diff --git a/src/main/kotlin/org/opensearch/replication/task/shard/FollowerClusterStats.kt b/src/main/kotlin/org/opensearch/replication/task/shard/FollowerClusterStats.kt
index 77cfb6720..d36dcb2fe 100644
--- a/src/main/kotlin/org/opensearch/replication/task/shard/FollowerClusterStats.kt
+++ b/src/main/kotlin/org/opensearch/replication/task/shard/FollowerClusterStats.kt
@@ -20,6 +20,7 @@ import org.opensearch.core.xcontent.ToXContentFragment
 import org.opensearch.core.xcontent.XContentBuilder
 import org.opensearch.common.xcontent.XContentType
 import org.opensearch.core.index.shard.ShardId
+import java.util.concurrent.ConcurrentHashMap
 import java.util.concurrent.atomic.AtomicLong
 
 class FollowerShardMetric  {
@@ -116,5 +117,6 @@ class FollowerShardMetric  {
 
 @Singleton
 class FollowerClusterStats {
-    var stats :MutableMap<ShardId, FollowerShardMetric> =  mutableMapOf()
-}
\ No newline at end of file
+    // ConcurrentHashMap: concurrent put/remove/get from multiple ShardReplicationTasks (issue #1660)
+    val stats: MutableMap<ShardId, FollowerShardMetric> = ConcurrentHashMap()
+}