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Influenza (Flu)
Influenza (Flu) Home
 1. Seasonal Flu
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KEY FACTS ABOUT SEASONAL FLU VACCINE

Key Facts About Flu Vaccines
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 * Flu Vaccination
 * Flu Vaccine Options
 * Vaccine Effectiveness
 * Vaccine Benefits
 * Vaccine Match
 * Vaccine Side Effects
 * Vaccine Supply and Distribution


FLU VACCINATION


WHY SHOULD PEOPLE GET VACCINATED AGAINST FLU?

Influenza (flu) is a potentially serious disease that can lead to
hospitalization and sometimes even death. Every flu season is different, and flu
can affect people differently, but during typical flu seasons, millions of
people get flu every year, hundreds of thousands of people are hospitalized and
thousands to tens of thousands of people die from flu-related causes. Flu can
mean a few days of feeling bad and missing work, school, or family events, or it
can result in more serious illness. Complications of flu can include bacterial
pneumonia, ear infections, sinus infections and worsening of chronic medical
conditions, such as congestive heart failure, asthma, or diabetes. An annual
seasonal flu vaccine is the best way to help reduce the risk of getting flu and
any of its potentially serious complications. Vaccination has been shown to have
many benefits including reducing the risk of flu illnesses, hospitalizations and
even the risk of flu-related death. While some people who get a flu vaccine may
still get sick with influenza, flu vaccination has been shown in several studies
to reduce severity of illness.


HOW DO FLU VACCINES WORK?

Flu vaccines cause antibodies to develop in the body about two weeks after
vaccination. These antibodies provide protection against flu illness.

Seasonal flu vaccines are designed to protect against the influenza viruses that
research indicates will be most common during the upcoming season. All flu
vaccines in the United States are “quadrivalent” vaccines, which means they
protect against four different flu viruses: an influenza A(H1N1) virus, an
influenza A(H3N2) virus, and two influenza B viruses.


FLU VACCINE OPTIONS

For people younger than 65 years, CDC does not preferentially recommend any
licensed, age-appropriate influenza (flu) vaccine over another during the
2022-2023 flu season. Options for this age group include inactivated influenza
vaccine [IIV], recombinant influenza vaccine [RIV], or live attenuated influenza
vaccine (LAIV), with no preference for any flu vaccine over another.

New for this season: For people 65 years and older, there are three flu vaccines
that are preferentially recommended over standard-dose, unadjuvanted flu
vaccines. These are Fluzone High-Dose Quadrivalent vaccine, Flublok Quadrivalent
recombinant flu vaccine and Fluad Quadrivalent adjuvanted flu vaccine. More
information is available at Flu & People 65 Years and Older.

All flu vaccines for the 2022-2023 season are quadrivalent vaccines, designed to
protect against four different flu viruses, including two influenza A viruses
and two influenza B viruses. Different vaccines are licensed for use in
different age groups, and some vaccines are not recommended for some groups of
people.

Available flu vaccines include:

 * Standard-dose flu shots that are manufactured using virus grown in eggs.
   Several different brands of standard dose flu shots are available, including
   Afluria Quadrivalent, Fluarix Quadrivalent, FluLaval Quadrivalent, and
   Fluzone Quadrivalent. These vaccines are approved for use in children as
   young as 6 months. Most flu shots are given in the arm (muscle) with a
   needle. Afluria Quadrivalent can be given either with a needle (for people 6
   months and older) or with a jet injector (for people 18 through 64 years
   only).
 * A cell-based flu shot (Flucelvax Quadrivalent) containing virus grown in cell
   culture, which is approved for people 6 months and older. This vaccine is
   completely egg-free.
 * A recombinant flu shot (Flublok Quadrivalent) which is a completely egg-free
   flu shot that is made using recombinant technology and is approved for use in
   people 18 years and older. This shot is made without flu viruses and contains
   three times the antigen (the part of the vaccine that helps your body build
   up protection against flu viruses) than other standard-dose inactivated flu
   vaccines, to help create a stronger immune response.
 * An egg-based high dose flu shot (Fluzone High-Dose Quadrivalent), which is
   approved for use in people 65 years and older. This vaccine contains four
   times the antigen (the part of the vaccine that helps your body build up
   protection against flu viruses) than other standard-dose inactivated flu
   vaccines, to help create a stronger immune response.
 * An egg-based adjuvanted flu shot (Fluad Quadrivalent), which is approved for
   people 65 years and older. This vaccine is made with an adjuvant (an
   ingredient that helps create a stronger immune response).
 * An egg-based live attenuated flu nasal spray vaccine (FluMist Quadrivalent)
   made with attenuated (weakened) live flu viruses, which is approved for use
   in people 2 years through 49 years. This vaccine is not recommended for use
   in pregnant people, immunocompromised people, or people with certain medical
   conditions.

