The Flu Shot: What You Need to Know

By Ashley McClure-Wolfson, PharmD
PGY1 Community Resident
Walgreen Co. and University of Illinois at Chicago
The Flu Shot: What You Need to Know

What is the flu?

Influenza (the flu) is a respiratory illness, which means that it affects your airways.1,2 It is spread easily from person to person through coughing, sneezing, and even talking. When you get the influenza virus it sometimes takes one to four days before you start noticing symptoms. This means that you could spread the flu to those around you before you even know that you're sick. Adults are able to spread the flu for up to seven days after first becoming sick, and children can continue to spread the flu for even longer.

Is the flu dangerous?

The symptoms of the flu could range from mild to severe.2 If you get the flu, you may experience a sudden fever, cough, aches, and fatigue. People who are 65 years or older, those with certain health conditions (such as asthma, heart disease, or diabetes), pregnant women, and young children are considered high risk and are especially vulnerable to the flu and its complications. Every year, thousands of people die in the United States due to the flu and even more are hospitalized.3,4

The flu can have a big economic impact as well.3-5 The direct costs of the flu (such as hospital stays, doctor's office visits, and medications) are an estimated $4.6 billion annually.5 Approximately 17 million workdays are missed every year due to the flu, which costs an estimated $7 billion.

How do I protect my family and myself from the flu?

Getting a flu shot each year is the best way to protect against the flu.4 The CDC recommends that, unless otherwise told by your healthcare provider, everyone who is 6 months of age or older should get the flu vaccine every year.

How does the flu vaccine work?

There are several different kinds of flu vaccines available today. The flu shot is either made with "inactivated" viruses or no virus at all. The vaccines that do not contain the virus are called "recombinant" vaccines and only contain certain proteins found in the influenza virus known as subunits.4,6 The nasal flu vaccine is made with live viruses that have been rendered too weak to cause any illness. This type of vaccine is called a "live-attenuated" vaccine.

Regardless of which type of vaccine you get, it trains your body to create antibodies that protect you from the flu by attacking the virus.

How effective is the flu shot?

The vaccine formulation changes every year to try to "match" the influenza viruses that are spreading in our communities.4,7 The vaccine is more effective when it has a good match. Even if a flu shot was not well-matched, it is still beneficial to get vaccinated. If you do get the flu after having received the vaccine, it's possible that it may be less severe than if you had not gotten vaccinated. Getting vaccinated also helps to protect your community by promoting herd immunity.7,8

How well the vaccine works varies from year to year.4,7 Overall, receiving the flu vaccine usually reduces the risk of getting the flu by between 40% and 60% among most people when the vaccine is well-matched with the isolated virus. Some people respond better than others to the flu vaccine and build stronger immunity. People that are older than 65 years typically build weaker immune responses to the influenza vaccine.7 That's why there are special flu vaccines made especially for seniors.

What is herd immunity?

Another term for herd immunity is "community immunity."8 When enough people in a community are vaccinated and immunized against a particular virus, the virus won't be able to spread. Some people can't get vaccines and some people get vaccines but don't have strong immune responses. These people are especially vulnerable and rely on herd immunity to stay safe.

Every time you get vaccinated you are protecting yourself, your family, and your community.

When should I get the flu shot?

The flu season varies from year to year, but it has been known to start as early as October, peak in the winter months between December and February, and then may continue as late as May.4 It's recommended to get the flu vaccine as soon as the vaccine becomes available in order to be protected before the start of the flu season, It takes about two weeks after vaccination for your body to build up the antibodies against the flu. It's better late than never, though. Even if you don't get the flu vaccine until January or later, you can still benefit from it.

It's important to get the flu vaccine every year because both you and the flu virus change.4 The antibodies that you create to build immunity to the flu will decrease with time, so you need a new vaccine to renew your supply of antibodies. The flu viruses are also constantly changing, which is why there is a new formulation every year.

Where can I get a flu shot?

There are more places than ever to get your flu vaccination. Doctor's office, clinics, and health departments offer the flu shot but you may need to make an appointment in advance.

Pharmacies have become an easy and convenient option to get your flu shot. At Walgreens, we offer flu shots whenever a pharmacist is on duty. Walk-ins are welcome but if you'd prefer, you can make an appointment online. Find a location near you to ask about vaccine availability or schedule an appointment online.

