A Look at Each Vaccine: Influenza Vaccine

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.

  • 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, such as: 

    • 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. The most recent 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.

  • The vaccine

    Who should get the influenza vaccine?

    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 1 dose of influenza vaccine last year (2013-14 influenza season) or have not received two doses of influenza vaccine since July 2010
    • Have an uncertain influenza vaccination history

    How is the influenza vaccine made?

    All of the vaccines listed below contain either three or four of the influenza strains circulating in the community during a particular year.

    One 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.

    Another influenza vaccine is made by taking influenza viruses and weakening it in the laboratory so that it can’t cause disease but can still induce protective antibodies. This vaccine, which is a live, weakened form of the virus, is given as a nasal spray and is recommended for healthy people between 2 and 49 years of age. Like the killed vaccine mentioned above, this vaccine is also grown in eggs. 

    Two other influenza vaccines are also given as shots. One is made using recombinants DNA technology, and contains two proteins that reside on the surface of influenza virus. The other is made by growing influenza viruses in mammalian cells (not eggs), and killing it with formaldehyde. Both of these vaccines are advantageous for people who have egg allergies.

    The influenza vaccine is unusual in that most years a different vaccine is made. Because strains of influenza virus that circulate in the community can 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) determines what strains of influenza are circulating, and makes sure that all the influenza vaccines that are made that season contain viruses that would protect against the circulating strains. For this reason, the influenza vaccine is probably the hardest vaccine to make.

    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 in the shot is "inactivated," it cannot cause respiratory symptoms, such as congestion and cough, which are common with influenza infections; however, people who receive the nasal spray may experience mild congestion or runny nose.

    Although the influenza vaccine is made in eggs and some people are severely allergic to eggs, the quantity of egg proteins in the vaccine is insufficient to cause a severe allergic response. But just to be sure, people with severe egg allergies should remain at their provider's office for about 30 minutes after receiving the influenza vaccine.

  • Other questions you might have

    Why has the age of the children who are supposed to get influenza vaccine changed?

    The ages of children recommended to get influenza vaccine have varied in the past:

    • 2002: 6- to 23-month-olds
    • 2006: 6- to 59-month-olds
    • 2009: 6- month to 18-year-olds

    These changes were coupled with increasing recommendations for adults, ultimately leading to the recommendation in 2010 for everyone older than 6 months of age to receive an annual influenza vaccine. Because there was not a supply of influenza vaccine to recommend it for the entire population, the recommendations had to gradually increase the number of people that should get the vaccine beginning with the most susceptible populations. As demand for the vaccine grew, so too did the quantity produced. Because the vaccine is made in eggs, this increase had to take into account the availability of eggs ─ and the chickens that produce them of course!

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

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

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

    Only two of the viruses mentioned above (influenza and rotavirus) 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.

    Should our family get the three-strain influenza vaccine or the four-strain version?

    Although the four-strain version protects against an additional strain of influenza virus, it is more important to be vaccinated early enough in the season to have immunity when influenza arrives in your community. So, you should get whichever vaccine is available at your earliest convenience, keeping in mind that it takes about two weeks to develop immunity after vaccination. It is likely that in the future, only the four-strain version of influenza vaccines will be available.

    Can the nasal spray influenza vaccine cause encephalitis?

    Even during influenza infections, encephalitis is not a complication because influenza viruses are not able to reproduce in brain cells. In addition, the way the intranasal influenza vaccines works is that the virus reproduces a small number of times in the cooler temperatures of the nasal passages, but cannot reproduce in the warmer temperatures in the lungs, which is where influenza viruses typically reproduce and cause infection.

    It is not likely that the virus would enter the brain; however, because the virus can only grow at the cooler temperatures of the nose, in the same way that it cannot reproduce in the lungs, it would not be able to reproduce in the brain either. For the same reason, the nasal spray influenza vaccine also doesn’t cause pneumonia.

    Does the influenza vaccine cause Guillain-Barré syndrome?

    The influenza vaccine does not cause Guillain-Barré syndrome. In fact, in a study published as recently as July 2013, researchers found people hospitalized with GBS were not more likely to have recently received the influenza vaccine (in the six weeks before onset) compared to those who received it longer ago (at least nine months before disease onset).

    Every year, 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 as soon as it is available. Because it takes about two weeks after receiving the vaccine to be fully protected, it’s important to get the vaccine early. This is especially important for children and adults with higher risk of suffering complications from influenza.

    People at increased risk of experiencing complications include 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 three or four influenza viruses expected to circulate during influenza season. However, two 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.

    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.
    Pregnant women should receive the injectable form of the vaccine (the shot) and not the nasal spray. Women who are breastfeeding may receive either version as long as they have no other conditions that preclude use of the nasal spray version.

  • 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 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
    • Mild congestion or runny nose (following nasal spray version)
  • Reference

    Plotkin SA, Orenstein W, and Offit PA. Inactivated influenza vaccines and influenza vaccine - live in Vaccines, 6th Edition. 2012, 257-311.

Reviewed by Paul A. Offit, MD on August 20, 2014

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.