| Summary: | <p>Non-pharmaceutical interventions (NPIs) during the COVID-19 pandemic significantly reduced influenza transmission. This study explores the hypothesis of "immunity debt" which suggests increased vulnerability to influenza following reduced exposure during the pandemic. We analyzed World Health Organization aggregated data in influenza from 116 countries and its association with NPI intensity as measured by the COVID-19 Stringency Index. Where individual-level data available (France, the United Kingdom, Spain, Italy, Belgium, and Romania), we replicated our analyses of influenza monthly rate in six Europe countries (France, the United Kingdom, Spain, Italy, Belgium, and Romania). Our results indicate globally a 46.3% (95%CI: 15.79 to 70.78%) reduction in influenza cases during COVID-19 restrictions in winter season, followed by a 131.7% (95%CI: 34.95 to 255.78%) increase in the first post-relaxation winter and a 161.2% (95%CI: 31.88 to 382.16%) increase in the summer as compared to the predicted level based on historical influenza epidemic trends. In addition, a positive association between the Stringency Index and post-relaxation influenza surge was observed globally (R² = 0.14-0.17) and replicated regionally. Our findings support the population immunity debt hypothesis for influenza and call for proactive preparations against its consequences in future pandemics.</p>
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