Definition and Overview

Influenza is a common infectious condition that can have serious, sometimes fatal, complications. Approximately 20% of the general population acquire influenza every year. Even previously healthy individuals can get this infection, and can spread the disease to other people, leading to unwanted hospitalizations and even death. At least 3,000 deaths every year are related to influenza; this number can reach more than 40,000 during really bad flu seasons. Majority of these deaths occur in the elderly, usually in those older than 65.

Getting an annual flu shot is a simple, effective way to reduce the risk of acquiring and transmitting the influenza virus. Individuals who receive influenza immunization are less likely to require medical treatment by a specialist or a health provider. Studies have shown that hospitalizations attributed to influenza are decreased by more than 70%. This significantly minimizes flu-related illness, hospitalizations, and death.

How Flu Vaccine Works

The flu vaccine protects against 3 or 4 strains of the disease, depending on which strains people are most likely to get infected with for the year, based on research. As with most vaccines, the flu vaccine exerts this protection by eliciting an immune response from the body, resulting in the production of antibodies against the influenza virus. It takes approximately two weeks from the time of immunization before the vaccine becomes effective. Vaccination should be performed as soon as the flu vaccine for the year becomes available, typically in October. It should still be offered even in the later months of the flu season, as long as there is still risk of contracting the virus.

Types of Available Flu Vaccine

There are several kinds of flu vaccines available. Flu vaccines may either be trivalent or quadrivalent. The trivalent vaccines act against three strains of the flu virus, which include two influenza A and one influenza B virus. The quadrivalent vaccines, meanwhile, act against four strains of the flu virus, which include two influenza A and two influenza B viruses.

The flu vaccine can be administered through several routes. The traditional flu shot is given intramuscularly, but some shots may also be administered into the skin, via the intradermal route. A high dose influenza vaccine may be given to elderly individuals, who are at higher risk of contracting the disease. Finally, one form of the quadrivalent vaccine may be given as a nasal spray, which is especially preferred for healthy children.

Why Annual Flu Shots are Important

The flu shot needs to be given once a year, in order to provide optimal protection to the individual. This is because the body's immunity to the virus decreases through time, and should thus be kept updated. Also, another reason for the annual vaccination is that the flu virus undergoes a phenomenon known as antigenic drift, wherein the virus changes constantly. Different types of flu viruses can become prominent one season and change the next. Every year, the formulation of the vaccine is evaluated and updated; hence, the need to get immunized each year.

The Centers for Disease Control and Prevention, or the CDC, recommends that all individuals who are older than six months should receive an annual influenza vaccine. It is especially recommended for individuals who are at risk of developing influenza and its complications. In particular, these include children less than 5 years old, adults older than 65 years old, those living in crowded areas and nursing homes, patients with chronic medical conditions, such as lung disease and kidney disease, and patients with potentially weak immune systems, such as those with HIV. Flu vaccination is also advised for people who are in close contact with individuals from these high-risk groups, such as caregivers, babysitters and health care workers. Flu vaccination can also protect pregnant women and their babies in the first few months of life.

Possible Side Effects

As with any other medication, you may experience some side effects after getting a flu shot. The usual side effects are minor, typically pain or slight redness on the vaccination site and mild fever. The nasal form of the vaccine can also cause headache and a runny nose. You will not get a flu from the influenza vaccine. It is also rare to experience serious complications from a flu vaccine. However, to be on the safe side, it is prudent to tell your doctor about any allergy or adverse reactions you've experienced in the past. Patients who have severe allergies, especially to eggs, may be given an egg-free vaccine to prevent allergic reactions. It is likewise important to tell your doctor if you've had Guillaine-Barre syndrome, a neurologic disease, in the past.

You can get immunized with the flu vaccine in various locations, including your local doctor's clinic, health centers, hospitals and primary care facilities, and even in schools. There are programs online, such as those in the CDC website, that can point you to areas in your vicinity offering flu immunizations.


  • Centers for Disease Control and Prevention
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