ARE ANY OF THE AVAILABLE FLU VACCINES RECOMMENDED OVER OTHERS?

Yes, for some people. For the 2022-2023 flu season, there are three flu vaccines
that are preferentially recommended for people 65 years and older. These are
Fluzone High-Dose Quadrivalent vaccine, Flublok Quadrivalent recombinant  flu
vaccine and Fluad Quadrivalent adjuvanted flu vaccine. This recommendation was
based on a review of available studies which suggests that, in this age group,
these vaccines are potentially more effective than standard dose unadjuvanted
flu vaccines. There is no preferential recommendation for people younger than 65
years.

WHAT IF A PREFERENTIALLY RECOMMENDED FLU VACCINE IS NOT AVAILABLE?

If none of the three flu vaccines preferentially recommended for people 65 years
and older is available at the time of administration, people in this age group
should get any other age-appropriate flu vaccine instead.

WHO SHOULD BE VACCINATED?

Everyone 6 months and older in the United States should get an influenza (flu)
vaccine every season with rare exception. CDC’s Advisory Committee on
Immunization Practices has made this “universal” recommendation since the
2010-2011 flu season.

Vaccination to prevent flu and its potentially serious complications is
particularly important for people who are at higher risk of developing serious
flu complications. A full list of age and health factors that confer increased
risk is available at People at Higher Risk of Developing Flu-Related
Complications.

More information is available at Who Needs a Flu Vaccine.

WHO SHOULD NOT BE VACCINATED?

Different influenza (flu) vaccines are approved for use in people in different
age groups. In addition, some vaccines are not recommended for certain groups of
people. Factors that can determine a person’s suitability for vaccination, or
vaccination with a particular vaccine, include a person’s age, health (current
and past) and any allergies to flu vaccine or its components. More information
is available at Who Should and Who Should NOT get a Flu Vaccine.

WHEN SHOULD I GET VACCINATED?

For most people who need only one dose of flu vaccine for the season, September
and October are generally good times to be vaccinated against flu. Ideally,
everyone should be vaccinated by the end of October. Additional considerations
concerning the timing of vaccination for certain groups of people include:

 * Most adults, especially those 65 years and older, and pregnant people in the
   first or second trimester should generally not get vaccinated early (in July
   or August) because protection may decrease over time. However, early
   vaccination can be considered for any person who is unable to return at a
   later time to be vaccinated.
 * Some children need two doses of flu vaccine. For those children it is
   recommended to get the first dose as soon as vaccine is available, because
   the second dose needs to be given at least four weeks after the first.
   Vaccination during July and August also can be considered for children who
   need only one dose.
 * Vaccination during July and August also can be considered for people who are
   in the third trimester of pregnancy during those months, because this can
   help protect their infants for the first few months after birth (when they
   are too young to be vaccinated).

WHERE CAN I GET A FLU VACCINE?




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Flu vaccines are offered in many doctor’s offices and clinics. Even if you don’t
have a regular doctor or nurse, you can get a flu vaccine somewhere else like a
health department, pharmacy, urgent care clinic, college health center, and even
in some schools and workplaces.

WHY DO I NEED A FLU VACCINE EVERY YEAR?

A flu vaccine is needed every year for two reasons. First, a person’s immune
protection from vaccination declines over time, so an annual flu vaccine is
needed for optimal protection. Second, because flu viruses are constantly
changing, the composition of flu vaccines is reviewed annually, and vaccines are
updated to protect against the viruses that research indicates will be most
common during the upcoming flu season. For the best protection, everyone 6
months and older should get vaccinated annually.

DOES FLU VACCINE WORK RIGHT AWAY?

No. It takes about two weeks after vaccination for antibodies to develop in the
body and provide protection against influenza virus infection. That’s why it’s
best to get vaccinated before influenza viruses start to spread in your
community.

Top of Page


VACCINE EFFECTIVENESS

Influenza (flu) vaccine effectiveness (VE) can vary. The protection provided by
a flu vaccine varies from season to season and depends in part on the age and
health status of the person getting the vaccine and the similarity or “match”
between the viruses in the vaccine and those in circulation. During years when
the flu vaccine match is good, it is possible to measure substantial benefits
from flu vaccination in terms of preventing flu illness and complications.
However, the benefits of flu vaccination will still vary, depending on
characteristics of the person being vaccinated (for example, their health and
age), what flu viruses are circulating that season and, potentially, which type
of flu vaccine was used. More information is available at Vaccine Effectiveness
– How well does the Flu Vaccine Work.

