Long before there was a COVID-19 pandemic, there were influenza (flu) pandemics. And we do mean long before.
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In the past 400 years alone, there have been 12 global outbreaks of the flu, with the 1918 flu pandemic — which killed an estimated 50 million people — being the most deadly, according to a study in the journal Biomedicine & Pharmacotherapy.
The flu (caused by the influenza virus) is a highly infectious disease that can attack the nose, throat and lungs. Its hallmark symptoms include fever, chills, fatigue and muscle aches. Worldwide, flu deaths per year number into the hundreds of thousands.
Today, the World Health Organization (WHO) heads a Global Influenza Programme (GIP) that collects data and statistics about the influenza virus from around the world. That information, including what types of virus are spreading where, can be used to help slow the spread of influenza and develop better vaccines for future seasons.
Here's a closer look at flu statistics, both past and present.
Strains and Types of Influenza
Scientists haven't been able to eliminate the flu largely because influenza viruses are constantly changing. The viruses change in two ways: through small genetic mutations that occur as the virus replicates over time (called "antigenic drift"), or through sudden, major shifts in the virus (called "antigenic shift").
Small mutations usually result in viruses that are closely related to each other — so much so that your immune system will likely recognize the "new" virus and may even be able to fight it off.
Humans usually don't have any immunity built up to a virus that has undergone a major change, though. One of the most well-known examples of abrupt shift occurred in the spring of 2009, when a new H1N1 virus (formerly called "swine flu") began circulating in the U.S.
Here's a look under the microscope at the different types of flu and flu strains.
Types of Flu
There are four types of influenza viruses: A, B, C and D, according to the Centers for Disease Control and Prevention (CDC).
- Influenza A: The most dangerous of the four, the influenza A virus is the only type that can cause a pandemic of flu disease in humans. Influenza A viruses tend to change more rapidly than other types. This virus is also responsible for most of the outbreaks of influenza that occur during the winter flu season in the U.S.
- Influenza B: Along with the influenza A virus, the influenza B virus is responsible for many of the flu infections that occur each year in the U.S., but it tends to change more slowly than the influenza A type.
- Influenza C: The influenza C virus usually only causes mild illnesses in humans.
- Influenza D: Unlike the other three types, influenza D primarily infects cattle and doesn't occur in humans.
Flu Strains and Subtypes
What some people refer to as a "flu strain" is scientifically known as a flu subtype.
The influenza A virus is divided into subtypes based on two proteins on the surface of the virus: hemagglutinin (H) and neuraminidase (N). There are thought to be 198 different subtype combinations, although only 131 of them have been found in nature, according to the CDC.
The most common influenza A subtypes that circulate in humans include H1N1 and H3N2.
The name of a flu virus typically includes the type of influenza virus, the host of origin (if it comes from an animal), the geographical origin, the strain number and the year it was collected. For example, a seasonal influenza virus may look like this:
- seasonal influenza A (H3N2), A/Perth/16/2019
The influenza B virus isn't divided into subtypes; rather, it's divided into two lineages: B (Victoria) and B (Yamagata).
What Strain of Flu Is Going Around?
As of September 1, 2021, it’s too early to tell which strain of flu will be circulating during the upcoming flu season. However, the CDC recommends that the current flu vaccine contain an influenza A (H1N1)-like virus, an influenza A (H3N2)-like virus and two influenza B viruses.
Last season, almost all the influenza viruses that were circulating were influenza A and B. From September 2020 to May 2021, public health labs in the U.S. reported that influenza A accounted for 61.4 percent of all influenza samples; influenza B accounted for 38.6 percent.
Of the influenza A viruses, more than half were H3N2, and the majority of influenza B viruses were of Victoria lineage, according to the CDC.
Like the 2020 to 2021 season, the upcoming year’s flu activity may be low — a result of COVID-19 prevention protocols such as wearing face masks, reduced travel and school closures that have been put in place since the start of the novel coronavirus pandemic.
The flu vaccine may also have helped prevent the spread of the illness. The CDC says that a record number of influenza vaccines — 193.8 million in total — were distributed during the 2020 to 2021 season.
How Did the Flu Start?
The influenza virus has likely been circulating for thousands of years. In 412 BCE, Hippocrates wrote about a pandemic that modern experts believe was influenza, according to the study in Biomedicine & Pharmacotherapy.
