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  • Jessica Craig

Seasonal influenzas vaccine

Updated: May 16, 2023

By Jessica Craig

What is influenza (flu)?

Influenza, commonly called the flu, is a viral infection that attacks your respiratory system, including your nose, throat and lungs. For most people, the flu resolves on its own. But every so often influenza can be deadly.

Flu viruses are divided into four broad categories: influenza A, B, C or D. Influenza A is the most common type. H1N1 flu is a subtype of influenza A. Subtypes of influenza A are categorized based on two proteins on the surface of the virus, hemagglutinin (H) and neuraminidase (N).

Further, there are many H and N subtypes, and each one is numbered.

  • All H and N flu subtypes are influenza A viruses. Each influenza subtype has many different strains of influenza virus. However, not all these strains infect people.

  • The subtypes of influenza A viruses currently found in people are the strains of H1N1 (One hemagglutininprotein and one neuraminidase protein) and H3N2 (three hemagglutinin protein and two neuraminidase protein).

  • Variations of these two antigens produce the waves of influenza cases each year in people who have no immunity from prior exposure to new strains.[1]

Seasonal influenzas vaccine


In recent studies, the CDC estimates that, in a six-month period from October 1, 2019, through April 4, 2020, there have been:

  • 39 million to 56,000,000 million flu illnesses

  • 18 million to 26 million flu medical visits

  • 410,000 to 740,000 flu hospitalizations

  • 24,000 to 62,000 flu deaths.

Some people who get vaccinated may still get sick with the flu. However, in some studies, a flu vaccination has been shown to reduce the severity of the infection in people who have been vaccinated but still get sick. A 2021 study showed that among adults, vaccination was associated with a 26 percent lower risk of ICU admission and a 31 percent lower risk of death from flu, compared with those who were unvaccinated. An earlier 2017 study showed that a flu vaccination reduced deaths, intensive care unit (ICU) admissions, ICU length of stay and the overall duration of hospitalization among those adults admitted for care with flu.[2]

And, age is a factor in one’s susceptibility to flu, says the CDC: “In recent years, for example, it’s estimated that between 70 percent and 85 percent of seasonal flu-related deaths have occurred in people 65 years and older, and between 50 percent and 70 percent of seasonal flu-related hospitalizations have occurred among people in this age group.”[3]

Signs and symptoms of the flu

Initially, the flu may seem like a common cold – symptoms include runny nose, sneezing and sore throat. But colds typically develop slowly, the flu tends to come on suddenly. And, while a cold can be a hassle, flu symptoms are much more extreme and uncomfortable. These symptoms include fever, cough, shortness of breath, sore throat, eye pain, runny or stuffy nose, body aches, headaches, chills with sweats, and fatigue or weakness. Some individuals may experience vomiting and diarrhea as well, though this is more common in children than in adults.

It is possible for people to be infected with flu, suffer the respiratory symptoms, but never experience a fever.[3]

People with the virus are likely contagious from about a day before symptoms appear until about five days after they start. Children and people with weakened immune systems may be contagious for a slightly longer time.[4]

Why does the vaccine change every year?

TEach year's flu vaccine includes variations with both strains H1N1 and H3N2 of influenza A, as well as influenza B.[5]

The H1N1 and H3N2 subtypes of influenza A have been the major viruses spreading across the human population. Of the 144 possible antigenic combinations – any substance that causes the body to make an immune response against that substance is an antigen – the H1N1 and H3N2 subtypes are the only ones that have become accustomed to human hosts. When antigenic change happens, new hemagglutinin and neuraminidase antigens appear; the human population has no immunity to these. Says one M.D., “These large alterations in the viral surface of antigens occur every ten to thirty years, whereas smaller changes (antigenic drift) in the existing circulating subtypes appear every one to three years. Antigenic shift produces epidemics or pandemics, and antigenic drift results in outbreaks or less widespread epidemics.”[1]

How the vaccine works

Influenza vaccines are constructed to activate an immune response to hemagglutinin and neuraminidase, the two proteins found on the surface of the influenza virus. These proteins are always mutating, so every year, seasonal influenza vaccines must be redeveloped with the three strains that are likely to be most successful in combating new influenza strains.

The World Health Organization’s Global Influenza Program screens the influenza viruses spreading among humans worldwide and promptly detects new strains so that new, appropriate vaccines can be produced for a particular year. Most vaccinated persons develop post-vaccination hemagglutination inhibition antibody titers. These antibodies are protective against illness caused by strains like those in the

The live attenuated influenza vaccine (LAIV), which contains the same three influenza viruses as TIV, is administered via the nose. LAIV viruses are also grown in chicken egg. LAIV is preservative-free and is provided in a single-dose sprayer device with one-half of the dosage sprayed into each nostril. LAIV is not recommended to be used during some flu seasons, depending on the severity of the strain and the effectiveness of the vaccine.

Enhanced technology and modernization have allowed better methods of administering influenza vaccines, including a reduced-dose injectable made possible by adding substances to enhance the body’s immune response and the use of a cell culture vaccine. Scientists are also exploring new routes of administration, such as transcutaneous; a patch delivers the vaccine through micro-needles that can hardly pierce the skin before dissolving and releasing the vaccine.[2]

When you should get the vaccine

It’s best to be vaccinated before flu cases begins circulating in the community. September and October are commonly good times to be vaccinated against flu. In an ideal world, everyone should be vaccinated by the end of October. However, even if one is not able to get vaccinated until November or later, vaccination is still recommended, because flu most commonly peaks in February, and significant activity can continue into May.[3]

Who should get the vaccine?

Persons aged 65 and older are at greater risk of developing serious flu problems, compared to young, healthy adults. This heightened danger is caused to some extent by changes in a person’s immune defenses with growing age. While flu seasons fluctuate in seriousness, during most seasons people 65 years and older carry the greatest weight of severe flu disease. Though the annual influenza vaccine isn't 100 percent effective, it's still the best defense against the flu.[3]


  1. Hawley HB MD. Influenza. Magill’s Medical Guide (Online Edition). 2019. Accessed December 21, 2021.

  2. Center for Disease Control and Prevention. (2021). Key Facts About Seasonal Flu Vaccine. Influenza (flu). Retrieved from

  3. Center for Disease Control. (2021). Influenza. Retrieved from

  4. Mayo Clinic. (2021). Influenza (flu). Symptoms and causes. Retrieved from

  5. Tosh, P.K. (2021). What's the difference between H1N1 flu and influenza A? Retrieved from

  6. Oyelola OPD. S. Influenza vaccine. Salem Press Encyclopedia of Health. 2020. Accessed December 21, 2021..

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