diff --git a/changelog.d/5220.feature b/changelog.d/5220.feature
new file mode 100644
index 000000000000..747098c16624
--- /dev/null
+++ b/changelog.d/5220.feature
@@ -0,0 +1 @@
+Add experimental support for relations (aka reactions and edits).
diff --git a/synapse/events/utils.py b/synapse/events/utils.py
index 27a2a9ef986e..e2d4384de199 100644
--- a/synapse/events/utils.py
+++ b/synapse/events/utils.py
@@ -330,12 +330,13 @@ def __init__(self, hs):
         )
 
     @defer.inlineCallbacks
-    def serialize_event(self, event, time_now, **kwargs):
+    def serialize_event(self, event, time_now, bundle_aggregations=True, **kwargs):
         """Serializes a single event.
 
         Args:
             event (EventBase)
             time_now (int): The current time in milliseconds
+            bundle_aggregations (bool): Whether to bundle in related events
             **kwargs: Arguments to pass to `serialize_event`
 
         Returns:
@@ -350,7 +351,7 @@ def serialize_event(self, event, time_now, **kwargs):
 
         # If MSC1849 is enabled then we need to look if thre are any relations
         # we need to bundle in with the event
-        if self.experimental_msc1849_support_enabled:
+        if self.experimental_msc1849_support_enabled and bundle_aggregations:
             annotations = yield self.store.get_aggregation_groups_for_event(
                 event_id,
             )
diff --git a/synapse/handlers/events.py b/synapse/handlers/events.py
index 6003ad9cca87..eb525070cff8 100644
--- a/synapse/handlers/events.py
+++ b/synapse/handlers/events.py
@@ -122,6 +122,9 @@ def get_stream(self, auth_user_id, pagin_config, timeout=0,
 
             chunks = yield self._event_serializer.serialize_events(
                 events, time_now, as_client_event=as_client_event,
+                # We don't bundle "live" events, as otherwise clients
+                # will end up double counting annotations.
+                bundle_aggregations=False,
             )
 
             chunk = {
diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index 7b2c33a9228f..7e40cb6502a1 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -166,6 +166,9 @@ def get_state_events(
         now = self.clock.time_msec()
         events = yield self._event_serializer.serialize_events(
             room_state.values(), now,
+            # We don't bother bundling aggregations in when asked for state
+            # events, as clients won't use them.
+            bundle_aggregations=False,
         )
         defer.returnValue(events)
 
diff --git a/synapse/rest/client/v2_alpha/sync.py b/synapse/rest/client/v2_alpha/sync.py
index c701e534e7b8..d3025025e337 100644
--- a/synapse/rest/client/v2_alpha/sync.py
+++ b/synapse/rest/client/v2_alpha/sync.py
@@ -358,6 +358,9 @@ def encode_room(
         def serialize(events):
             return self._event_serializer.serialize_events(
                 events, time_now=time_now,
+                # We don't bundle "live" events, as otherwise clients
+                # will end up double counting annotations.
+                bundle_aggregations=False,
                 token_id=token_id,
                 event_format=event_formatter,
                 only_event_fields=only_fields,