diff --git a/RecoVertex/ConfigurableVertexReco/src/VertexFitterManager.cc b/RecoVertex/ConfigurableVertexReco/src/VertexFitterManager.cc
index 5d12f1cdc4190..724cd0a75256d 100644
--- a/RecoVertex/ConfigurableVertexReco/src/VertexFitterManager.cc
+++ b/RecoVertex/ConfigurableVertexReco/src/VertexFitterManager.cc
@@ -53,7 +53,10 @@ VertexFitterManager::VertexFitterManager ( const VertexFitterManager & o )
 
 VertexFitterManager & VertexFitterManager::Instance()
 {
-  static VertexFitterManager singleton;
+  //The singleton's internal structure only changes while
+  // this library is being loaded. All other methods are const.
+  
+  [[cms::thread_safe]] static VertexFitterManager singleton;
   return singleton;
 }
 
diff --git a/RecoVertex/ConfigurableVertexReco/src/VertexRecoManager.cc b/RecoVertex/ConfigurableVertexReco/src/VertexRecoManager.cc
index c7efff397317b..cd139948baed7 100644
--- a/RecoVertex/ConfigurableVertexReco/src/VertexRecoManager.cc
+++ b/RecoVertex/ConfigurableVertexReco/src/VertexRecoManager.cc
@@ -44,7 +44,9 @@ VertexRecoManager::VertexRecoManager ( const VertexRecoManager & o )
 
 VertexRecoManager & VertexRecoManager::Instance()
 {
-  static VertexRecoManager singleton;
+  //The singleton's internal structure only changes while
+  // this library is being loaded. All other methods are const.
+  [[cms::thread_safe]] static VertexRecoManager singleton;
   return singleton;
 }