Flu season in the United States is between October and May each year. Because of this, consider getting a flu shot as early as October to protect yourself.

The flu is a highly contagious respiratory illness that causes a range of symptoms. Common symptoms include:

  • sore throat
  • coughing
  • runny nose
  • fatigue
  • chills
  • body aches
  • headache

Some infections are mild, and symptoms can improve in 1 to 2 weeks. But life threatening complications can occur in people who have a weakened immune system, such as adults ages 65 and older.

Flu shots are safe for most people ages 6 months and older. Here’s a look at the different types of flu shots, as well as information on who’s eligible for each type.

Trivalent flu vaccines protect against three strains of the virus:

  • influenza A (H1N1)
  • influenza A (H3N2)
  • an influenza B virus

Options are below.

Regular standard-dose trivalent shots

These are egg-grown flu vaccines administered by a needle into a muscle in your arm. Standard-dose vaccines are for people ages 18 to 64.

Trivalent and quadrivalent shots made with adjuvant

These shots, called Fluad (trivalent) and Fluad Quadrivalent, are another option for flu vaccines that are approved for people ages 65 and older. It includes an ingredient called adjuvant, which also creates a stronger immune system response.

These flu vaccines are slightly different because they protect against four different strains of the flu virus (two influenza A viruses and two influenza B viruses).

Because of this, these vaccinations can provide broader protection from infection.

Options are below.

Regular standard-dose quadrivalent shot

The standard-dose flu shot is available to people ages 6 months and older. There’s also the option of a quadrivalent shot that contains the virus grown in a cell culture. This particular vaccine is only available to people ages 4­­ years and older.

High-dose quadrivalent shot

The high-dose quadrivalent vaccine (Fluzone) is designed specifically for people ages 65 and older. Flu-related complications increase with age because older individuals have a weaker immune system.

Fluzone contains four times the amount of flu virus antigen as a standard-dose shot. Antigen refers to the part of the vaccine that stimulates your immune system to respond and protect against the flu virus.

A high-dose vaccine is recommended for older adults because up to 85 percent of flu-related deaths occur in people ages 65 and older.

Jet injection quadrivalent shot

AFLURIA Quadrivalent is a type of flu shot now approved for administration by needle for people ages 6 months or older, or by jet injector for people ages 18 to 64.

A jet injector is a medical device that uses a high-pressure stream of fluid to get into the skin rather than a needle.

Recombinant quadrivalent shot

This vaccine isn’t manufactured or grown from eggs, making it a suitable alternative for those who have severe egg allergy. It’s approved for people ages 18 and older.

Live attenuated intranasal spray

This vaccine is made using eggs and administered as a nasal spray for people between 2 and 49 years who aren’t pregnant.

It includes a dose of attenuated flu viruses. Instead of killed flu, the flu included in this vaccine is severely weakened, making it unable to cause a widespread infection.

Similar to other types of vaccinations, there’s a risk of side effects with the flu shot. Common side effects may include tenderness or redness at the injection site.

In addition, some people experience mild flu-like symptoms for 1 to 2 days after vaccination. This can include weakness, body aches, or a fever, but this isn’t the flu.

You might have issues if you’re severely allergic to eggs or another ingredient in the vaccine.

Signs of a serious reaction include:

  • breathing difficulty
  • wheezing
  • hives
  • fast heartbeat
  • dizziness

Life threatening allergic reactions are rare after getting the flu shot, though.

Symptoms of a reaction occur within a few hours of vaccination. If you do have symptoms of an allergic reaction, seek immediate medical attention.

The CDC recommends people with an egg allergy still get a flu shot. If you have a severe egg allergy, you may consider getting your flu shot in a medical setting that can treat allergic reactions. You can also request a vaccine that doesn’t contain egg protein.

You may have to avoid vaccination if you’re allergic to another ingredient in the vaccine.

In rare cases, Guillain-Barré syndrome may develop within days or weeks of a vaccination.

Guillain- Barré syndrome is a neurological disorder where the body’s immune system attacks the peripheral nervous system. This condition can cause muscle weakness and paralysis.

Among those who receive a vaccination, there’s only one or two cases per 1 million people.

Getting an annual flu vaccination is one of the best ways to protect yourself from the flu virus.

Vaccination is also important because the flu can progress and cause a secondary infection, such as bronchitis, pneumonia, or an ear infection.

Complications can occur in people with a weak immune system, such as young children, older adults, and those with chronic conditions.

Talk with your doctor to see which flu vaccine is right for you, and get vaccinated as early as possible. On average, it takes about 2 weeks for the vaccine to offer protection.

The flu vaccine is about 40 to 60 percent effective when the virus type in the vaccine aligns with the circulating virus. For those who do get sick after getting a flu shot, vaccination may reduce the severity of symptoms.