A Look at Each Vaccine: Influenza Vaccine

VEC Influenza infographic thumbnail To view the full-size infographic, click this photo or the link in the text. Every year in the United States, influenza kills thousands to tens of thousands of people. Probably the best example of how devastating influenza can be was the influenza pandemic in 1918 — this worldwide outbreak killed between 50 and 100 million people in a single influenza season.

For information about influenza vaccine and people with egg allergies, see “Does the influenza vaccine have side effects?” in "The Vaccine" section.

Click here to view a full-size image of the infographic [PDF, 115KB], or read the accompanying article about influenza vaccine ingredients.

The disease

What is influenza?

Commonly known as the flu, influenza is a virus that infects the trachea (windpipe) or bronchi (breathing tubes). Symptoms come on suddenly and include high fever, chills, severe muscle aches and headache. The onset of shaking chills is often so dramatic that many people will remember the exact hour that it started. The virus also causes runny nose and a cough that can last for weeks.

Complications of influenza include severe, and occasionally fatal, pneumonia.

Animals can be infected with influenza

Some diseases are only found in humans; however, influenza can infect many types of animals, specifically: 

  • Birds and poultry, such as chickens and turkeys
  • Aquatic birds, such as ducks
  • Pigs
  • Horses
  • Dogs
  • Cats
  • Sea mammals, such as seals and whales

When animals are infected with different strains of influenza at the same time, a new type can emerge. If the new type can infect humans, be easily passed from one person to another and causes illness, a pandemic, or worldwide epidemic can occur.

What is the difference between an epidemic and a pandemic?

Influenza epidemics occur every year. An epidemic does not affect all people because many people have at least some immunity. On the other hand, when new strains emerge, people do not have immunity, and, therefore, almost everyone is susceptible. 

Flu pandemics occur about three times every century. Pandemics have occurred in 1889, 1900, 1918, 1957, 1968, and 2009. The pandemics in 1957 and 1968 each claimed four to six million lives, but the pandemic in 1918 was the most devastating. Between 50 and 100 million people died from the strain of influenza known as "Spanish flu" during that pandemic.

Watch this video clip related to making a vaccine in advance of the arrival of influenza during the fall 1968 pandemic. The clip is from the award-winning documentary, Hilleman: A Perilous Quest to Save the World’s Children.

The vaccine

Who should get the influenza vaccine and when?

The Centers for Disease Control and Prevention (CDC) recommends that everyone 6 months of age and older receive the influenza vaccine each year.

Children 6 months to 8 years of age require two doses of influenza vaccine separated by four weeks if they:

  • Have never received an influenza vaccine
  • Have not received at least two doses of influenza vaccine before July 1, 2021 (during any influenza season)
  • Have an uncertain influenza vaccination history

Most people should receive the influenza vaccine in the fall, preferably September or October. A couple of groups are recommended to get the vaccine if it is ready earlier (July or August):

  • Infants and children getting their first influenza vaccine can get vaccinated earlier since they need two doses separated by a month. This will increase the likelihood that they will be protected before influenza virus starts circulating.
  • Pregnant people in their third trimester.

Children of any age who require a single dose can also be vaccinated in the summer, particularly if there is concern that they will not be vaccinated later in the season.

For people who do not receive the vaccine in the fall, they can still get vaccinated later in the season because the peak incidence of influenza can often occur as late as February or March.

Watch this video in which Dr. Paul Offit answers why and when to get the influenza vaccine.

A note about influenza vaccine and GBS: Most people with a history of GBS can be protected against influenza through vaccination. The only people with a history of GBS that typically prevents them from getting influenza vaccine are those whose GBS was diagnosed within six weeks of receipt of a previous influenza vaccine. In some cases, even these individuals may opt for vaccination after consulting with their healthcare provider. For example, they may have other conditions that increase their risk for complications if infected with influenza. In these situations, the patient and their healthcare provider should evaluate the potential risks and benefits to decide whether they should get vaccinated. For more information about GBS and vaccines, see this page of our website.

How is the influenza vaccine made?

All of the vaccines listed below contain four influenza strains that circulated in the last year, either in the U.S. or in the Southern hemisphere.

