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Flu and Flu Vaccines

December 17, 2021

What is the flu?


Flu is an infectious respiratory disease, caused by influenza viruses A, B, C and D. A and B influenza viruses cause seasonal epidemics and are responsible for most of the disease burden in humans, C virus usually causes a mild disease and D virus primarily affects the cattle and doesn’t cause any diseases in humans. 


Influenza viruses A and B are further divided into clades, such as A(H1N1) and A(H3N2), and lineages, such as B(Victoria) and B(Yamagata). 


Flu may be mild to severe and typical symptoms include:

  • Fever and chills
  • Sore throat and cough
  • Runny or stuffy nose
  • Muscle or body aches
  • Headaches
  • Tiredness 
  • Sometimes vomiting or diarrhea, which are more common in children


Flu and common cold are not the same diseases. They are caused by different viruses – colds are caused by 10 different viruses, such as rhinoviruses, parainfluenza, and seasonal coronaviruses (different from SARS-CoV-2). Colds are usually milder than flu. Flu symptoms are typically more intense and begin more abruptly. Unlike flu, colds generally do not result in serious health problems, such as pneumonia, bacterial infections, or hospitalizations. 


Flu spreads through very small droplets made when infected people sneeze, cough or talk, or through contaminated surfaces.


Symptoms and seasonality


About 8% of the U.S. population gets sick from flu each season, ranging from 3% to 11%, depending on the season. Children below 18 are the most likely to get sick from flu and people above 65 are the least likely to get sick. However, anyone can get the flu, even very healthy people. Some groups are at high risk of complications (bacterial pneumonia, ear infections, sinus infections, worsening of chronic medical conditions, such as congestive heart failure, asthma, or diabetes) – 65 years and older, people of any age with certain chronic medical conditions (such as asthma, diabetes, or heart disease), pregnant women, and children younger than 5 years.


Influenza viruses circulate year-round, but most infections occur in fall and winter in the USA. The exact timing varies and is difficult to predict, but it usually peaks between December and February, although there were years when flu was still active in May. 


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Influenza - Prevention and Vaccines

Influenza - Prevention and Vaccines

Vaccines against flu


All flu vaccines available in the US are quadrivalent (they have 4 components), to prevent infection and illness caused by 2 types A (H1N1 and H3N2) and 2 types B viruses (B/Victoria and B/Yamagata). Getting a flu vaccine can protect against these viruses as well as additional flu viruses that are antigenically similar to the viruses used to make the vaccine. Seasonal flu vaccines do not protect against influenza C or D viruses or against zoonotic (animal-origin) flu viruses that can cause human infections, such as an avian flu. In addition, flu vaccines will not protect against infection caused by viruses causing the common cold.


Influenza viruses change through a process called an “antigenic drift”, meaning accumulation of small mutations over time. Mutations change the virus surface proteins, and they are not so easy to be recognized by our immune system, even if we had flu in the past. Therefore, we need to update flu vaccines every year, to better protect against the currently circulating viruses. 


Flu vaccines prevent up to 60% of disease caused by a flu virus and are the best protection against flu and its serious complications. Vaccines prevent serious disease, hospital admissions, ICU admissions and deaths in all age groups, but mostly in people with high risk of complications.


Flu vaccines can be given at the same time as COVID-19 vaccines.




Annual flu vaccination is recommended to everybody 6 months of age and older. Flu vaccination is especially important for people who are at higher risk of flu or complications from flu:

  • 65 years of age and older
  • Children younger than 2 years of age
  • Adults with comorbidities (e.g., asthma, neurological and neurodevelopmental disorders, blood disorders, chronic lung disease, diabetes, heart, kidney and liver diseases, obesity, stroke, diseases and treatments leading to a weakened immune system)
  • Pregnant people, up to 2 weeks after delivery. Flu vaccination in pregnancy also protects the child after birth for several months, when they are too young to get a flu vaccine.
  • People living in nursing homes
  • People from racial and ethnic minority groups (e.g., non-Hispanic Black persons, Hispanic or Latino persons, and American Indian or Alaska Native persons) because they have a higher probability of hospitalizations due to flu


There are very few people who should not get a flu vaccine: 

  • Children younger than 6 months
  • People with severe, life-threatening allergies to any ingredient in flu vaccines
  • Some people with a history of Guillain-Barre Syndrome, but they should talk to their your doctor


Most people with egg allergies can get any approved flu vaccine, as the severe allergic reactions are unlikely. However, if the egg allergy is very severe, the flu shot should be given in an inpatient or outpatient medical setting, under the supervision of a health care provider.


There are two flu vaccines, which are not manufactured in eggs:

  • Flublok Quadrivalent (licensed for people 18 years and older)
  • Flucelvax Quadrivalent (licensed for people 6 months and older)


It’s best to be vaccinated before flu begins spreading in your community, in September and October. Ideally, everyone should be vaccinated by the end of October. However, even a vaccination in November or later is still recommended because flu most commonly peaks in February and significant flu activity can continue into May.


Flu vaccines available in the US


There are many flu vaccines to choose from in the US, depending on age and other preferences. For example:

  • People who are afraid of needles can get other options, such as a nasal spray vaccine (Flumist) or a vaccine delivered by a jet injector, which uses a high-pressure, narrow stream of fluid to penetrate the skin instead of a needle (Afluria Quadrivalent). They are approved for most age groups, but there are some limitations of use, especially for the nasal spray vaccine.
  • There are 2 vaccines developed for people 65 year and older – Fluzone High Dose Quadrivalent and adjuvanted flu vaccine, Fluad Quadrivalent (adjuvant is an ingredient improving immune response).  Both are intended to give older people a better immune response to vaccination and therefore, a better protection against flu.  



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