I've heard there are different flu vaccines available. Which one is right for me?

There are several different flu vaccines to choose from, and your pharmacist or other healthcare provider would be happy to talk with you to help you find the right vaccines for you. The CDC recommends that all persons ages 6 months and older receive the flu shot. The CDC does not have a recommendation for one flu vaccine over another; the most important thing is that you get a flu vaccine every year.4,6

The flu vaccine can be split into two categories: trivalent or quadrivalent. The trivalent flu vaccines have two strains of the Influenza A virus (H1N1 and H3N2) and one strain of the Influenza B virus. The quadrivalent flu vaccines protect against the same three strains, but also have an additional B virus.4

The flu shots are injections that are administered into your upper arm. The flu shots do not contain a live virus and are either inactivated or recombinant (contain no influenza virus at all).

There is also a nasal spray. The nasal spray contains live-attenuated (weakened) influenza virus and is quadrivalent.4 The nasal spray flu vaccine may be an option for healthy persons between the ages of 2 and 49.6

Persons who are 65 years and older have additional choices. Older adults have the option of receiving either a high dose or adjuvanted flu vaccine. These flu shots are specifically formulated for seniors and in past studies they have demonstrated better protection against the flu in persons ages 65 and older.4,6,9

What do seniors (65 years and older) need to know about the flu vaccine?

Compared to younger adults, people who are 65 years and older have a greater risk of developing serious complications from the flu.9 It has been estimated that in recent years seniors have accounted for up to 70% of seasonal-flu related hospitalizations and up to 85% of seasonal flu-related deaths. Also alarming is that seniors typically develop weaker immune responses to the flu vaccine. That's why there are special flu vaccines available specifically designed for seniors: the high dose flu vaccine and the adjuvanted flu vaccine.

Compared to a regular flu shot, the high dose vaccine has four times the amount of antigen.9 The adjuvanted flu vaccine contains an extra substance called an adjuvant that helps the vaccine work better by giving you a stronger immune response. Both the high dose vaccine and the adjuvanted flu vaccine have demonstrated stronger immune responses in persons 65 years and older as compared to seniors who received the standard flu shot.

Can I get the flu shot if I'm pregnant?

Yes, a flu shot during pregnancy is recommended. In fact, it is recommended that all women who are pregnant or may become pregnant during the flu season receive a flu shot during any trimester.4,6 A flu shot protects both the mother and the baby from getting the flu, since the mother can pass some of the antibodies onto the baby. Any of the age-appropriate flu shots may be used. It is important that pregnant women only receive a flu shot; the nasal flu vaccine is not recommended during pregnancy.

What are the flu vaccine side effects?

Common flu shot reactions include redness, soreness, or swelling in the area where the shot was administered.4 The nasal spray could cause different side effects, such as runny nose, sore throat, or a cough. These side effects are usually mild and short-lived.

Some people may also develop a low-grade fever or body aches. Similar to the other side effects, these symptoms are usually mild and quickly go away on their own. While some of these side effects may feel flu-like, it's important to remember that the flu shot will not make you sick with the flu.

Allergic reactions to the flu shot are rare.4

Can you get the flu from the flu shot?

No, you cannot get the flu from the flu shot.4

The flu shot is either recombinant (does not contain the influenza virus) or it contains an inactivated virus.4,6 The nasal flu vaccine is made with live viruses but the virus is "attenuated," meaning it has been made too weak to cause any illness. Regardless of which type of vaccine you get, it trains your body to create antibodies that protect you from the flu by attacking the virus but it does not get you sick.

While some people may experience a low-grade fever or body aches as a side effect, that does not mean the vaccine gave them the flu.4 For example, if you got sick with the flu shortly after receiving a flu vaccine, it means you were exposed to the flu before your body had the chance to fully develop the antibodies needed. It takes about two weeks after receiving the vaccine before you're protected against the influenza virus.

Who should not get the flu vaccine?