Other influenza outbreaks likely occurred in the Middle Ages, although an article in the European Journal of Epidemiology notes that medical historians can only be confident about epidemics that occurred after 1500 AD. The name "influenza" originated in the 1400s in Italy, from an epidemic caused by the "influence of the stars."
Although multiple pandemics spread across Asia, Europe and North America throughout the 1700s and 1800s, the deadliest influenza outbreak occurred in 1918, according to a study in the Journal of Applied Microbiology. It's unclear where the 1918 virus first sprang up, but scientists do know that it was an H1N1 virus that originated in birds. The CDC estimates that about 500 million people — about 1 in 3 people alive at the time — were infected, and at least 50 million people died from it.
It wasn't until the 1930s that the influenza virus was discovered, which then paved the way for the development of the first inactivated flu vaccine in the 1940s by researchers at the University of Michigan.
Since 2010, the CDC estimates that flu in the U.S. has resulted in an annual:
- 9 million to 45 million illnesses
- 140,000 to 810,000 hospitalizations
- 12,000 to 61,000 deaths
How Flu Spreads
The flu virus is primarily spread through droplets that are made when people cough, sneeze or talk. These droplets can land in other people's mouths and noses (or be inhaled) up to about 6 feet away, according to the CDC.
Less commonly, a person might touch a surface — such as a tabletop — that has the flu virus on it, then transfer the bug to their mouth, nose or eyes when they touch their face.
Here's one possible timeline of how a virus can infect people, and when the symptoms start to appear.
Flu Infection Timeline
- Day 1: The influenza virus enters your body.
- Day 2: Even though you haven't developed any flu symptoms yet, you may still be contagious and able to infect other people.
- Day 3: Flu symptoms such as fever and fatigue can abruptly start to develop.
- Day 4 and 5: You may be at your most contagious up until this day, although oftentimes, the symptoms can subside.
- Day 6 and 7: You may feel like you've made a full recovery, but you may still be able to infect others.
Flu Season and Timing
In the Northern and Southern Hemispheres, flu season primarily occurs in the winter months, though it can start in the fall and last until the spring. In the tropics, the flu can infect people all year long, according to the WHO.
Still, because people travel internationally, it's possible to catch the flu (and spread it to others) even if flu season hasn't started in your own country. The CDC recommends people get the influenza vaccine at least two weeks before they travel, because it takes about two weeks to build up antibodies after getting vaccinated.
Flu Season in the U.S.
Flu season in the U.S. typically starts during the fall and winter, often picking up in October, peaking between December and February and tapering off in the spring (although sometimes lasting as late as May), according to the CDC.
Flu Season in the Northern Hemisphere
Like the U.S., flu season in countries that are located north of the equator — including Canada, most of Europe and Russia — can begin as early as October and linger into April or May.
Flu Season in the Tropics
Influenza season in tropical countries, like those in Central America and the Caribbean, can take place all year long.
Flu Season in the Southern Hemisphere
In countries south of the equator, such as Argentina and Australia, flu season starts in April and can last into September. This coincides with fall and spring in the Southern Hemisphere.
Vaccination Facts and Stats
The best way to avoid getting the flu is to get the influenza vaccine. Most of the flu vaccines in the U.S. are given via a shot (i.e., with a needle), but there is also a nasal spray flu vaccine available. The flu shot contains an inactive virus, while the nasal spray contains a weakened, live virus. Neither vaccine will give you the flu.
The effectiveness of annual flu vaccines varies from season to season, but according to the CDC, when the vaccine is a good "match" for the viruses that are circulating during flu season, getting vaccinated can reduce the risk of being sickened with the flu by about 40 to 60 percent. A vaccine that is a good "match" is made from many of the same viruses that are circulating during flu season.
Specifically, the CDC states that during the 2019 to 2020 influenza season, the flu vaccine prevented:
- 105,000 hospitalizations
- 3.69 million medical visits
- 7.52 million illnesses
Even if the vaccine doesn't prevent a person from catching the flu, it can lessen the severity of the illness. One research review in Vaccine found that the flu vaccine can lower the risk of being admitted to the intensive care unit (ICU) by 26 percent.
Vaccine Status by Age
Toward the end of the 2020 to 2021 flu season, the following percentages of people had received the flu vaccine, according to the CDC:
- 58.2% of children ages 6 months to 17 years
- 55% of all adults ages 18 and older
About 80 percent of flu-related deaths in children occur in children who weren't vaccinated, according to the CDC.