A few different types of influenza vaccine are available:

  • One type of influenza vaccine is made by growing influenza viruses in hen's eggs, purifying it, and completely killing it with a chemical (formaldehyde). Historically, this influenza shot has been administered into the muscle. However, a newer version of the vaccine, available for adults between 18 and 64 years old, is administered into the skin. Since the newer version uses a much smaller needle, it may be preferred for adults who are apprehensive about getting needles.
  • One is made using recombinant DNA technology and contains one protein that resides on the surface of influenza virus, called hemagglutinin. It is given as a shot.
  • One is made by growing influenza viruses in mammalian cells (canine kidney cells), instead of eggs, and killing them with formaldehyde.
  • The intranasal version of the influenza vaccine contains live weakened versions of four strains of influenza virus and can be given to people between 2 and 49 years of age.

The influenza vaccine is unusual in that most years a different vaccine is made. Because strains of influenza virus that circulate in the community often differ from one season to the next, the vaccine must change to best protect against those different strains. Every year in the United States, the Centers for Disease Control and Prevention (CDC) and the Food and Drug Administration (FDA) determine which strains of influenza are circulating, and make sure that all the influenza vaccines that are made that season contain viruses that should protect against the circulating strains. For this reason, the influenza vaccine is probably the hardest vaccine to make.

Learn more about why the influenza vaccine changes with each season by watching this short video part of the series Talking About Vaccines with Dr. Paul Offit.

Watch this video in which Dr. Offit answers, “Why Can't We Make a Better Flu Vaccine?”.

Does the influenza vaccine have side effects?

Side effects from the influenza vaccine are extremely rare. Fever or muscle aches generally occur in those who never had influenza or the influenza vaccine before. These symptoms do not mean that you have "the flu." Because the vaccine virus is "inactivated" in all but the nasal spray version, it cannot cause respiratory symptoms, such as congestion and cough, which are common with influenza infections.

The nasal spray version contains live weakened influenza viruses that can reproduce in the nose, but not the lungs. As such, it can cause mild symptoms, such as runny nose and low-grade fever.

Although most versions of the influenza vaccine are made in eggs and some people are severely allergic to eggs, the quantity of egg proteins in the influenza vaccine is insufficient to cause a severe allergic response. For this reason, all versions of the influenza vaccine can be given to people with egg allergies. As is recommended with other vaccines, people should remain at their provider's office for about 15-30 minutes after receiving the influenza vaccine.

Learn more about the concern that the influenza vaccine can cause influenza in this short video, part of the series Talking About Vaccines with Dr. Paul Offit.

Other questions you might have

What is the difference between winter viruses, colds and "the flu?"

With winter and the holidays comes cold and flu season. We see different people, more people, and tend to stay in warm, confined places. All of these things can lead to more sicknesses. Often when people become ill during the winter, they assume they have "the flu." While symptoms for some of these illnesses are "flu-like," the cause may not be influenza. Here is a list of common winter symptoms and the viruses that can cause them:

Respiratory symptoms (congestion, runny nose, sneezing, coughing, sore throat) can be caused by:

  • Influenza virus
  • Parainfluenza virus
  • Respiratory syncytial virus (RSV)
  • Rhinoviruses ("common cold" - over 100 different types)
  • Adenoviruses
  • Metapneumoviruses
  • Human coronaviruses

Gastrointestinal symptoms (vomiting, nausea, diarrhea) can be caused by:

  • Rotavirus
  • Norovirus
  • Enteroviruses (may also cause respiratory symptoms)
  • Adenoviruses
  • Caliciviruses
  • Astroviruses

Only three of the viruses mentioned above (influenza, rotavirus, and one version of coronavirus (SARS-CoV-2, which causes COVID-19)) are currently preventable by immunization. Washing your hands frequently and thoroughly, getting enough rest, staying home when ill, coughing into your elbow, and eating a well-balanced diet may decrease your chances of getting sick or passing an illness to others.

And remember, if you think you have "the flu," you may not actually have influenza.

In this short video, Dr. Lori Handy explains the difference between the common cold and influenza.

Does the influenza vaccine cause Guillain-Barré syndrome?

