That's not a good comparison. If you want to use Alertness as a basis for a feat that grants immunity, you need to consider what surprise is most like. In my opinion, surprise most closely resembles some of the weaker conditions. I could definitely see a feat that gives immunity to the Frightened condition, for example. It should probably have some kind of kicker as well (a bonus to Intimidation perhaps?).
A Feat that makes you immune to the "Frightened" condition would be in my eyes OP, Advantage on Frightened condition like the Halfling's Race Ability and or +5 on save vs. Frightened/Fear condition would make you near immune but not quite