Influenza (flu) is a viral respiratory infection that affects millions of people each year. The Centers for Disease Control and Prevention (CDC) estimates that the number of yearly hospitalizations due to influenza since 2010 has ranged from 140,000 to 710,000.

Every year, a flu vaccine is developed that aims to protect against the most commonly circulating strains. Receiving the seasonal flu vaccine is the best way to protect yourself against becoming ill with the flu.

But how does the vaccine work? How long does it last, and when’s the best time to get it? Here’s what you need to know.

Development of the seasonal flu vaccine actually begins many months ahead of flu season. The viruses used in the vaccine are based off of extensive research and surveillance into which strains will be most common during the upcoming season.

Seasonal flu vaccines protect against two types of influenza viruses: influenza A and influenza B. They can also be either trivalent or quadrivalent.

The trivalent vaccine protects against three flu viruses: two influenza A viruses and an influenza B virus.

The quadrivalent vaccine protects against the same three viruses as the trivalent vaccine, but also includes an additional influenza B virus.

Once you receive your flu shot, it takes two weeks for your body to develop antibodies that provide protection.

It’s important to remember that during this period, you’re still vulnerable to becoming ill with the flu.

During those two weeks you should be extra careful to:

  • practice good hygiene
  • avoid touching your nose or mouth whenever possible
  • avoid crowds if flu is circulating in your community

Your body’s immunity to the flu decreases over time. This is true whether you’ve had a vaccination or a flu infection.

Additionally, influenza viruses are constantly changing. Because of this, a vaccine from the previous flu season may not protect you through an upcoming flu season.

Generally speaking, receiving the seasonal influenza vaccine should help to protect you for the duration of the current flu season.

You’ll need to receive a seasonal influenza vaccine every year in order to have the best protection against influenza viruses.

The flu vaccine is produced by a number of private manufacturers and typically begins to ship to healthcare providers during August. However, there’s some evidence that it may not be advantageous to receive your vaccine this early.

A recent study indicated that maximum immunity is achieved shortly following vaccination and decreases with each passing month. Therefore, if you get your vaccine in August, you may be more susceptible to infection late in the flu season, around February or March.

The CDC recommends getting the flu vaccine before influenza activity begins to pick up within your community, ideally by the end of October.

If you receive your vaccine later, don’t worry. Late vaccination can still provide adequate protection, as influenza can circulate within your community through March.

There are several side effects that you may experience after receiving the seasonal flu vaccine.

It’s important to note that the flu shot is made with inactivated virus. It can’t give you the flu.

Side effects from the flu shot are typically mild and only last a few days. They can include:

  • redness, swelling, or soreness at the injection site
  • low-grade fever
  • general aches and pains

Influenza viruses are constantly changing and evolving rapidly. Circulating influenza viruses can mutate from one season to the next.

Researchers need to select the specific influenza viruses to include in the vaccine many months before flu season begins. This means what’s in the vaccine may not always match what’s actually circulating during flu season. This can decrease the effectiveness of the seasonal flu vaccine.

Age can also play a factor in vaccine efficacy because your immune system tends to become weaker as you age. A high-dose influenza shot is available for people 65 and older. The higher dose is aimed at providing a better immune response and therefore better protection within this age group.

The CDC also recommends that some children between the ages of 6 months and 8 years receive two doses of the influenza vaccine in order to have sufficient protection.

Everyone over 6 months of age should receive the flu shot each year.

It’s particularly important for people who are at an increased risk for flu-related complications to be vaccinated. This includes:

  • people ages 65 and older
  • pregnant women
  • children younger than 5 years old
  • people with health conditions such as asthma or cancer

Children under 6 months of age shouldn’t receive the influenza vaccine. To protect these children from potential exposure to the virus, all family members or caregivers should be vaccinated.

Additionally, if you’re currently sick with an acute illness, you may need to wait until you’re better to receive the vaccine.

Before you’re vaccinated, you should let your doctor know if you’ve had:

Influenza viruses cause seasonal epidemics of respiratory illness every year. While some people might experience mild illness, others may experience a more serious infection requiring hospitalization.

Getting your flu shot each year is the best way to decrease your chances of getting sick with the flu. Additionally, when more people receive the flu vaccine, the virus is less able to circulate in the community.

You should aim to receive your flu shot every fall before influenza virus activity begins to pick up within your area.