What Is a Flu Epidemic and Should You Be Concerned?

Protect yourself from a flu epidemic. Ghislain & Marie David de Lossy/Cultura/Getty Images

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?

Epidemic vs. Pandemic

An epidemic in medical terms is also called a disease outbreak and is defined by the World Health Organization (WHO) as "the occurrence of cases of disease in excess of what would normally be expected in a defined community, geographical area or season.

An outbreak may occur in a restricted geographical area, or may extend over several countries. It may last for a few days or weeks, or for several years."

In contrast, a pandemic is a ​global outbreak of a disease.

During flu pandemics, a new, mutated strain of influenza A appears and causes outbreaks of disease around the world. These pandemic outbreaks usually last a year or more with two or three peaks in illness, as opposed to the few months that seasonal flu outbreaks cause illness.

What Does It Mean When a Flu Epidemic is Declared?

When the Centers for Disease Control and Prevention (CDC) declares the flu to be at epidemic levels in the United States, it means that the percentage of people with the flu has risen significantly higher than the levels typically seen in the community. The epidemic threshold level differs by week. The percentage is calculated based on averages for the same week during the previous 5 years.

For example, when the CDC announced that the flu had reached an epidemic level for the week ending on December 20th, 2014, 6.8% of all deaths reported through the 122 Cities Mortality Reporting System were due to pneumonia or influenza. 6.8% was the epidemic threshold for the 51st week of the year, meaning the flu was at epidemic levels.

The following week the epidemic threshold increased to 6.9% but the reported percentage of deaths remained at 6.8%, meaning it was just below the epidemic threshold level for the 52nd week of the year.

Should You Be Concerned About a Flu Epidemic?

Seasonal flu deaths reach epidemic level nearly every year. The amount of time spent above the epidemic threshold and how high above the threshold the rates go varies greatly by flu season. When the strain of flu is more severe and the vaccine is not well matched to the circulating strain of influenza, the flu epidemic levels may be higher than in other years.

Regardless of the epidemic status, the flu is a serious illness that everyone should take steps to avoid. Get your annual flu vaccine, wash your hands frequently and take steps to keep yourself and your family healthy throughout cold and flu season.


"Disease Outbreaks". Health Topics 2015. World Health Organization. 8 Jan 15.

"Flu Deaths Hit Epidemic Threshold". Infectious Disease 30 Dec 15. MedPage Today. 8 Jan 15.

"Weekly US Influenza Surveillance Report". Seasonal Influenza (Flu) 9 Jan 15. US Centers for Disease Control and Prevention. Department of Health and Human Services. 10 Jan 15.

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