Soft Head is a flaw. Mayday is the unique Medic who has this flaw.
It gives the soldier +15% damage taken from headshots, which means 60% damage taken from headshots (because headshots normally do +45% damage than normal hits). This flaw can make you die easily from enemy Snipers, especially those with AWP rifles, that have +50% headshot damage as an additional effect.
Luckily, there are two ways to fix this flaw: by unlocking/equipping Hard Headed defensive upgrade or Hard Hat defensive equipment. Both items work opposite of Soft Head (-15% damage from headshots).