Flu season in the United States is between October and May each year. Because of this, you should 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, and headache.

Some infections are mild and symptoms improve in one to two 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 is eligible for each type.

Trivalent flu vaccines protect against three strains of the virus: influenza A (H1N1), influenza A (H3N2), and an influenza B virus. Options include:

Regular standard-dose trivalent shots

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

High-dose trivalent shot

The high-dose trivalent 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 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.

Trivalent shot made with adjuvant

This shot, called Fluad, is another high-dose flu vaccine 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 include:

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 those ages four­­ and older.

Intradermal quadrivalent shot

This flu shot is administered into the skin as opposed to a muscle. It’s approved for people ages 18 to 64.

Recombinant quadrivalent shot

This vaccine isn’t manufactured or grown from eggs, making it a suitable alternative for those who have an 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. 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 one to two days after vaccination. This can include weakness, body aches, or a fever, but this isn’t the flu.

You may also have problems if you’re allergic to egg protein or another ingredient in the vaccine. Signs of a serious reaction include breathing difficulty, wheezing, hives, fast heartbeat, and 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 have symptoms of an allergic reaction, see a doctor immediately.

If you’re allergic to eggs, you’ll need 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 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 two 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.