The impact of the 2009 influenza pandemic on the seasonality of human respiratory syncytial virus: A systematic analysis

BACKGROUND: Several local studies showed that the 2009 influenza pandemic delayed the RSV season. However, no global‐level analyses are available on the possible impact of the 2009 influenza pandemic on the RSV season. OBJECTIVES: We aim to understand the impact of the 2009 influenza pandemic on the RSV season. METHODS: We compiled data from published literature (through a systematic review), online reports/datasets and previously published data on global RSV seasonality and conducted a global‐level systematic analysis on the impact of the 2009 influenza pandemic on RSV seasonality. RESULTS: We included 354 seasons of 45 unique sites, from 26 countries. Globally, the influenza pandemic delayed the onset of the first RSV season by 0.58 months on average (95% CI: 0.42, 0.73; maximum delay: 2.5 months) and the onset of the second RSV season by a lesser extent (0.25 months; 95% CI: 0.12, 0.39; maximum delay: 3.4 months); no delayed onset was observed for the third RSV season. The delayed onset was most pronounced in the northern temperate, followed by the southern temperate, and was least pronounced in the tropics. CONCLUSIONS: The 2009 influenza pandemic delayed the RSV onset on average by 0.58 months and up to 2.5 months. This suggests evidence of viral interference as well as the impact of public health measures and has important implications for preparedness for RSV season during the ongoing COVID‐19 pandemic and future pandemics.