CAN I GET SEASONAL FLU EVEN THOUGH I GOT A FLU VACCINE THIS YEAR?

Yes. It’s possible to get sick with flu even if you have been vaccinated
(although you won’t know for sure unless you get a flu test). This is possible
for the following reasons:

 * You may be exposed to a flu virus shortly before getting vaccinated or during
   the period that it takes the body to gain protection after getting
   vaccinated. This exposure may result in you becoming ill with flu before the
   vaccine begins to protect you. (Antibodies that provide protection develop in
   the body about 2 weeks after vaccination.)
 * You may be exposed to a flu virus that is not included in the seasonal flu
   vaccine. There are many different flu viruses that circulate every year. A
   flu vaccine is made to protect against the four flu viruses that research
   suggests will be most common.
 * Unfortunately, some people can become infected with a flu virus that the
   vaccine is designed to protect against, despite getting vaccinated.
   Protection provided by flu vaccination can vary widely, based in part on
   health and age factors of the person getting vaccinated. In general, a flu
   vaccine works best among healthy younger adults and older children. Some
   older people and some people with certain chronic illnesses may develop less
   immunity after vaccination. Flu vaccination is not a perfect tool, but it is
   the best way to protect against flu virus infection.

WHAT PROTECTION DOES A FLU VACCINE PROVIDE IF I DO GET SICK WITH FLU?

Some people who get vaccinated may still get sick with flu. However, flu
vaccination has been shown in studies to reduce severity of illness in people
who get vaccinated but still get sick. A 2021 study showed that among adults,
flu vaccination was associated with a 26% lower risk of ICU admission and a 31%
lower risk of death from flu compared with those who were unvaccinated. A 2017
study showed that flu vaccination reduced deaths, intensive care unit (ICU)
admissions, ICU length of stay, and overall duration of hospitalization among
hospitalized adults with flu.

Top of Page


VACCINE BENEFITS

WHAT ARE THE BENEFITS OF FLU VACCINATION?

Below is a summary of the benefits of flu vaccination and selected scientific
studies that support these benefits.

 * Flu vaccination can keep you from getting sick with flu.
   * Flu vaccine prevents millions of illnesses and flu-related doctor’s visits
     each year. For example, during 2019-2020, the last flu season prior to the
     COVID-19 pandemic, flu vaccination prevented an estimated 7.5 million
     influenza illnesses, 3.7 million influenza-associated medical visits,
     105,000 influenza-associated hospitalizations, and 6,300
     influenza-associated deaths.
   * During seasons when flu vaccine viruses are similar to circulating flu
     viruses, flu vaccine has been shown to reduce the risk of having to go to
     the doctor with flu by 40% to 60%.
 * Flu vaccination has been shown in several studies to reduce severity of
   illness in people who get vaccinated but still get sick.
   * A 2021 study showed that among adults hospitalized with flu, vaccinated
     patients had a 26% lower risk of intensive care unit (ICU) admission and a
     31% lower risk of death from flu compared with those who were unvaccinated.
   * A 2018 study showed that among adults hospitalized with flu, vaccinated
     patients were 59% less likely to be admitted to the ICU than those who had
     not been vaccinated. Among adults in the ICU with flu, vaccinated patients
     on average spent four fewer days in the hospital than those who were not
     vaccinated.
 * Flu vaccination can reduce the risk of flu-associated hospitalization.
   * Flu vaccine prevents tens of thousands of hospitalizations each year. For
     example, during 2019-2020 flu vaccination prevented an estimated 105,000
     flu-related hospitalizations.
   * A 2018 study showed that from 2012 to 2015, flu vaccination among adults
     reduced the risk of being admitted to an ICU with flu by 82%.
   * A 2017 study found that during 2009-2016, flu vaccines reduced the risk of
     flu-associated hospitalization among older adults by about 40% on average.
   * A 2014 study showed that flu vaccination reduced children’s risk of
     flu-related pediatric intensive care unit (PICU) admission by 74% during
     flu seasons from 2010-2012.
 * Flu vaccination is an important preventive tool for people with certain
   chronic health conditions.
   * Flu vaccination has been associated with lower rates of some cardiac
     events among people with heart disease, especially among those who have had
     a cardiac event in the past year.
   * Flu vaccination can reduce the risk of a flu-related worsening of chronic
     lung disease (for example, chronic obstructive pulmonary disease (COPD)
     requiring hospitalization).
   * Among people with diabetes and chronic lung disease,flu vaccination has
     been shown in separate studies to be associated with reduced
     hospitalizations from a worsening of their chronic condition.
 * Flu vaccination during pregnancy helps protect pregnant people from flu
   during and after pregnancy and helps protect their infants from flu in their
   first few months of life.
   * A 2013 study showed that during the 2010–2011 and 2011–2012 flu seasons
     vaccination reduced the risk of flu-associated acute respiratory infection
     in pregnant people by about one-half.
   * A 2018 study showed that getting a flu shot reduced a pregnant person’s
     risk of being hospitalized with flu by an average of 40% from 2010-2016.
   * A number of studies have shown that in addition to helping to protect
     pregnant people from flu, a flu vaccine given during pregnancy helps
     protect the baby from flu for several months after birth, when babies are
     too young to be vaccinated.
 * Flu vaccine can be lifesaving in children.
   * A 2022 study showed that flu vaccination reduced children’s risk of severe
     life-threatening influenza by 75%.
   * A 2020 study found that during the 2018-2019 flu season, flu vaccination
     reduced flu-related hospitalization by 41% and flu-related emergency
     department visits by half among children (aged 6 months to 17 years old).
   * A 2017 study was the first of its kind to show that flu vaccination can
     significantly reduce children’s risk of dying from flu.
 * Getting vaccinated yourself may also protect people around you, including
   those who are more vulnerable to serious flu illness, like babies and young
   children, older people, and people with certain chronic health conditions.