Vaccine Status by Race and Ethnicity
Toward the end of the 2020 to 2021 flu season, the following percentages of people, by race and ethnicity, had received the flu vaccine, according to the CDC:
- 76.9% of Asian, non-Hispanic people
- 62.8% of Hispanic people
- 61.3% of white, non-Hispanic people
- 60% of other, non-Hispanic people
- 36.5% of Black, non-Hispanic people
Vaccine Status Among Pregnant People
Pregnant people are more likely to be sickened with the flu than those who aren't pregnant — which is why the CDC recommends they get a seasonal flu shot (not the nasal spray).
Here's a closer look at flu vaccination during pregnancy:
- 61.3% of pregnant people received the flu vaccine by the end of the 2020 to 2021 flu season.
- Getting a flu shot can reduce a pregnant person's chances of being hospitalized with the flu by an average of 40 percent.
- Flu vaccinations can reduce the risk of flu-related acute respiratory infections (such as pneumonia) by about one-half.
Flu Death Rate Demographics
The WHO estimates that 290,000 to 650,000 people die of the flu each year. (Flu death rates have been rounded to the nearest whole number.)
Flu Death Rate Globally
Here's a closer look at the influenza death rate by region, according to a December 2019 study in the Journal of Global Health, which analyzed statistics from 2002 to 2011:
- Americas: 6 people per 100,000
- Eastern Mediterranean: 5 people per 100,000
- Europe: 5 people per 100,000
- South-East Asia: 6 people per 100,000
- Sub-Saharan Africa: 6 people per 100,000
- Western Pacific: 5 people per 100,000
Flu Deaths in England
According to the United Kingdom's Office for National Statistics:
- In 2018, 1,598 people died of flu.
- In 2019, 1,223 people died of flu.
Flu Deaths in Wales
According to the U.K.'s Office for National Statistics:
- In 2018, 73 people died of flu.
- In 2019, 53 people died of flu.
Flu Deaths in Canada
According to the Canadian government:
- During the 2019 to 2020 flu season, 120 people died of flu.
Flu Deaths in Australia
According to the Australian Government Department of Health:
- In 2019, 812 people died of flu.
Flu Death Rate in the U.S.
Here's a look at the number of people per 100,000 who died of flu during 2019, according to the CDC:
- Alabama: 17 per 100,000
- Alaska: 7 per 100,000
- Arizona: 10 per 100,000
- Arkansas: 17 per 100,000
- California: 13 per 100,000
- Colorado: 8 per 100,000
- Connecticut: 11 per 100,000
- Delaware: 11 per 100,000
- Florida: 8 per 100,000
- Georgia: 12 per 100,000
- Hawaii: 17 per 100,000
- Idaho: 11 per 100,000
- Iowa: 13 per 100,000
- Kansas: 14 per 100,000
- Kentucky: 16 per 100,000
- Louisiana: 12 per 100,000
- Maine: 15 per 100,000
- Maryland: 11 per 100,000
- Massachusetts: 13 per 100,000
- Michigan: 13 per 100,000
- Minnesota: 8 per 100,000
- Mississippi: 23 per 100,000
- Missouri: 13 per 100,000
- Montana: 11 per 100,000
- Nebraska: 15 per 100,000
- Nevada: 13 per 100,000
- New Hampshire: 10 per 100,000
- New Jersey: 11 per 100,000
- New Mexico: 13 per 100,000
- New York: 17 per 100,000
- North Carolina: 14 per 100,000
- North Dakota: 14 per 100,000
- Ohio: 13 per 100,000
- Oklahoma: 14 per 100,000
- Oregon: 9 per 100,000
- Pennsylvania: 13 per 100,000
- Rhode Island: 12 per 100,000
- South Carolina: 11 per 100,000
- South Dakota: 16 per 100,000
- Tennessee: 16 per 100,000
- Texas: 11 per 100,000
- Utah: 11 per 100,000
- Vermont: 6 per 100,000
- Virginia: 11 per 100,000
- Washington: 10 per 100,000
- West Virginia: 16 per 100,000
- Wisconsin: 10 per 100,000
- Wyoming: 15 per 100,000
Flu Deaths in the U.S. by Flu Season
Some flu seasons are deadlier than others. The 2017 to 2018 flu season, for example, killed about five times as many Americans as the 2011 to 2012 flu season. Here's a look at the statistics for the past 10 years.
