diff --git a/config/references_for_titer_plots_h1n1pdm.txt b/config/references_for_titer_plots_h1n1pdm.txt
index 0c65de55..dc665569 100644
--- a/config/references_for_titer_plots_h1n1pdm.txt
+++ b/config/references_for_titer_plots_h1n1pdm.txt
@@ -1,3 +1,4 @@
+A/Sydney/5/2021 # C.1, vaccine strain
 A/Sydney/604/2023 # C.1
 A/Victoria/650/2023 # C.1
 A/Wisconsin/67/2022 # C.1.1, vaccine strain
@@ -9,7 +10,7 @@ A/Washington/22/2023 # C.1.3
 A/Darwin/7/2023 # C.1.6
 A/SouthAustralia/59/2023 # C.1.6
 A/Darwin/23/2023 # C.1.7
-A/Sydney/5/2021-egg # C.1
+A/Sydney/5/2021-egg # C.1, vaccine strain
 A/Norway/31694/2022-egg # C.1.1
 A/SouthAfrica/R05722/2022-egg # C.1.1
 A/Victoria/4897/2022-egg # C.1.1.1, vaccine strain