The data have been mixed regarding whether influenza vaccine can cause Guillain-Barré syndrome (GBS); however, it is now generally agreed that very rarely, people can get GBS after receiving the influenza vaccine. However, people can also experience GBS after viral infections, including influenza, and the rates of GBS are about 17 times higher after infection than vaccination. For more information about GBS, see the September 2021 Parents PACK Feature Article, “Guillain-Barré Syndrome (GBS) & Vaccines: The Risks and Recommendations.”

Every year, between 75 and 150 previously healthy children die after becoming infected with influenza; therefore, the benefits of getting the influenza vaccine outweigh the risks.

When is the best time to get the influenza vaccine?

Influenza season can start as early as the fall, so the American Academy of Pediatrics (AAP) and the Centers for Disease Control and Prevention (CDC) recommend receiving the influenza vaccine in September or October. Because it takes about two weeks after receiving the vaccine to be fully protected, it’s important to get the vaccine early during the fall. Only a few groups of people should be offered the influenza vaccine in August if it is available. These include:

  • Women who are in their third trimester, so their baby can benefit from maternal antibodies before it is old enough to get vaccinated.
  • Young children, particularly those who require two doses.

Some people are at increased risk of experiencing complications and as such, it is particularly important for them to get the influenza vaccine, including young children, pregnant women, adults 65 years of age and older, and individuals with underlying medical conditions, such as chronic heart, lung and kidney conditions.

Why do people still get the flu when they have had the influenza vaccine?

Influenza vaccine protects against the four influenza virus strains expected to circulate during influenza season. However, other factors should be considered.

First, some years the vaccine strains are not as good of a match with the circulating strains as we would like. In these cases, people who were vaccinated may still get influenza. For most of them, their disease will be less severe and shorter than if they had not been vaccinated.

Second, during the period when influenza is common, many other viruses are also circulating. Often these viruses cause colds and other illnesses that cause flu-like symptoms, such as congestion, runny nose, sneezing, coughing, sore throat, vomiting, nausea and diarrhea. Sometimes people with these symptoms will attribute them to influenza even though the virus is not to blame for the illness.

Third, because it takes about two weeks to be protected after getting vaccinated, some people may get the flu before the vaccine took effect.

Should I get the flu vaccine if I’m pregnant?

Yes. Pregnant women infected with influenza virus are more likely to suffer severe illness and complications that require hospitalization than women of the same age who are not pregnant. Physical changes including increased blood volume and added stress on the heart and lungs make influenza infections particularly dangerous during pregnancy. Pregnant women infected with influenza are also at increased risk of premature labor.

Getting an influenza vaccine during pregnancy also provides a woman’s newborn with protection for up to six months after birth when he or she is too young to get an influenza vaccine.

What should I do if influenza enters our home?

While the influenza vaccine is not perfect, it’s important to remember that someone who received a flu vaccine and still gets the flu is likely to have a shorter or less severe illness compared with an unvaccinated person.

Unfortunately, because of low vaccination rates and the imperfect influenza vaccine, vaccinated people might still get the flu. So, what should you know if influenza enters your house?

What to consider at home

Most people with the flu, particularly those who got an influenza vaccination, will have a mild illness and not require medical care or antiviral drugs. If you think you have the flu:

  • Get plenty of rest.
  • Drink plenty of fluids.
  • Return to work or school only after being fever-free for at least 24 hours.
  • Stay home — Not only does this help with rest, but it also aids in decreasing the spread of influenza.

However, some people are at greater risk for complications and hospitalization when infected with influenza, including:

  • Children younger than 5 years old — especially those younger than 2 years old.
  • Adults older than 65 years of age.
  • Children or adults with underlying or chronic conditions of the heart, lungs, endocrine system, kidneys, liver or metabolic system. The CDC offers a comprehensive list of high-risk factors for influenza.
  • Children with household members who have compromised immune systems, such as family members receiving chemotherapy for cancer.
  • Pregnant women.

Those at increased risk should seek medical attention if they suspect infection with influenza. Early medical attention may provide an opportunity for treatment with antiviral medications or other specific advice depending on the individual’s situation.

Even though you may be caring for someone with influenza at home, it is important to know and recognize the emergency warning signs of influenza illness regardless of risk factors or vaccine status.


