Impact of Non-Pharmaceutical Interventions Targeted at COVID-19 Pandemic on Influenza Burden-A Systematic Review and...

Seasonal influenza imposes a substantial global health burden. The COVID-19 pandemic, accompanied by widespread non-pharmaceutical interventions (NPIs), profoundly disrupted respiratory virus circulation, offering a unique opportunity to assess their broader epidemiological impact. This study aimed to quantify changes in influenza burden between the pre-pandemic and intra-pandemic periods. Within the RESPINOW project, we conducted a systematic review following PRISMA guidelines. Studies reporting absolute influenza case counts before and during the pandemic were included. Data extraction and quality assessment were performed using a modified NHLBI tool for before-after studies. Influenza cases were normalized by reporting period length, and relative changes were estimated using incidence rate ratios. Subgroup analyses explored age, setting, hemisphere, human development index, influenza transmission zones, WHO regions, and viral strains. Of 20,676 screened records, 115 studies from 98 countries were included. Globally, influenza incidence declined by -92% (95% CI: -94 to -90) following the onset of the pandemic. Reductions varied geographically, ranging from near-elimination in several countries to more modest declines (e.g., South Korea: -61%, 95% CI: -79 to -26). Decreases were observed across all transmission zones and WHO regions, with descriptively larger reductions observed in high-income countries (-96%, 95% CI: -98 to -94) than in low-income settings (-82%, 95% CI: -88 to -73). Age-specific declines appeared smaller among young children (< 6 years: -66%, 95% CI: -79 to -45) compared with adults aged 18-64 years (-80%, 95% CI: -90 to -63). Influenza A appeared to decline more strongly than influenza B. The COVID-19 pandemic was associated with an unprecedented global reduction in influenza incidence. Data limitations and the need for robust epidemiological indicators highlight the importance of strengthened, integrated global surveillance to inform future pandemic responses.