If you have ever had a severe allergic reaction to any influenza vaccine you should not get another the flu vaccine again, even if it's a different formulation.6,10

If you have experienced the neurological disorder Guillain-Barré syndrome that causes paralysis within six weeks following a previous dose of the influenza vaccine, you should talk with your doctor before getting the flu vaccine.6,10,11

It's a good idea to be sure you're feeling well before getting the flu vaccine. If you are experiencing a moderate to severe illness (with or without a fever) it may be beneficial to wait to receive the flu vaccine until you are feeling better.6,10 For example, if you have the flu and you feel bad enough to miss work, then you should wait until you've improved, but if you only have a mild cold, you could still get the flu vaccine.

If you are interested in the influenza vaccine nasal spray, there are a few more things to know. Because it contains a live virus, you should not get the influenza nasal spray if you are pregnant, have a weakened immune system or are in close contact with persons who have a weakened immune system (such as those receiving chemotherapy), or have taken an influenza antiviral medication within the last 48 hours.6

Children and adolescents who receive aspirin or salicylate-containing medication should not receive the influenza nasal spray, nor should young children (between the ages of 2 and 4) who have experienced an episode of wheezing or who have been diagnosed with asthma. People with chronic health conditions such as COPD or diabetes should talk with their doctor before receiving the influenza nasal spray.

What if I have an egg allergy?

People that have experienced mild allergic reactions or hives because of eggs can receive any influenza vaccine.4,6,10 If eggs have given you serious allergic reactions, such as trouble breathing, you can still receive any of the available influenza vaccines but you should receive it in a doctor's office. Your doctor may recommend that you receive the egg-free influenza vaccine.

Is there mercury in the flu shot?

Vials of vaccine that contain more than one dose contain thimerosal, which is a mercury-based preservative.12 These multi-dose vials have a preservative because every time a needle enters the vial there is a risk of introducing bacteria or fungi, which could be dangerous. Thimerosal protects the vaccines from contamination. Our bodies remove thimerosal quickly and easily, so it does not stay or build up in our system. Most pre-filled syringes and nasal spray formulations of the flu vaccine do not contain thimerosal.

I need to get my pneumonia shot this year. Which one should I get first, the pneumonia shot or the flu shot?

Actually, you can receive both vaccines at the same time. The flu shot can be given at the same time with as any other vaccine you may need.

What are other ways I can protect against the flu?

In addition to your yearly flu vaccine, there are a few other things you can do to protect your family and you from the flu. There are daily precautions you can take such as limiting contact with sick people as much as possible and washing your hands with soap and water often.13 If you have received a prescription for flu antiviral medications, it's important to take them as directed by your doctor.

But remember, the number one way to protect you and your family against the flu virus is to get vaccinated every year.

Published on September 19, 2018


1. Influenza (Seasonal). World Health Organization. Published January 31, 2018.

2. How Flu Spreads. Centers for Disease Control and Prevention. Published October 5, 2017.

3. Disease Burden of Influenza. Centers for Disease Control and Prevention. Published May 22, 2018.

4. Key Facts about Seasonal Flu Vaccine. Centers for Disease Control and Prevention. Published October 30, 2017.

5. Influenza (Flu) in the Workplace. The National Institute for Occupational Safety and Health (NIOSH). Published February 20, 2018.

6. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices (ACIP)–United States, 2017-18 Summary of Recommendations. Centers for Disease Control and Prevention. Published January 26, 2018.

7. Vaccine Effectiveness - How Well Does the Flu Vaccine Work? Centers for Disease Control and Prevention. Published October 3, 2017.

8. U.S. Department of Health and Human Services. Vaccines Protect Your Community. Published October 11, 2006.

9. What You Should Know and Do this Flu Season If You Are 65 Years and Older. Centers for Disease Control and Prevention. Published April 11, 2018.

10. Vaccination: Who Should Do It, Who Should Not and Who Should Take Precautions. Centers for Disease Control and Prevention. Published October 3, 2017.

11. Guillain-Barré Syndrome Fact Sheet. National Institute of Neurological Disorders and Stroke.é-Syndrome-Fact-Sheet. Published June 2018.

12. Thimerosal in Flu Vaccine. Centers for Disease Control and Prevention. Published November 2, 2017.

13. CDC Features. Protect Yourself & Your Family Against Flu. Published January 22, 2018.

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