diff --git a/cara/models.py b/cara/models.py
index 157eb256..c4a6005a 100644
--- a/cara/models.py
+++ b/cara/models.py
@@ -682,6 +682,9 @@ class _PopulationWithVirus(Population):
     #: The virus with which the population is infected.
     virus: Virus
 
+    #: The percentage of host immunity
+    host_immunity: float
+
     @method_cache
     def fraction_of_infectious_virus(self) -> _VectorisedFloat:
         """