Merck Manual

Please confirm that you are not located inside the Russian Federation

honeypot link

Influenza Vaccine


Margot L. Savoy

, MD, MPH, Lewis Katz School of Medicine at Temple University

Reviewed/Revised Jul 2023

The influenza virus vaccine helps protect against influenza Influenza (Flu) Influenza (flu) is a viral infection of the lungs and airways with one of the influenza viruses. It causes a fever, runny nose, sore throat, cough, headache, muscle aches (myalgias), and a general... read more . Two types of influenza virus, type A and type B, regularly cause seasonal epidemics of influenza in the United States. There are many different strains within each type. The strains of virus that cause influenza outbreaks change each year. Thus, a new vaccine is needed each year. Each year's vaccine is directed against the 3 or 4 strains that scientists predict will be most common in the coming year.

Influenza can be mild, causing fever, aches, and fatigue, but it can be serious. Influenza can cause severe pneumonia, worsening of chronic heart and lung disorders, organ failure, and death. The number of deaths caused by influenza varies greatly from year to year, with about 20,000 to 50,000 deaths annually in the United States (see Centers for Disease Control and Prevention's [CDC] impact of influenza in past years). Occasionally, severe outbreaks, called pandemics, cause even more deaths, especially among young people. In 1918, influenza killed millions of people worldwide.

Administration of Influenza Vaccine

The influenza vaccine is recommended for

  • All people 6 months old and over

Influenza vaccine is usually given as an injection of inactivated virus into the muscle. It is also available as a nasal spray, which contains live but weakened (attenuated) virus.

Influenza epidemics usually begin in late December or midwinter. Therefore, the best time to get the vaccine is in September through November. A vaccine against avian influenza (bird flu Bird Flu Bird flu is a viral infection with strains of influenza virus that normally occur in wild birds and domestic poultry. The bird flu virus rarely spreads from animals to people (spread to people... read more ) has been developed in case that virus becomes able to spread from person to person.

An influenza vaccine that has a higher dose of inactivated virus is recommended for people 65 years old and over. It is given as an injection.

The main reason for not giving either influenza vaccine to someone is

Certain other conditions may affect whether and when people are vaccinated (see also CDC: Who Should NOT Get Vaccinated With These Vaccines?). For example, the nasal spray vaccine, which contains weakened live virus, is not given to the following:

If people have a temporary illness, doctors usually wait to give the vaccine until the illness resolves.

The influenza vaccine may contain small amounts of egg because many of the formulations are made from viruses grown in eggs. Recommendations for giving the vaccine to people with an egg allergy vary according to the severity of the allergy:

Side Effects of Influenza Vaccine

Occasionally, the injection site becomes sore. Fever and muscle aches occur occasionally.

Whether the influenza vaccine increases the risk of developing Guillain-Barré syndrome Guillain-Barré Syndrome (GBS) Guillain-Barré syndrome is a form of polyneuropathy causing muscle weakness, which usually worsens over a few days to weeks, then slowly improves or returns to normal on its own. With treatment... read more , a progressive nerve disorder, is unclear. However, if this rare syndrome develops within 6 months after influenza vaccination, people should talk to their doctor about whether future vaccinations are advisable.

The live-virus nasal spray vaccine sometimes causes a runny nose, sore throat, and mild wheezing.

More Information

The following English-language resources may be useful. Please note that THE MANUAL is not responsible for the content of these resources.

Drugs Mentioned In This Article

Generic Name Select Brand Names
Anacin Adult Low Strength, Aspergum, Aspir-Low, Aspirtab , Aspir-Trin , Bayer Advanced Aspirin, Bayer Aspirin, Bayer Aspirin Extra Strength, Bayer Aspirin Plus, Bayer Aspirin Regimen, Bayer Children's Aspirin, Bayer Extra Strength, Bayer Extra Strength Plus, Bayer Genuine Aspirin, Bayer Low Dose Aspirin Regimen, Bayer Womens Aspirin , BeneHealth Aspirin, Bufferin, Bufferin Extra Strength, Bufferin Low Dose, DURLAZA, Easprin , Ecotrin, Ecotrin Low Strength, Genacote, Halfprin, MiniPrin, St. Joseph Adult Low Strength, St. Joseph Aspirin, VAZALORE, Zero Order Release Aspirin, ZORprin
Adrenaclick, Adrenalin, Auvi-Q, Epifrin, EpiPen, Epipen Jr , Primatene Mist, SYMJEPI, Twinject
quiz link

Test your knowledge

Take a Quiz!