Compared with standard flu vaccines, high-dose flu vaccines have more of an ingredient that boosts the immune response. The ingredient is a flu virus antigen called hemagglutinin. High-dose flu vaccines may have 3 to 4 times as much flu virus antigen as the standard flu vaccine.

There's also a flu vaccine that boosts immune response using an ingredient called an adjuvant. These shots have the same amount of flu virus antigen as a standard flu shot. But the adjuvant helps the body respond to the vaccine. This type of flu shot is called an adjuvanted flu vaccine.

High-dose and adjuvanted flu vaccines can help people have a stronger immune response against influenza.

For this reason, high-dose and adjuvanted flu vaccines are recommended for people age 65 and older. These vaccines also may be available to people ages 18 to 64 who have had solid organ transplants.

Why are high-dose or adjuvanted flu vaccines needed?

Flu, also called influenza, is an infection of the nose, throat and lungs, which are part of the respiratory system. The flu is caused by a virus. Most people with the flu get better on their own. But influenza can cause serious complications for some groups of people.

Based on past flu seasons, standard flu vaccines may not work as well for people older than age 65 and people who've had organ transplants.

• People age 65 and older are at higher risk than others for severe flu and complications. Rates for hospital stays and death linked to flu are typically higher in this age group than they are for younger people.
• When it comes to people who've had a transplant, past flu seasons suggest they have a higher than average risk of needing hospital care for flu.

Among people age 65 and older, high-dose and adjuvanted flu vaccines help prevent flu-related hospital stays better than the standard flu vaccines do. That's based on research from past flu seasons.

Among people who've had a solid organ transplant, these types of flu vaccines may boost the immune system's ability to respond to an infection.

But any flu vaccination is better than none. So if you can't get a high-dose or adjuvanted flu vaccine, get the flu vaccine that is offered.

Like other flu vaccines, high-dose and adjuvanted flu vaccines are updated every year. This helps protect against the flu strains most likely to cause the flu during the upcoming flu season.

Are there side effects?

Compared with the standard flu vaccine, people who get the high-dose vaccine are more likely to have side effects. These include:

• Soreness or pain the site of the shot.
• Headache.
• Muscle aches.
• Feeling very tired.

Side effects after the high-dose vaccine most often happened during the week after getting the vaccine.