History, Spread, and Prevention of Influenza in the U.S.

Influenza, commonly known as the flu, is a contagious respiratory illness caused by influenza viruses. These viruses infect the nose, throat, and sometimes the lungs, leading to symptoms ranging from mild to severe. Understanding the causes, prevalence, transmission, and historical impact of influenza in the United States is crucial for effective prevention and control. Additionally, awareness of the nation’s detection and monitoring programs underscores the importance of vaccination in mitigating the flu’s impact.

Causes of Influenza

Influenza is primarily caused by two types of viruses: Influenza A and Influenza B. Influenza A viruses are further categorized into subtypes based on two proteins on their surface: hemagglutinin (H) and neuraminidase (N). For example, the H1N1 and H3N2 subtypes have been predominant in recent seasons. Influenza B viruses are not divided into subtypes but are classified into lineages, mainly B/Yamagata and B/Victoria. These viruses undergo frequent genetic changes, leading to new strains that can evade the immune system, necessitating annual updates to the flu vaccine².

Prevalence of Influenza in the United States

Influenza remains a significant public health concern in the United States. The Centers for Disease Control and Prevention (CDC) estimates that, during the 2019–2020 flu season, there were approximately 35 million flu illnesses, 16 million flu-related medical visits, 380,000 hospitalizations, and 20,000 deaths¹. These figures highlight the substantial burden influenza places on the healthcare system and the population. The prevalence of influenza varies annually, influenced by factors such as vaccine effectiveness, circulating virus strains, and public health interventions.

How Does Influenza Spread?

Influenza spreads primarily through respiratory droplets when an infected person coughs, sneezes, or talks. These droplets can land in the mouths or noses of people nearby or be inhaled into the lungs. Less commonly, a person might contract the flu by touching a surface or object contaminated with the virus and then touching their own mouth, nose, or eyes. The virus can survive on surfaces for several hours, making hand hygiene a critical preventive measure. Individuals are most contagious in the first three to four days after their illness begins, but some may spread the virus before symptoms appear and up to a week after becoming sick.

Historical Impact of Influenza in the United States

Influenza has had a profound impact on U.S. history, with several notable pandemics. The 1918 H1N1 influenza pandemic resulted in an estimated 675,000 deaths in the United States. Subsequent pandemics, including those in 1957 (H2N2), 1968 (H3N2), and 2009 (H1N1), caused significant morbidity and mortality, though on a lesser scale than the 1918 pandemic. These events show the need for robust public health infrastructure and effective surveillance systems to detect and respond to influenza outbreaks promptly.

Detection and Monitoring Programs in the United States

The United States has established comprehensive surveillance systems to monitor influenza activity, detect outbreaks, and inform public health interventions. The CDC’s Influenza Division leads these efforts, collaborating with state and local health departments, public health and clinical laboratories, healthcare providers, and other partners. Key components of the U.S. influenza surveillance system include:

1. Virologic Surveillance: Approximately 100 public health and over 300 clinical laboratories across the country participate in virologic surveillance. These laboratories test respiratory specimens for influenza viruses and report the findings to the CDC, providing data on circulating strains and their geographic distribution³.
2. Outpatient Illness Surveillance: The U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) monitors patient visits to healthcare providers for influenza-like illness (ILI). This system helps track the level of outpatient illness and identifies trends in influenza activity³.
3. Hospitalization Surveillance: The Influenza Hospitalization Surveillance Network (FluSurv-NET) collects data on laboratory-confirmed influenza-associated hospitalizations. This network covers approximately 9% of the U.S. population and provides valuable information on the severity of influenza seasons⁴.
4. Mortality Surveillance: The National Center for Health Statistics (NCHS) collects and reports data on deaths attributed to pneumonia and influenza. This information helps assess the impact of influenza on mortality rates nationwide³.
5. Geographic Spread Surveillance: State and territorial epidemiologists report the geographic spread of influenza activity in their jurisdictions, which the CDC compiles and publishes in weekly FluView reports³.

These surveillance components work synergistically to provide a comprehensive picture of influenza activity, guiding vaccine formulation, public health messaging, and resource allocation.

The Importance of Influenza Vaccination

Vaccination remains the most effective strategy to prevent influenza and its complications. The CDC recommends annual flu vaccination for everyone aged six months and older, with rare exceptions⁵.

The benefits of flu vaccination include:

• Reducing Illness: Flu vaccines prevent millions of illnesses and flu-related doctor visits each year. For instance, during the 2019–2020 season, vaccination prevented an estimated 7 million influenza illnesses⁵.
• Preventing Hospitalizations: Vaccination reduces the risk of flu-associated hospitalizations. A 2018 study showed that from 2012 to 2015, flu vaccination among adults reduced the risk of being admitted to an intensive care unit with flu by 82%⁵.
• Protecting Vulnerable Populations: Vaccination is particularly important for preventing the illness from spread to the elderly or people with compromised immune systems.

 

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References:

1. https://www.cdc.gov/flu-burden/php/data-vis/index.html
2. https://www.mayoclinic.org/diseases-conditions/flu/symptoms-causes/syc-20351719
3. https://stacks.cdc.gov/view/cdc/43971/cdc_43971_DS1.pdf
4. https://www.cdc.gov/fluview/overview/influenza-hospitalization-surveillance.html
5. https://www.cdc.gov/flu/vaccines/vaccinations.html