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Understanding Influenza: ILI, SARI, Economic Burden, and Global Impact

Updated: Nov 27, 2023


Influenza is an infectious disease that affects millions of people worldwide every year. In this post, we will explore the differences between ILI (Influenza-Like Illness) and SARI (Severe Acute Respiratory Infection), WHO's criteria for measuring economic burden, annual influenza infection rates in the United States and globally, and the economic losses experienced by different countries.

economic impact of influenza

Differences between ILI and SARI

ILI (Influenza-Like Illness) and SARI (Severe Acute Respiratory Infection) both refer to respiratory infections, but they differ in their definitions and severity. ILI is a term used to describe patients who exhibit symptoms similar to influenza, including fever, cough, and sore throat. On the other hand, SARI represents severe acute respiratory infections, which can involve serious symptoms such as difficulty breathing and pneumonia. Generally, SARI is considered more dangerous and potentially fatal compared to ILI. (1)

WHO's Criteria for Measuring Economic Burden

The World Health Organization (WHO) considers various factors when measuring the economic burden of influenza. These factors include Costs of illness, Direct costs, Indirect costs, and direct medical costs, direct non-medical costs. (1)

components of economic burdern proposed by WHO
Components of economic burden, WHO, 2016

the economic burden estimiation for the seasonal influenza, WHO, 2016
Total economic burden equation, WHO, 2016

Additionally, the WHO takes into account the extent of influenza outbreaks in each country and the health status of the population to assess the economic burden. This enables a better understanding of the impact of influenza on national and global economies.

Annual Influenza Infection Rates in the US and Worldwide

In the United States, annual influenza infection rates are estimated through consistent monitoring and research. For example, in recent years, the influenza infection rate in the US has been estimated 14.4 million in 18 to 49-year-olds group and 13.2 million cases in 50 to 64-year-olds group.(2) Moreover, influenza infection rates vary from country to country worldwide, ranging from 3% to 13%. (3) These numbers can be interpreted in numerical size: 3 - 5 million cases of severe illness and others are assumed as influenza like illness infection. (4) These infection rate estimations are made possible through the collaboration of global infectious disease monitoring networks and research institutions.

Projected Economic Losses in Various Countries

The economic losses caused by influenza vary depending on several factors in each country. Projected loss figures are calculated by considering the severity of influenza outbreaks, population composition, and healthcare systems. For instance, it is estimated that the United States experiences annual economic losses of around $11.2 billion dollars (Direct US$3.2 billion and Indirect US$8.0 billion) due to influenza in 2018. In Europe, the cost of seasonal influenza is suspected as €6 billion to €14 billion annually. (2) This includes expenses related to healthcare, productivity losses, and costs associated with infection prevention. Other countries or continents also anticipate and prepare for similar economic losses using comparable approaches.


In conclusion, understanding ILI and SARI differences, measuring economic burden by WHO criteria, and analyzing annual influenza infection rates are vital for comprehending influenza's global impact. The distinctions between ILI and SARI highlight severity levels and the need for appropriate medical attention. WHO's criteria shed light on influenza's impact on healthcare, productivity, and prevention. Examining infection rates in the US and worldwide provides insights into prevalence and global surveillance. Projected economic losses underscore influenza's financial impact and the need for investment. By exploring these aspects, we enhance efforts to mitigate influenza's impact and protect global public health.


  1. World Health Organization. Who manual for estimating the economic burden of seasonal influenza. World Health Organization.

  2. de Courville, C., Cadarette, S. M., Wissinger, E., & Alvarez, F. P. (2022). The economic burden of influenza among adults aged 18 to 64: A systematic literature review. Influenza and Other Respiratory Viruses, 16(3), 376–385.

  3. Tokars, J. I., Olsen, S. J., & Reed, C. (2017). Seasonal incidence of symptomatic influenza in the United States. Clinical Infectious Diseases, 66(10), 1511–1518.

  4. World Health Organization. (n.d.-a). Influenza (seasonal). World Health Organization.

This content is intended for educational purposes only and is not a substitute for professional medical advice. If you have any questions or concerns about your health, please consult with a healthcare professional.

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