’t imagine a situation where I’d wear body armor.

Be aware of your surroundings and try to avoid situations where you’re more likely to get shot. Know where the exits are in a crowded building, and stay near the edges of any outdoor crowd so you can make a quick escape. Always trust your instincts if you get a bad feeling or someone seems suspicious. Take some self defense classes. With enough training you can even learn how to disarm an attacker, but don’t try it without extensive training.