skip to Main Content

Influenza (Flu)

Influenza (more commonly known as the flu) is a highly contagious viral illness that is spread person-to-person through sneezes and coughs. It usually is a seasonal illness (October – May in the U.S.). One of the reasons the flu is so contagious is that people can pass it on before they even know they are sick.

Symptoms from the flu range from mild to severe and include:

  • fever (although not everyone with flu will have a fever) or feeling feverish/chills
  • cough
  • sore throat
  • runny or stuffy nose
  • muscle or body aches
  • headaches
  • fatigue (tiredness)
  • vomiting and diarrhea (more common in children than adults)

Symptoms usually appear from one to four days after exposure to the virus, and they last five to seven days.

What is the difference between a cold and the flu?

Signs and Symptoms Cold Influenza (Flu)
Symptom onset Gradual Abrupt
Fever Rare Usual; lasts 3-4 days
Aches Slight Usual; often severe
Chills Uncommon Fairly common
Fatigue, weakness Sometimes Usual
Sneezing Common Sometimes
Chest discomfort, cough Mild to moderate; hacking cough Common; can be severe
Stuffy nose Common Sometimes
Sore throat Common Sometimes
Headache Rare Common

On average, 140,000 to 960,000 people are hospitalized each year due to the flu. Between 12,000 – 79,000 people die from flu complications each year, depending on how severe flu season it is.

Pneumonia is a serious complication of the flu. Other serious risks include inflammation of the:

  • heart (myocarditis)
  • brain (encephalitis)
  • muscle (myositis, rhabdomyolysis) tissues

Multi-organ failure (for example, respiratory and kidney failure) and sepsis (when a body has a serious and life-threatening response to infection) can also occur.

The flu can be serious for anyone, but those most at risk for serious complications are:

  • children under 5 (particularly those under 2)
  • pregnant people
  • people over 65 years of age
  • people with certain chronic medical conditions (such as asthma, diabetes, or heart disease)

Emergency warning signs for children:

  • Fast breathing or trouble breathing
  • Bluish lips or face
  • Ribs pulling in with each breath
  • Chest pain
  • Severe muscle pain (child refuses to walk)
  • Dehydration (no urine for 8 hours, dry mouth, no tears when crying)
  • Not alert or interacting when awake
  • Seizures
  • Fever above 104°F
  • Any fever in children younger than 12 weeks
  • Fever or cough that improved but then returns or worsens
  • Worsening of chronic medical conditions

If your child has any of the above symptoms, get them medical care immediately.

The flu vaccine’s effectiveness varies from year to year because the flu virus mutates (changes) over time, so last year’s vaccine won’t be fully effective against this year’s flu. It takes time to produce enough vaccines to protect the United States. For that reason, the annual flu vaccine is developed BEFORE the flu arrives in the United States. Scientists look at the Southern hemisphere during its flu season (April – September) for clues on an effective vaccine. Some years’ vaccines are a better match than others to the flu virus that ends up circulating in the U.S.

Estimates are that, on average, the flu vaccine reduces the risk of getting flu symptoms from 40 – 60%. That may not seem like much, and scientists are working on improving the effectiveness. But even if someone does get the flu after receiving the flu vaccine, their symptoms are likely to be milder than if they had not received the vaccine.

Another factor in the flu vaccine’s effectiveness is the age and health of the person receiving the vaccine. Older individuals and those with weakened immune systems may not produce as strong an immune response. Given that older people are at high risk for complications due to flu, they should always get vaccinated and should opt for the higher dose vaccine to help their immune systems mount a defense against influenza.

The flu vaccine is very safe. Like any vaccine or medication, there can be side effects, but they are usually mild and go away on their own in a day or two. The most common side effect is soreness in the arm where the vaccination is received.

You cannot get the flu from the flu vaccine.

We’ve all heard someone say, “I got the flu vaccine last year, and then I got the flu from it.” The most common vaccine only contains inactivated (killed) flu virus so it cannot infect you and give you the flu.

Even the flu-mist, which contains a weakened live virus, can’t give you the flu – it’s too weak to give you the flu.

There is a newer, third kind of flu vaccine called a recombinant vaccine. This vaccine only contains small pieces of the virus that can’t infect you to help your body recognize the virus. 

So why did you feel “flu-ish” after getting the flu vaccine? One possible explanation might simply be that the vaccine is working! Mild vaccine reactions are normal and signs the body is mounting an immune response to protect against the flu.

Also, it’s important to remember that it takes up to two weeks for the vaccine to work, so it’s important to get the flu shot as soon as you can before you are exposed to the flu to get as much protection from the vaccine that you can!

Your child (and everyone in your family) should get a flu vaccine every fall, ideally before October. It can take up to two weeks for the flu shot to offer the best protection, so get it as early as possible.

Babies older than 6 months old can get the flu vaccine. Children younger than 8 may need 2 doses for the best protection. Pregnant people should receive the vaccine to help protect themselves and their newborn babies.

Talk to your healthcare provider about what vaccines your family needs. To find a vaccine, visit Vaccine Finder.

Time to get a flu shot? Download our Influenza vaccine fact sheet to prepare for your appointment.

Learn more

Want more information?

How do I know this information is credible?

We work for parents so we make sure that parent concerns are addressed using facts and science and our content is reviewed by experts who have spent their careers studying vaccines. Learn more about how we ensure we are bringing you the best information to help you make healthy choices for your family.

Every year the flu causes: Tens of thousands of deaths $10.4 billion in direct medical expenses $15.3 billion in lost wages million sick days The flu vaccine prevents an average of 5.3 million flu cases each year 2.6 million flu-related medical visits each year 40% reduction in risk for pregnant women Numbers don’t lie – the flu vaccine saves lives and money. Make sure everyone in your family gets their flu shot. Every year.
Back To Top