/effect @a wither 1 5 = -2 health points but it would do more damage:

"Wither effect does 1 (Half Heart) damage every 12/6/3/1 ticks (Heart × 0.8/1.66/3.33/10 per second) at level 2/3/4/5. No extra effect at levels 6 and up."
http://minecraft.gamepedia.com/Effects

level 5: 1Heart x 10 = 10Hearts per second

Also It must remove 10 hearts.