diff --git a/examples/01-seir/main.cpp b/examples/01-seir/main.cpp
index 7728f5f89..ae071a85a 100644
--- a/examples/01-seir/main.cpp
+++ b/examples/01-seir/main.cpp
@@ -14,7 +14,7 @@ int main() {
 
     // Adding a bernoulli graph as step 0
     model.agents_from_adjlist(
-        rgraph_smallworld(500000, 5, .001, false, model)
+        rgraph_smallworld(1000000, 5, .001, false, model)
     );
 
     // Running and checking the results
diff --git a/include/epiworld/model-meat.hpp b/include/epiworld/model-meat.hpp
index b3df43f65..2b665645e 100644
--- a/include/epiworld/model-meat.hpp
+++ b/include/epiworld/model-meat.hpp
@@ -1792,6 +1792,15 @@ inline void Model<TSeq>::update_state() {
 template<typename TSeq>
 inline void Model<TSeq>::mutate_virus() {
 
+    // Checking if any virus has mutation
+    size_t nmutates = 0u;
+    for (const auto & v: viruses)
+        if (v->mutation_fun)
+            nmutates++;
+
+    if (nmutates == 0u)
+        return;
+
     if (use_queuing)
     {