diff --git a/app/models/class_import.rb b/app/models/class_import.rb
index 4e59873b4..caf69065d 100644
--- a/app/models/class_import.rb
+++ b/app/models/class_import.rb
@@ -56,10 +56,12 @@ def postprocess_rows!
 
     unknown_patients = session.patients - patients
 
-    unknown_patients.each do |unknown_patient|
-      unknown_patient.update!(school: nil)
-    end
-
-    session.patients.delete(unknown_patients)
+    session
+      .patient_sessions
+      .where(patient: unknown_patients)
+      .find_each do |patient_session|
+        patient_session.patient.update!(school: nil)
+        patient_session.destroy! if patient_session.added_to_session?
+      end
   end
 end
diff --git a/app/models/immunisation_import.rb b/app/models/immunisation_import.rb
index e08be63b3..b9d1adf78 100644
--- a/app/models/immunisation_import.rb
+++ b/app/models/immunisation_import.rb
@@ -148,9 +148,12 @@ def postprocess_rows!
         .includes(:vaccination_records)
         .select { _1.vaccinated?(programme) }
 
-    PatientSession.where(
-      session: team.sessions.upcoming,
-      patient: already_vaccinated_patients
-    ).delete_all
+    PatientSession
+      .where(
+        session: team.sessions.upcoming,
+        patient: already_vaccinated_patients
+      )
+      .select(&:added_to_session?)
+      .each(&:destroy!)
   end
 end
diff --git a/app/models/session.rb b/app/models/session.rb
index 162651f74..1d860df1a 100644
--- a/app/models/session.rb
+++ b/app/models/session.rb
@@ -131,6 +131,7 @@ def create_patient_sessions!
         required_programmes.all? { |programme| patient.vaccinated?(programme) }
       end
 
+    # First we remove patients from any other upcoming sessions.
     unvaccinated_patients.each do |patient|
       sessions_other_than_self = patient.upcoming_sessions.reject { _1 == self }
       next if sessions_other_than_self.empty?
@@ -142,6 +143,7 @@ def create_patient_sessions!
         .each(&:destroy!)
     end
 
+    # Next we can add the unvaccinated patients to this session.
     PatientSession.import!(
       %i[patient_id session_id],
       unvaccinated_patients.map { [_1.id, id] },
diff --git a/spec/models/class_import_spec.rb b/spec/models/class_import_spec.rb
index 335426133..7ca59329f 100644
--- a/spec/models/class_import_spec.rb
+++ b/spec/models/class_import_spec.rb
@@ -355,6 +355,20 @@
         expect { record! }.to change { existing_patient.reload.school }.to(nil)
         expect(session.reload.patients).not_to include(existing_patient)
       end
+
+      context "when the patient has been seen" do
+        let(:existing_patient) do
+          create(:patient, :consent_given_triage_not_needed, session:)
+        end
+
+        it "doesn't remove the patient from the session" do
+          expect(session.patients).to include(existing_patient)
+          expect { record! }.to change { existing_patient.reload.school }.to(
+            nil
+          )
+          expect(session.reload.patients).to include(existing_patient)
+        end
+      end
     end
   end
 end