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Influenza (or flu) is a respiratory virus that affects the throat, nose, bronchi and, sometimes, the lungs. There are different types of influenza viruses, and they evolve and change from year to year. For most people, the flu is an inconvenience that subsides in a few days.

The seasonal flu vaccine typically changes each year. Each seasonal vaccine is generally designed to protect against three strains of influenza: two “A” strains and one “B” strain. The development process for the seasonal flu vaccine can take up to eight months.


Influenza surveillance centers, comprised of four World Health Organization (WHO) Collaborating Centers and 112 institutions in 83 countries, monitor the circulating influenza strains for trends year-round. Genetic data is collected, and new mutations are identified. The WHO is then responsible for selecting three strains most likely to genetically resemble strains circulating in the coming winter flu season. Sometimes, one of the strains used in the previous year’s vaccine may be chosen again if that strain continues to circulate.


Four to five months after the three vaccine strains have been identified, they are separately tested for purity and potency. Only after individual testing is completed are the three strains combined into a single seasonal vaccine.


An additional vaccine may be created to protect against a particularly virulent or widespread strain of influenza. The need for a 2009 H1N1 influenza vaccine became apparent after the strains for the seasonal flu vaccine were selected; therefore, a separate vaccine was created.


A quadrivalent inactivated influenza vaccine (works by stimulating an immune response against four different antigens or viruses) was licensed in the United States in 2012. A quadrivalent live virus nasal spray vaccine was licensed in 2013. These formulations include two influenza B strains in addition to the A strains. These vaccines began to be available, along with trivalent vaccines, in the 2013-14 influenza season.


The flu shot is made from an inactivated virus that can't transmit infection. So, people who get sick after receiving a flu vaccination were going to get sick anyway. It takes a week or two to get protection from the vaccine.

Influenza can cause mild to severe illness, and at times can lead to death. Flu is different from a cold. Flu usually comes on suddenly. People who have flu may feel some or all of these symptoms:

  • fever* or feeling feverish/chills

  • cough

  • sore throat

  • runny or stuffy nose

  • muscle or body aches

  • headaches

  • fatigue (tiredness)

  • some may have vomiting and diarrhea, though this is more common in children than adults

    *It’s important to note that not everyone with flu will have a fever.


The most obvious benefit of having a flu vaccine is prevention from being infected with the flu. But a flu vaccine also helps keep the disease from spreading to other people. Once you have the flu virus, it can take a few days before symptoms begin. You can spread the infection to others during that time, including family members and people at high risk for complications and death.


Annual flu vaccines offer benefits to those at risk for flu complications. People with asthma, COPD, or heart disease, for example, are all more likely to become seriously ill, which could result in hospitalization or even death. If you’re pregnant, having a flu shot can protect you and your baby for several months after birth. 


For all persons aged 6 months or older, annual flu vaccination is recommended, with rare exceptions. 

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