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The flu, or influenza, is a contagious respiratory illness caused by viruses that infect the nose, throat, and sometimes the lungs. The flu spreads mostly from person to person, and people with flu are most contagious in the first three to four days after their illness begins.

The flu can come on suddenly. Early symptoms can include fatigue, body aches and chills, cough, sore throat, and fever. For most people, influenza resolves on its own, but sometimes, the flu, and its complications, can be deadly.

Flu viruses travel through the air in droplets when someone with the infection coughs, sneezes, or talks. You can inhale the droplets directly, or you can pick up the germs from an object and then transfer them to your eyes, nose, or mouth. People with flu can spread it to others up to about six feet away.

At the time of publishing this article, influenza activity in the United States for the 2018-2019 flu season remained low. The proportion of outpatient visits for influenza-like illness increased slightly to 1.7 percent, which is below the national baseline of 2.2 percent.

The 2017-2018 flu season, however, was one of the deadliest in decades, with high levels of outpatient clinic and emergency department visits for flu-like illness and high flu-related hospitalization rates.

You can find out more about flu facts and statistics, below.

Prevalence

There are four types of influenza viruses: A, B, C, and D. Human influenza A and B viruses cause seasonal epidemics almost every winter in the United States.

Influenza C infections generally cause a mild respiratory illness and are not thought to cause epidemics. Meanwhile, influenza D viruses primarily affect cattle and are not known to infect or cause illness in humans.

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Most people who get sick with flu will have mild illness, will not need medical care or antiviral drugs, and will recover in less than two weeks. People at higher risk of developing flu complications include:

  • children younger than age 5, especially those younger than age 2
  • adults older than age 65
  • residents of nursing homes and other long-term care facilities
  • pregnant women and women up to two weeks postpartum
  • people with weakened immune systems
  • people who have chronic illnesses, such as asthma, heart disease, kidney disease, liver disease, and diabetes
  • people who are very obese, with a body mass index of 40 or higher

The flu has resulted in 9.3 million to 49 million illnesses each year in the United States since 2010. Each year, on average, five to 20 percent of the United States population gets the flu.

It is estimated that the flu results in 31.4 million outpatient visits and more than 200,000 hospitalizations each year.

During the severe 2017-2018 flu season, one of the longest in recent years, estimates indicate that more than 900,000 people were hospitalized and more than 80,000 people died from flu.

Additionally, as of late October 2018, 185 pediatric deaths had been reported to CDC during the 2017-2018 season. Approximately 80 percent of these deaths occurred in children who had not received a flu vaccination.

Last season took the greatest toll on adults age 65 years and older. About 58 percent of the estimated hospitalizations occurred in that age group.

Costs

The flu costs an estimated $10.4 billion a year in direct medical expenses and another $16.3 billion in lost earnings annually.

Additionally, the flu causes United States employees to miss approximately 17 million workdays due to flu, which costs an estimated $7 billion a year in sick days and lost productivity.

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One report put the estimate of the cost of lost productivity to employers due to the flu in 2017-2018 at more than $21 billion, according to employment consulting firm Challenger, Gray & Christmas. Moreover, an estimated 25 million workers got sick, while $855.68 was the average amount of wages lost due to missing shifts.

A 2018 report estimated the average annual total economic burden of seasonal influenza to the American healthcare system and society was $11.2 billion. Direct medical costs were estimated to be $3.2 billion and indirect costs $8 billion.

Vaccine

The single best way to protect against the flu is to get vaccinated each year. The CDC recommends an annual flu shot for everyone older than six months old.

The flu vaccine is available as an injection or as a nasal spray. The most common way that flu vaccines are made is using an egg-based manufacturing process that has been used for more than 70 years.

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There is also a cell-based production process for flu vaccines, which was approved by the U.S. Food and Drug Administration in 2012. A third type of vaccine was approved for use in the United States in 2013; this version involves using recombinant technology.

While the annual influenza vaccine isn't 100 percent effective, it's still the best defense against the flu. Vaccine effectiveness can vary from season to season and among different age and risk groups and even by vaccine type.

Recent studies, however, show that flu vaccination reduces the risk of flu illness by between 40 percent and 60 percent among the overall population during seasons when most circulating flu viruses are well-matched to the flu vaccine.

During the 2016-2017 influenza season, the CDC estimates that flu vaccine prevented an estimated 5.3 million illnesses, 2.6 million medical visits, and 85,000 hospitalizations associated with influenza.

A 2017 study concluded that flu vaccination reduced the risk of flu-associated death by half among children with underlying high-risk medical conditions. For healthy children, it cut the risk by nearly two-thirds.

Another study published in 2018 showed getting a flu shot lessened the risk of severe flu among adults and also reduced the severity of illness.

Among adults who were admitted to the hospital with flu, vaccinated adults were 59 percent less likely to have a very severe illness that resulted in intensive care unit admission than those who had not been vaccinated.

The overall vaccine effectiveness of the 2017-2018 flu vaccine against both influenza A and B viruses is estimated to be 40 percent. This means the flu vaccine reduced a person’s overall risk of having to seek medical care at a doctor’s office for flu illness by 40 percent.

For the last several seasons, flu vaccination coverage among children age 6 months to 17 years has remained steady, but has fallen short of national public health goals, which is 80 percent.

During the 2017-2018 season, coverage dropped to 57.9 percent compared with 59 percent the year before. In the same time period, flu vaccination coverage among adults was 37.1 percent, a decrease of 6.2 percentage points from the year before.

For the 2018-2019 season, vaccine manufacturers have estimated that up to 168 million doses of influenza vaccine will be available in the United States.

Complications and mortality

Most people who get flu will recover anywhere from a few days to less than two weeks, but high-risk children and adults may develop complications such as:

  • pneumonia
  • bronchitis
  • asthma flare-ups
  • sinus infections
  • heart problems
  • ear infections

The flu is a common cause of pneumonia, especially among younger children, the elderly, pregnant women, or those with certain chronic health conditions or who live in a nursing home. In 2016, influenza and pneumonia were the eighth-leading cause of death in the United States.

People 65 years and older are at greater risk of serious complications from the flu. Estimates of the number of flu patients who also develop a bacterial infection range from as low as 2 percent to as high as 65 percent, according to one 2016 report.

It is estimated that between 70 and 85 percent of seasonal flu-related deaths have occurred in people 65 years and older. Between 50 and 70 percent of seasonal flu-related hospitalizations have occurred among people in that age group.

In addition to the flu shot, the CDC recommends everyday preventative actions like staying away from people who are sick, covering coughs and sneezes, and frequent handwashing.

If you do get the flu, antiviral drugs — which can make the illness milder and shorten the time you are sick — can be prescribed by a doctor and can mean the difference between having a milder illness versus a very serious illness that could result in a hospital stay.