Despite the many benefits offered by flu vaccination, only about half of
Americans get an annual flu vaccine. During an average flu season, flu can cause
millions of illnesses, hundreds of thousands of hospitalizations and tens of
thousands of deaths. Many more people could be protected from flu if more people
got vaccinated.

*References for the studies listed above can be found at Publications on
Influenza Vaccine Benefits.

Top of Page


VACCINE MATCH

WHAT IS MEANT BY A “GOOD MATCH” BETWEEN VIRUSES IN THE VACCINE AND CIRCULATING
INFLUENZA VIRUSES?

A “good match” is said to occur when the flu vaccine viruses used to produce flu
vaccine and the viruses circulating among people during a given flu season are
“like” one another such that the antibodies induced by vaccination protect
against infection caused by circulating viruses.

WHAT IF CIRCULATING FLU VIRUSES ARE DIFFERENT FROM VACCINE VIRUSES?

During seasons when one or more of the circulating viruses are different or
“drifted” from the vaccine viruses, vaccine effectiveness against the drifted
viruses can be reduced. It’s important to remember that flu vaccine protects
against four different flu viruses and multiple different viruses usually
circulate during any one season. Even if the effectiveness of the vaccine is
reduced against one virus, vaccination can still be effective at preventing flu
illness caused by the other circulating viruses. For these reasons, CDC
continues to recommend flu vaccination for everyone 6 months and older even if
vaccine effectiveness against one or more viruses is reduced.

WHY IS THERE SOMETIMES NOT A GOOD MATCH BETWEEN A VACCINE VIRUS AND CIRCULATING
VIRUSES?

Flu viruses are constantly changing (called “antigenic drift”) – they can change
from one season to the next or they can even change within the course of one flu
season. Experts must pick which viruses to include in the vaccine many months in
advance in order for vaccine to be produced and delivered on time. (For more
information about the vaccine virus selection process visit Selecting the
Viruses in the Influenza (Flu) Vaccine.) Because of these factors, there is
always the possibility of a less than optimal match between circulating viruses
and the viruses used to produce vaccine.

The production process for some seasonal vaccines also may impact how well
vaccine works against certain viruses, especially influenza A(H3N2) viruses.
Growth in eggs is part of the production process for many seasonal flu vaccines.
While all influenza viruses undergo changes when they are grown in eggs, changes
in influenza A(H3N2) viruses are more likely to result in antigenic changes
compared with changes in other influenza viruses. These so-called “egg-adapted
changes” are present in most of the vaccine viruses recommended for use in
egg-based vaccine production and may reduce their effectiveness against
circulating influenza viruses. Advances in vaccine production technologies (for
example, cell-based and recombinant technology) and advanced molecular
techniques are being explored as ways to improve flu vaccine effectiveness.
Learn more by visiting, Advancements in Influenza Vaccines.

