Influenza or “the flu” is one of the most common and dangerous viral diseases in the world. Every year, about 50% of Americans get immunized against the flu with a vaccine.
What are the types of influenza?
When is flu season in the United States?
What is bird flu?
What is swine flu?
How many people get the flu?
What are symptoms of the flu?
Is the flu serious?
What are complications of the flu?
Who is at high risk of flu complications?
How does the flu spread?
How long is the flu contagious?
How is the flu treated?
Can I get the flu again?
Why do I need a flu shot every year?
How effective is the flu shot?
Who needs a flu shot?
Who should NOT get vaccinated?
When is the best time to get a flu shot?
What type of flu shot can I get?
Do I need a flu shot for school or daycare?
Flu (Influenza) is a highly-contagious lung infection caused by viruses. It can occur in humans and animals. Some of the most serious types of flu started in animals and mutated to infect humans.
The deadliest flu pandemic in modern history was the “Spanish flu” in 1918, which killed 50 million people worldwide. It was caused by the H1N1 virus, which returned in 2009 and killed around 17,000 people.
There are four types of influenza viruses: A, B, C, and D. The only types that cause epidemics in people are Influenza A and B. They are further separated into subtypes, such as Influenza A(H1N1) or A(H3N2), and further broken down into individual strains of the virus.
In the United States, flu season starts in October and ends the next year in May. The “peak” season is from December to February. You can get the flu in any season, but the highest risk is the winter months.
Bird flu viruses usually only infect birds, but two types (H5N1 and H7N9) can infect humans. Death occurs in 50% of people who get avian influenza. The first case was reported in Hong Kong in 1997. There have been outbreaks in Asia, Africa, the Pacific, Middle East, and Europe.
Swine flu is caused by the H1N1 virus that was first found in pigs. It mutated in humans and spread worldwide, causing a pandemic in 2009. It is now considered a “normal” flu virus that is preventable with a flu shot.
During the 2015-2016 flu season, approximately 25 million people in the U.S. got the flu, 11 million people needed medical treatment, 310,000 people were hospitalized, and 12,000 people died of the flu.
From 2010 to 2016, hospitalizations for the flu ranged from a low of 140,000 (during 2011-2012) to a high of 710,000 (during 2014-2015).
The numbers vary depending on the type of flu virus circulating, how many people get a flu shot, and how well the flu shot works that year.
- Body aches
- Muscle pain
- Fever (not everyone will have a fever)
- Sore throat
- Runny or stuffy nose
- Fatigue (tiredness)
- Sometimes vomiting and diarrhea (mostly children)
Most people who get the flu recover on their own in under 2 weeks without treatment or antiviral medication. However, some people develop serious complications that lead to hospitalization or death. This is more likely in people with a chronic lung disease like asthma or heart disease. The flu can also exacerbate other health problems.
- Sinus infection
- Ear infection
- Heart inflammation (myocarditis)
- Brain inflammation (encephalitis)
- Muscle inflammation (myositis, rhabdomyolysis)
- Multi-organ system failure
- Kidney failure
- Children under 5, but especially under age 2
- Adults over 65 years old
- Pregnant women (and up to 2 weeks after birth)
- Residents of nursing home and long-term care facilities
- American Indians and Alaskan Natives
- People with chronic medical conditions (see below for a list)
The flu can make pre-existing health problems worse. The following medical conditions are linked to a higher risk of flu complications:
- Brain or developmental disorders
- Cerebral palsy
- Intellectual disabilities
- Muscular dystrophy
- Heart disease
- Blood disorders (sickle cell disease)
- Kidney or liver problems
- Weak immune system due to disease or medication (HIV/AIDS, steroids, etc.)
- People under age 19 on long-term aspirin therapy
The flu spreads in the air when an infected person coughs, sneezes, breathes, or talks. You can catch the flu up to 6 feet away from someone who is sick if you breathe in the virus or by touching your eyes, nose, or mouth after touching a contaminated surface.
