Systematic Review on Influenza Burden in Emerging Markets in 2018-2023-An Evidence Update to Guide Influenza Vaccination Recommendations.

Influenza is a contagious respiratory illness responsible for seasonal epidemics and with potential to cause pandemics. The decline in influenza-related studies published since 2018 resulted in data gaps, particularly in emerging markets. This systematic review searched for studies in six databases and gray literature sources to define the clinical burden of influenza and influenza-like illness (ILIs) and their associated sequelae among humans across emerging markets. Eligible studies were published in English, Spanish, or Chinese between January 2018 and September 2023 and conducted in Asia, the Middle East, Africa, and Latin America. In total, 256 articles were included, mostly on lab-confirmed influenza infections (n = 218). Incidences of lab-confirmed influenza cases in Asia (range 540-1279 cases/100,000 persons) and Sub-Saharan Africa (range 34,100-47,800 cases/100,000 persons) were higher compared to Latin America (range 0.7-112 cases/100,000 persons) and the Middle East and North Africa (range 0.1-10 cases/100,000 persons). Proportions of lab-confirmed influenza cases and influenza-associated outcomes (i.e., hospitalization, ICU admission and death) varied widely across regions. Temporal variation in influenza trend was observed before and during the COVID-19 pandemic. In conclusion, influenza causes significant disease burden in emerging markets. Robust large real-world studies using a similar methodology are needed to have more accurate estimates and compare studies within age groups and regions. Continuous monitoring of influenza epidemiology is important to inform vaccine programs in emerging markets with heavy influenza disease burden.