Joseph Wambaugh(who was a sworn police officer/LAPD) wrote in one of his popular "Hollywood Division" novels that about 30-40% of sworn LEOs killed in the US every year were wearing body armor.
Some spec ops troopers & armed professionals do not wear vests or body armor on a regular basis. Retired US Navy SEAL officer, Richard Marcinko, www.DickMarcinko.com
stated he didn't wear body armor because it would "slow him down".
I'm not saying protective vests or armor are not worth it, but they are not a replacement for proper tactics or training.