The flu is contagious 1-2 days before symptoms appear, and for 5-7 days after becoming sick. Children are contagious for more than 7 days. The symptoms start 1-4 days after the virus enters the body.
Two antiviral drugs are recommended for treating severe cases of the flu: Tamiflu (oseltamivir) and Relenza (zanamivir). Flu viruses can become resistant to these medications, so they are usually reserved for people at high risk of complications.
Yes. You can get the flu more than once, even in the same year. However, you can’t get exactly the same type of flu again because your body will be immune to it. There are many types of flu viruses and they evolve rapidly — just like the common cold.
Flu season is also when other dangerous non-flu viruses circulate, such as rhinovirus (“common cold”) and respiratory syncytial virus (RSV). The flu shot does NOT protect you from these other viruses.
The flu shot is a vaccine that is given with a needle, usually in the upper arm. New flu shots are created each year depending on what flu viruses are in South America before flu season starts in the U.S.
The vaccine protects against three or four types of viruses — usually Influenza A (H1N1), A (H3N2), and one or two influenza B viruses.
People in the U.S. can get these flu shots or nasal sprays:
- FluMist (not recommended for the 2016-2017 flu season)
- Influenza A (H1N1) 2009
- Influenza A (H5N1)
There are two reasons. One is that the vaccine wears off and you lose immunity over time. The other is that flu viruses are always changing, so the formulation of the flu vaccine is updated every year to keep up.
Flu shots are up to 50-60% effective at preventing the flu, but it changes from year to year. In the 2016-2017 flu season, it was 48% effective at preventing the flu, with similar effectiveness in 2015-2016, but only 19% effective in 2014-2015.
The nasal vaccine (FluMist) is not recommended because studies showed that it was not effective during several past flu seasons.
Everyone age 6 months or older is recommended for a flu shot by the government, with rare exceptions.
- Infants under 6 months old
- People who had a severe allergy to a previous flu shot
- People allergic to other ingredients in the flu shot (i.e., thimerosal)
Ask a doctor if you should get the flu shot if you have:
- Egg allergy
- Moderate or severe illness (with or without fever)
- Guillain-Barre Syndrome (GBS) within 6 weeks of a previous flu shot
The best time to get the flu shot is between Halloween and Thanksgiving. New flu shots are generally delivered by August, but immunity wears off and you need protection during “peak” flu season in late fall and winter. It also takes a few weeks to develop immunity.
There are many types of flu shots, including trivalent (three-virus immunity) and quadrivalent (four-virus immunity). There are nasal sprays that contain live attenuated (“weakened”) influenza viruses.
There are also less-common types, such as flu shots given under the skin rather than a muscle, egg-free shots for people who are allergic, or shots that use a high-pressure stream of water instead of a needle.
Connecticut, Rhode Island, and New Jersey require a yearly flu shot for children entering daycare. For more information, visit www.immunize.org/laws/
To learn more about the side effects associated with flu vaccines, please visit this page: Flu vaccine side effects.
- Pain, redness, or swelling from the shot
- Muscle aches
- Loss of appetite
To learn more about the side effects associated with the influenza/flu vaccine, please visit this page: Flu Vaccine Side Effects
- High fever (over 101°F)
- Injection-site swelling for more than 1 week
- Allergic reaction
- Guillain-Barré syndrome (GBS)
- Eye disorders
- Brain inflammation (encephalitis)
- Bell’s palsy
- Brachial neuritis
- Nerve damage
- Bleeding disorder
- Low blood count
- Stevens-Johnson Syndrome
- Erythema multiforme
Guillain-Barré Syndrome is estimated to occur in about 1 in 1,000,000 people who get the flu shot. This rare disorder occurs when the body’s immune system attacks nerve cells. It can cause nerve damage, muscle weakness, and permanent paralysis.
No. Flu shots can’t cause a flu illness. The shot contains a dead (inactivated) virus. The nasal spray vaccine has a weak flu virus that causes a mild infection in the nose — not the lungs.