I think there should be a grab mechanic for zombie.
The purpose of a such an attack is to make melee combat more difficult and to properly simulate just how easy it would be to be overrun.
Also addresses the absurdity of high levels of dodge where a late game character could be surrounded on all 8 sides by zombies yet still manage to dodge every attack.
By grab mechanic I mean the zombie should try and grab you, if they succeed you cannot attack (melee), move, or reload until you shoot/kill them, or break their grab, (subject to a strength/unarmed skill check).
For example if one zombie had a grab lock on you might need say 4 strength to have a 50% of breaking it per turn, if two zombies had a grab lock on you then you would need 8 strength to have a 50% of breaking it per turn.
(A zombie that had a grab lock on you can only attempt to bite you)
(Being “grabbed” will reduce your dodge skill massively to point where it’s impossible to dodge any incoming attacks)
(Zombie grabbers and brutes will have a much stronger grab attack - requiring 9 - 11 strength to have a 50% of breaking it per turn.)
Edit
I suppose one alternative would be to simulate grabbing by giving the player +1 encumbrance (Torso, legs, arms) for every adjacent zombie, +4 for a grabber zombie.
Edit 2
Another alternative could be a passive speed penalty based on how many adjacent zombies there are.