Flu Deaths in the U.S. by Season
Number of deaths
2010 to 2011
Influenza A (H3N2)
Influenza A (H1N1pdm09)
2011 to 2012
Influenza A (H3N2)
2012 to 2013
Influenza A (H3N2)
2013 to 2014
Influenza A (H1N1pdm09)
2014 to 2015
Influenza A (H3N2)
2015 to 2016
Influenza A (H1N1pdm09)
2016 to 2017
Influenza A (H3N2)
*2017 to 2018
Influenza A (H3N2)
*2018 to 2019
Influenza A (H1N1pdm09)
*2019 to 2020
Flu Death Rate in the U.S. by Age
In the U.S., older adults are more likely to be hospitalized and die from the flu than younger people.
Of the people who died of flu during the 2019 to 2020 flu season:
- 62.4% were 65 and older
- 23.4% were ages 50 to 64
- 12.2% were ages 18 to 49
- 0.8% were ages 5 to 17
- 1.2% were 4 and younger
Of the people who were hospitalized:
- 43.7% were 65 and older
- 22.1% were ages 50 to 64
- 21.3% were ages 18 to 49
- 5.6% were ages 5 to 17
- 7.4% were 4 and younger
Fast Flu Stats
The CDC estimates that the 2019 to 2020 flu season caused:
- 38 million illnesses: About the same as the population of California
- 400,000 hospitalizations: About the same as the population of Miami, Florida
- 22,000 deaths: Enough people to fill Madison Square Garden in New York City
- Biomedicine & Pharmacotherapy: "The Epidemiology and History of Influenza"
- World Health Organization: "Global Influenza Programme"
- World Health Organization: "Burden of Disease"
- Centers for Disease Control and Prevention: "How the Flu Virus Can Change: 'Drift' and 'Shift'"
- World Health Organization: "Seasonal Influenza FAQs"
- Centers for Disease Control and Prevention: "Types of Influenza Viruses"
- Centers for Disease Control and Prevention: "Frequently Asked Influenza (Flu) Questions: 2021-2022 Season"
- Centers for Disease Control and Prevention: "2020-2021 Flu Season Summary"
- European Journal of Epidemiology: "Introduction to Pandemic Influenza Through History"
- Journal of Applied Microbiology: "A History of Influenza"
- Centers for Disease Control and Prevention: "History of 1918 Flu Pandemic"
- Centers for Disease Control and Prevention: "Influenza"
- Centers for Disease Control and Prevention: "Disease Burden of Influenza"
- Centers for Disease Control and Prevention: "How Flu Spreads"
- Centers for Disease Control and Prevention: "Flu Symptoms & Complications"
- Centers for Disease Control and Prevention: "The Flu Season"
- Centers for Disease Control and Prevention: "Estimated Influenza Illnesses, Medical Visits, and Hospitalizations Averted by Vaccination in the United States - 2019 - 2020 Influenza Season"
- Vaccine: "Does Influenza Vaccination Attenuate the Severity of Breakthrough Infections? A Narrative Review and Recommendations for Further Research"
- Centers for Disease Control and Prevention: "Frequently Asked Questions About Estimated Flu Burden"
- Centers for Disease Control and Prevention: "Influenza Vaccination Coverage Among Pregnant Persons Overall and by Race/Ethnicity, United States"
- Centers for Disease Control and Prevention: "How Flu Vaccine Effectiveness and Efficacy Are Measured"
- Centers for Disease Control and Prevention: "Influenza (Flu) Vaccine and Pregnancy"
- Journal of Global Health: "Global Mortality Associated With Seasonal Influenza Epidemics: New Burden Estimates and Predictors From the GLaMOR Project"
- Office for National Statistics: "Influenza Deaths in 2018, 2019 and 2020"
- Australian Government Department of Health: "Australian Influenza Surveillance Report"
- Government of Canada: "FluWatch Annual Report: 2019-2020 Influenza Season"
- Centers for Disease Control and Prevention: "Influenza/Pneumonia Mortality by State"
- Centers for Disease Control and Prevention: "What You Should Know About Flu Antiviral Drugs"
- Centers for Disease Control and Prevention: "2017-2018 Estimated Influenza Illnesses, Medical Visits, Hospitalizations, and Deaths and Estimated Influenza Illnesses, Medical Visits, Hospitalizations, and Deaths Averted by Vaccination in the United States"
- Centers for Disease Control and Prevention: "Estimated Influenza Illnesses, Medical Visits, Hospitalizations, and Deaths in the United States - 2018 - 2019 Influenza Season"