  • Being unable to eat
  • Trouble breathing
  • No tears when crying (a sign of dehydration)
  • Significantly fewer wet diapers than normal (another sign of dehydration)


  • Fast breathing or trouble breathing
  • Bluish skin color
  • Not drinking enough fluids
  • Not waking up or not interacting
  • Being so irritable that they don’t want to be held
  • Flu-like symptoms improve but then return with fever and worse cough
  • Fever with a rash


  • Difficulty breathing or shortness of breath
  • Pain or pressure in the chest or abdomen
  • Sudden dizziness
  • Confusion
  • Severe or persistent vomiting
  • Flu-like symptoms that improve but then return with fever and worse cough

What is available if you seek medical care

If you seek medical attention for influenza infection, a first step is usually confirming the influenza infection. This is done with a “rapid flu test,” which detects genetic material of the virus from a nasal swab within 15 minutes.

The test helps healthcare providers determine whether to administer antiviral treatments. Generally, antivirals are most effective when taken within 48 hours of beginning symptoms. If administered properly, antivirals have the potential to:

  • Shorten the duration of influenza infection
  • Lessen the symptoms
  • Prevent those at high risk from developing serious, life-threatening complications

Does influenza infection cause people to be more vulnerable to other infections?

Viruses and bacteria sometimes adopt a “kick them while they’re down” approach, leveraging the weakened state of a person’s body to cause further infection. The pathogens that cause the second infection are often referred to as “opportunistic.”

One example is that when people have influenza, they are more susceptible to pneumococcal infections. Some other examples of opportunistic pathogens include:  

  • Group A streptococcus (GAS), more commonly known as “flesh-eating bacteria,” can infect sores caused by chickenpox infection.
  • Tuberculosis (TB) infections are one of the leading causes of death among people living with human immunodeficiency virus (HIV) because the bacteria that causes TB, Mycobacterium tuberculosis, invades the lungs of infected individuals.

Let’s face it, when you are lying in bed with “the flu,” your first thought isn’t that you could get sick with something else. However, the reality is that when influenza virus causes an infection, it often opens the door for a pneumococcal infection to follow. This is why older adults are particularly at risk of hospitalization or death following infection with influenza. While some will be hospitalized or die from pneumonia caused by influenza, most will have complications due to pneumococcal infection. If you or someone you know has ever had the flu, seemed to be getting better and then ended up even more ill a few days later, you have witnessed what can happen when pneumococcus infection follows infection with influenza.

Fortunately, influenza and pneumococcal vaccines are available to protect older adults from this one-two punch.

I remember that at one point the intranasal flu vaccine wasn't available. What happened?

Since 2003, when the nasal spray flu vaccine, known as FluMist, was introduced, many — especially those wanting to avoid an injection — preferred it. However, in June 2016, the Centers for Disease Control and Prevention (CDC) stopped recommending this version due to ineffectiveness.

Watch “Understanding What Happened with the Intranasal Flu Vaccine” in which Dr. Offit addresses why FluMist was taken off the market in 2016 and the changes that were made to make it more effective, so that it could be used again beginning 2018.

Relative risks and benefits

Do the benefits of the influenza vaccine outweigh the risks?

The influenza vaccine can cause mild side effects. On the other hand, influenza typically hospitalizes and kills more people in this country than any other vaccine-preventable disease — about 200,000 hospitalizations and thousands to tens of thousands of deaths occur every year. Therefore, the benefits of the influenza vaccine clearly outweigh its risks.

Disease risks

  • High fever and chills
  • Severe muscle aches
  • Headaches
  • Pneumonia
  • Runny nose and coughing for weeks
  • Disease can be fatal

Vaccine risks

  • Pain, redness and swelling at the injection site
  • Fever or muscle aches


Plotkin SA, Orenstein W, and Offit PA. Inactivated influenza vaccines and influenza vaccine - live in Vaccines, 7th Edition. 2018, 456-510.

Reviewed by Paul A. Offit, MD on October 21, 2021

Materials in this section are updated as new information and vaccines become available. The Vaccine Education Center staff regularly reviews materials for accuracy.

You should not consider the information in this site to be specific, professional medical advice for your personal health or for your family's personal health. You should not use it to replace any relationship with a physician or other qualified healthcare professional. For medical concerns, including decisions about vaccinations, medications and other treatments, you should always consult your physician or, in serious cases, seek immediate assistance from emergency personnel.