Experts recommend getting the flu vaccine in September or October, before influenza activity begins to pick up. Your immunity to the flu may decrease slowly over time, but the vaccine will protect you throughout flu season.

Influenza (flu) is a viral respiratory infection that affects millions of people each year. As we head into flu season in the United States, it’s important to know what to expect and how to prevent the flu.

Every year, manufacturers develop flu vaccines to protect against the most commonly circulating strains at that time. Receiving the seasonal flu vaccine is the best way to reduce your chances of getting the flu.

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

Development of the seasonal flu vaccine begins many months before flu season. The viruses used in the vaccine are based on 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.

Trivalent vaccines protect against three flu viruses: two influenza A viruses and an influenza B virus.

Quadrivalent vaccines protect against the same three viruses as the trivalent vaccine, but they also include another influenza B virus.

As of the 2024 to 2025 flu season, all available flu vaccines are trivalent.

According to the World Health Organization (WHO)Trusted Source, quadrivalent flu vaccines are not currently in use. This is because public health precautions during the COVID-19 pandemic effectively eliminatedTrusted Source one of the influenza B strains previously included in those vaccines. But it’s possible that quadrivalent vaccines will be used again in the future.

Once you receive your flu vaccine, it takes 2 weeks for your body to develop antibodies that provide protection.

During those 2 weeks, you can still contract the flu, so you should be extra careful to:

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

These precautions can also help you avoid other respiratory conditions, such as COVID-19. You can contract the flu along with other infections, and it’s important to protect yourself and others.

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

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

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

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

Private manufacturers produce flu vaccines each year and typically begin to ship them to healthcare professionals in August. But the vaccines may not be available to you that early in the season.

The Centers for Disease Control and Prevention (CDC)Trusted Source recommends that healthcare professionals offer vaccines to the public in September or October. Although immunity from the vaccine may decrease slightly over time, research has found that flu vaccines protect against the influenza virus throughout flu season.

And if you get your vaccine later in the season, don’t worry. Late vaccination can still provide enough protection, since influenza can circulate in your community until the end of March or even longer.

The seasonal flu vaccine is made with an inactivated virus, which means that the vaccine cannot cause the flu. But you might experience several side effects after receiving the vaccine.

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

Flu vaccine side effects can include:

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

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

Researchers need to choose the specific influenza viruses to include in the vaccine many months before flu season starts. This means that the virus strains in the vaccine may not always match the ones that are actually circulating during flu season. If the seasonal flu vaccine does not align with the circulating viruses, it will be less effective.

Age can also play a role in vaccine effectiveness because your immune system tends to become weaker as you age. The Food and Drug Administration (FDA)Trusted Source has approved a high dose flu vaccine (Fluzone High Dose) for people 65 years old and older.

The higher dose is aimed at providing a better immune response and therefore better protection for people in this age group. Multiple studies have shown increased effectiveness for those ages 65 and older with the high dose vaccine.

The CDC also recommendsTrusted Source that some children between the ages of 6 months and 8 years receive two doses of the influenza vaccine during the first season in which they’re vaccinated in order to have enough protection.

It’s still possible to get the flu after being vaccinated, but research has shown that the condition may be less severe and that people who receive a flu vaccine may be less likelyTrusted Source to be admitted to the hospital if they get the flu.

Most people over 6 months of age should receive the flu vaccine each year.

Vaccination is especially important for people who are at an increased risk of flu-related complications. This includes the following groups:

  • people over 50 years old
  • anyone who has a chronic health condition
  • anyone with a weakened immune system
  • children between 6 months and 5 years of age
  • people 18 years old and younger who receive aspirin therapy
  • people who are pregnant or have given birth in the last 2 weeks
  • people with a body mass index of 40 or higher
  • people of American Indian or Alaska Native descent
  • healthcare workers
  • anyone who lives or works in a nursing home or long-term care facility
  • caregivers of people in any of the above categories

Children under 6 months old should not receive the influenza vaccine. To protect these children from possible exposure to the virus, all family members and caregivers should be vaccinated. This is called herd immunity and will help protect those who can’t get the vaccine.

Additionally, if you currently have an acute health condition, you may need to wait until you feel better to get the vaccine.

Before getting vaccinated, let your doctor know if you’ve previously experienced any of the following:

These factors may indicate that you should not get the flu vaccine. But check with your doctor to see what they recommend.

Many flu vaccines contain a small amount of egg protein. But even if you have an allergy to eggs, the CDC recommendsTrusted Source that you get an annual flu vaccine. It’s generally best to get your vaccine in a setting where healthcare professionals can monitor you for an allergic reaction and provide treatment if necessary.

Influenza viruses cause seasonal respiratory infections every year, most often in the fall and winter. While some people might experience a mild case of the flu, others (especially those in certain high risk groups) may develop a more serious infection that requires hospitalization.

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

It’s best to get your flu vaccine every fall before influenza virus activity begins to pick up within your area.

If you experience any symptoms of a cold or flu, it’s important to avoid contact with others and get tested for the flu and COVID-19.