The Importance of Real-World Evidence in Understanding Influenza Vaccine Effectiveness

© 2022 The Authors. Published by SEEd srl. This is an open access article under the CC BY-NC 4.0 license (https://creativecommons.org/licenses/by-nc/4.0) BACKGROUND Seasonal influenza is an acute viral respiratory disease which circulates globally with constantly evolving epidemiology [1]. Influenza viruses can infect up to 20% of the global population each year, resulting in more than 650,000 annual deaths [2]. Seasonal influenza is therefore a major cause of illness, associated with substantial health and economic burdens, due to its impact on healthcare – with increased medical resource utilization and costs, and on society – with enormous death toll and loss of production [3]. Effective vaccines and timely vaccination programs are crucial strategies for the control of seasonal influenza, the spread and severity of which is unpredictable. Influenza viruses are constantly changing due to different types of mutation in viral genes that result in altered surface glycoproteins (hemagglutinin and neuraminidase), which enables viruses to escape the antigen-specific immunity that is induced by prior infections and/or vaccination [1]. Changes in antigenic sites result from either common point mutations during viral replication, which cause small changes in surface proteins (antigenic drift), or from less frequent significant genetic reassortment and the consequent emergence of novel virus subtypes or clades (antigenic shift) [4]. Antigenic evolution and regional variations in environmental strain / clade dominance require that the antigen composition of seasonal influenza vaccines is changed annually [1,4]. Annual vaccination is the best way to prevent influenza disease in people aged 6 months and older [3]. Influenza vaccines are proven to be economically favorable in specific populations including older adults, people with chronic medical problems, pregnant women, and children – from both healthcare and societal perspectives [3]. Therefore, the appropriate evaluation of influenza vaccines based on epidemiological and economic data is of the utmost importance for accurately informing policymakers and allocating resources for seasonal influenza vaccination programs [2,3].