What Is the Difference Between a Flu Epidemic and a Flu Pandemic?

You may read about or hear in the news that there is a flu epidemic occurring, but do you know what that means? How do public health officials determine that flu levels are high enough to declare an epidemic? And how does it differ from a pandemic?

Sick woman blowing her nose, she covered with blanket
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What is a Flu Epidemic?

Although the word "epidemic" sounds scary, it isn't uncommon for the flu to reach epidemic levels. In fact, seasonal flu epidemics happen almost every year.

An epidemic is a rise in the number of cases of disease beyond what is normally expected in a specific geographic area. The increase in cases is sudden, the disease stays contained to a specific geographic area—like a city or region—and doesn't spread across countries and continents. It may not even spread across an entire state.

Each week, the Centers for Disease Control and Prevention (CDC) compares the percentage of flu-related deaths to an epidemic threshold value. If this number exceeds the threshold, the CDC declares the flu to be at epidemic levels. When the percentage of flu deaths drops below the threshold, the epidemic is over.

What is a Flu Pandemic?

Flu pandemics happen when a new strain of the flu A virus appears. A new viral strain quickly spreads because most people aren't immune and a vaccine that offers immediate protection isn't widely available. As a result, it spreads across several countries around the world, causing widespread illness.

The World Health Organization (WHO) says the following conditions can lead to a flu pandemic:

  • The virus is able to infect humans
  • The virus is able to spread easily from human to human
  • The virus must cause serious illness or death in humans

While flu pandemics are rare, they do occur periodically. There have been four flu pandemics since the beginning of the 20th century, the most severe being the Spanish flu of 1918:

  • 1918 Spanish flu pandemic: Killed approximately 675,000 in the U.S.
  • 1957-1958 Asian flu pandemic: Caused an estimated 116,000 U.S. deaths
  • 1968 Hong Kong flu pandemic: Linked to around 100,000 deaths in the U.S.
  • 2009 H1N1 flu pandemic: Responsible for an estimated 12,469 U.S. deaths

Just like with an epidemic, a flu pandemic is over when the number of cases is no longer above a defined threshold. However, the virus can continue to circulate in humans even after the pandemic is over. Take the 2009 H1N1 flu pandemic: Even though WHO declared an end to this pandemic in 2010, H1N1 continues to circulate as a seasonal flu virus, causing illness and hospitalization worldwide every year.

A new flu strain can cause a pandemic, with high rates of infection worldwide because no vaccine is available and people have not been exposed or developed immunity to it. An existing flu strain may cause an epidemic if it spreads among more people than usual in a specific geographic region, but it is not expected to spread worldwide in higher numbers than usual because most people are already immune, and a vaccine is available.

Differences Between a Flu Epidemic and a Flu Pandemic

A seasonal flu epidemic is usually caused by an existing flu strain that increases in a certain geographic area. A pandemic flu virus is a new flu strain that hasn't circulated for a long time, if ever. Because of this, humans have little to no immunity against the virus and it spreads quickly and globally, causing widespread sickness and deaths.

Flu Epidemic
  • Happens every year, usually in winter

  • Caused by flu viruses that are similar to those already affecting people

  • Specific to one city, region, or country

  • Vaccine available at the beginning of flu season

  • Causes an average of between 12,000 and 52,000 deaths each year in the U.S.

  • Infants and elderly most at-risk for serious complications

Flu Pandemic
  • Rarely happens (only four times since 1918)

  • Caused by a new flu virus that people have not been exposed to before

  • Spreads worldwide

  • Vaccine not available in the early stages of a pandemic

  • Number of deaths can be significantly higher. The Spanish flu of 1918 caused approximately 675,000 deaths in the U.S.

  • Healthy people at-risk for serious complications

A Word From Verywell

Flu vaccines are the best way to protect people during flu epidemics and pandemics. Seasonal flu vaccines are available every year before the flu season. These vaccines only protect against epidemic-causing flu viruses. Vaccines against pandemic flu strains typically aren't available in the early stages of a pandemic.

Regardless of the epidemic status, the flu is a serious illness that everyone should take steps to avoid. By properly washing your hands, covering your nose and mouth when coughing or sneezing, and disinfecting hard surfaces, you can keep yourself and your family healthy throughout the flu season.

8 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Dicker R, Coronado F, Koo D, Parrish RG. Principles of Epidemiology in Public Health Practice, 3rd Edition. Atlanta, GA: CDC; 2006.

  2. Xu X, Blanton L, Elal AIA, et al. Update: Influenza Activity in the United States During the 2018-19 Season and Composition of the 2019-20 Influenza Vaccine. MMWR Morb Mortal Wkly Rep. 2019;68(24):544-551. doi:10.15585/mmwr.mm6824a3

  3. Pandemic Influenza Risk Management: A WHO guide to inform and harmonize national and international pandemic preparedness and response. Geneva: World Health Organization; 2017.

  4. Centers for Disease Control and Prevention. 1918 Pandemic (H1N1 virus). Updated March 20, 2019.

  5. Centers for Disease Control and Prevention. 1957-1958 Pandemic (H2N2 virus). Updated January 2, 2019.

  6. Centers for Disease Control and Prevention. 1968 Pandemic (H3N2 Virus). Updated January 2, 2019.

  7. Centers for Disease Control and Prevention. 2009 H1N1 Pandemic (H1N1pdm09 virus). Updated June 11, 2019.

  8. Khanna M, Kumar B, Gupta A, Kumar P. Pandemic Influenza A H1N1 (2009) Virus: Lessons from the Past and Implications for the Future. Indian J Virol. 2012;23(1):12-17. doi:10.1007/s13337-012-0066-3

Additional Reading

By Kristina Duda, RN
Kristina Duda, BSN, RN, CPN, has been working in healthcare since 2002. She specializes in pediatrics and disease and infection prevention.