WILL THIS SEASON’S VACCINE BE A GOOD MATCH FOR CIRCULATING VIRUSES?

It’s not possible to predict with certainty if a flu vaccine will be like
circulating flu viruses because flu viruses are constantly changing. A flu
vaccine is made to protect against the flu viruses that research and
surveillance indicate will likely be most common during the upcoming season.
Over the course of the flu season, CDC studies samples of circulating flu
viruses to evaluate how close a match there is between viruses used to make the
flu vaccine and circulating flu viruses. More information about the 2022-2023
flu season and recommended vaccines is available.

Top of Page


VACCINE SIDE EFFECTS (WHAT TO EXPECT)

CAN A FLU VACCINE GIVE ME FLU?

No, a flu vaccine cannot cause flu illness. Flu vaccines that are given with a
needle (flu shots) are currently made in two ways: the vaccine is made either
with a) flu vaccine viruses that have been killed (inactivated) and are
therefore not infectious, or b) with proteins from a flu virus (which is the
case for recombinant influenza vaccine). Nasal spray vaccine is made with
weakened (attenuated) live flu viruses and also cannot cause flu illness. The
weakened viruses are cold-adapted, which means they are designed to only
reproduce at the cooler temperatures found within the nose. The viruses cannot
reproduce in the lungs or other areas where warmer temperatures exist.

WHAT SIDE EFFECTS CAN OCCUR AFTER GETTING A FLU VACCINE?

While a flu vaccine cannot give you flu illness, there are different side
effects that may be associated with getting a flu shot or a nasal spray flu
vaccine. These side effects are usually mild and short-lasting, especially when
compared to symptoms of flu.

A flu shot: The viruses in a flu shot are killed (inactivated), so you cannot
get flu from a flu shot. Some minor side effects that may occur are:

 * Soreness, redness, and/or swelling where the shot was given
 * Headache (low grade)
 * Fever
 * Muscle aches
 * Nausea
 * Fatigue

The nasal spray: The viruses in the nasal spray vaccine are weakened and do not
cause influenza illness. In children, side effects from the nasal spray may
include:

 * Runny nose
 * Wheezing
 * Headache
 * Vomiting
 * Muscle aches
 * Fever (low grade)

In adults, side effects from the nasal spray vaccine may include:

 * Runny nose
 * Headache
 * Sore throat
 * Cough

If these problems occur, they begin soon after vaccination and usually are mild
and short-lived. A flu shot, like other injections, can occasionally cause
fainting. Tell your provider if you feel dizzy or have vision changes or ringing
in the ears. As with any medicine, there is a very remote chance of a vaccine
causing a severe allergic reaction, other serious injury, or death. People who
think that they have been injured by a flu vaccine can file a report with the
Vaccine Adverse Event Reporting System (VAERS).

More information about the safety of flu vaccines is available at Influenza
(Flu) Vaccine Safety.

Top of Page


VACCINE SUPPLY AND DISTRIBUTION

HOW MUCH INFLUENZA VACCINE IS PROJECTED TO BE AVAILABLE FOR THE 2022-2023
INFLUENZA SEASON?

Flu vaccine is produced by private manufacturers, so supply depends on
manufacturers. Vaccine manufacturers have projected that they will supply the
United States with as many as 173.5 million to 183.5 million doses of influenza
vaccines for the 2022-2023 season. These projections may change as the season
progresses. All flu vaccines for the 2022-2023 season will be quadrivalent (four
component). Most will be thimerosal-free or thimerosal-reduced
vaccine (93%), and about 20% of flu vaccines will be egg-free.

WHERE CAN I FIND INFORMATION ABOUT VACCINE SUPPLY?

Information about vaccine supply is available on CDC’s Vaccine Supply &
Distribution.

Top of Page

SPECIAL CONSIDERATION REGARDING EGG ALLERGY

People with egg allergies can receive any licensed, recommended age-appropriate
influenza (flu) vaccine (IIV4, RIV4, ccIIV4, or LAIV4) that is otherwise
appropriate. People who have a history of severe egg allergy (those who have had
any symptom other than hives after exposure to egg) should be vaccinated in a
medical setting, supervised by a health care provider who is able to recognize
and manage severe allergic reactions. Two completely egg-free flu vaccine
options are available: Flublok Quadrivalent recombinant flu vaccine and
Flucelvax Quadrivalent cell-based flu shot.

Top of Page

Last Reviewed: December 19, 2022, 01:00 PM
Source: Centers for Disease Control and Prevention, National Center for
Immunization and Respiratory Diseases (NCIRD)
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