Flu season is starting, in case you didn't know.

October 1 marks the beginning of flu season. If you haven't already had your annual flu shot, it's a smart idea to roll up your arm for one soon, before flu cases start to peak later this year. But what about the high-dose flu vaccine—is that available right now, and who should get it?

Whatever you do, it's crucial to get the flu shot. "There was almost no flu last year and people's immune systems may have forgotten about influenza," Timothy Murphy, MD, senior associate dean for clinical and translational research at the University at Buffalo Jacobs School of Medicine and Biomedical Sciences, tells Health. "This could be a particularly severe flu season, and it may come earlier."

As for the high-dose flu vaccine…it's not available to everyone. Here's what infectious disease experts say.

What is the high-dose flu shot, and who should get it?

This year's high-dose flu shot is called the Fluzone High-Dose Quadrivalent, and it's the only licensed high-dose inactivated flu vaccine, according to the Centers for Disease Control and Prevention (CDC). This vaccine has four components to help boost your immunity to the flu.

For adults 65 and older, the high-dose flu vaccine is recommended. "This type of vaccine contains a lot more viral protein, which is used to stimulate immunity, than the regular flu vaccine," infectious disease expert Amesh A. Adalja, MD, senior scholar at the Johns Hopkins Center for Health Security in Maryland, tells Health. Basically, the high-dose flu shot works in the same way as the regular flu shot—delivering inactivated virus to prompt the body to make antibodies. But it's associated with a stronger immune response.

Why should seniors get the high-dose vaccine? Because older adults are at a higher risk of flu complications. CDC studies estimate that this age group accounts for 70% to 85% of flu-related deaths and 50% to 70% of flu-related hospitalizations each flu season.

Your immune system typically becomes weaker as you age and, as a result, people who are 65 and older usually don't have immune systems that are as robust as their younger counterparts, Richard Watkins, MD, an infectious disease physician and professor of internal medicine at the Northeast Ohio Medical University, tells Health. A weakened immune system also means that older people don't respond as well to the flu vaccination, the CDC says—as a result, they're given a stronger vaccine.

So it offers more protection from the flu?

Exactly. A 2014 study published in The New England Journal of Clinical Medicine, which involved more than 30,000 adults aged 65 and older, found that participants who received the high-dose flu vaccine had 24% fewer flu illnesses compared to those who got the standard flu vaccine.

Another study, carried out during the 2013-2014 flu season and published in The Lancet Respiratory Medicine in 2017, found that the high-dose flu shot was associated with a lower risk of hospital admissions compared with the regular flu shot in people age 65 and over. This was particularly true for those living in long-term care facilities.

Does the high-dose flu shot have any risks or side effects?

The CDC specifically states that the high-dose flu vaccine isn't recommended for people with a history of severe allergic reactions to the vaccine or to ingredients other than eggs. (You can ask your care provider about the ingredients in the vaccine—which should be in the package inserts—if you have concerns.)

While studies have shown that the high-dose flu vaccine is more effective than the standard dose formulations for older adults, they've also shown an increased rate of mild to moderate local reactions, Richard Seidman, MD, MPH, chief medical officer at LA Care Health Plan, tells Health.

The most common adverse events experienced during clinical studies were mild and temporary; these included pain, redness at the injection site, headache, muscle aches, and weakness. Like the regular flu shot, the high-dose shot isn't recommended for people with a history of a severe allergy to the vaccine or an allergy to ingredients in the vaccine.

What if the high-dose flu shot isn't available?

Manufacturers have increased flu shot production to meet the high demand for the 2020-2021 season. According to the CDC, between 188 million and 200 million doses will be distributed. All vaccines will be quadrivalent (meaning they have four components), most will be thimerosal-free or thimerosal-reduced vaccines (87%) and about 18% of flu vaccines will be free of eggs.

But if you or your loved ones can't get the high-dose vaccine from your health care provider, pharmacy, or another vaccine site, you can absolutely get the standard-dose vaccine as an alternative, says Dr. Seidman.

There are two senior vaccines this year

Keep in mind that there are two different flu vaccines that are designed for people aged 65 and up. One vaccine has the exact same amount of antigen (the part of the virus that triggers an immune response in your body) as the injection for younger adults. It also contains an adjuvant, which is an ingredient added that's designed to help you get a stronger immune response from the vaccine. The other shot is a high-dose version of the vaccine—it has four times the amount of antigen.

"The adjuvanted flu vaccine is another vaccine specially formulated for older adults and would also be preferable to the ordinary flu vaccine in older age groups," says Dr. Adalja. The adjuvanted flu vaccine is made with MF59 adjuvant, an additive that triggers a stronger immune response to vaccination.

If you want to get the high-dose flu vaccine, it's important to specify that vs. just assuming you'll be given it based on your age alone. Still, "either one is fine for people over 65," Dr. Murphy says. "Both are considered equally effective."

Can people under 65 get the high-dose flu vaccine?

Currently, the high-dose flu vaccine is licensed only for persons aged 65 years and older. However, you might be able to get it if you're younger than 65. "You could ask your doctor," suggests Dr. Adalja. "It would be off-label and it might not be covered by insurance, so you might need to pay cash or the difference in cost between it and the standard dose."

Dr. Adalja also makes the point that younger people simply don't benefit from the high-dose shot as much as older people do. "Older age groups are at higher risk of complications from the flu," he says.

Before using a vaccine for off-label indications, Dr. Seidman recommends discussing your flu vaccine options with your doctor and considering the risks and benefits to help you make an informed decision.

The CDC recommends that everyone 6 months and older gets a flu vaccine every season, and advises going for a flu vaccine that's approved for your age group. Some inactivated flu vaccines, which use the inactivated version of the germ that causes a disease and provide less protection than live vaccines, are approved for people as young as 6 months. Other flu vaccines are only approved for adults, such as the recombinant influenza vaccine (RIV), which is approved for people aged 18 years and older.

Remember, no vaccine is a guarantee

No vaccine is 100% effective in preventing the flu. "The effectiveness of the flu vaccine last year is estimated at around 45 percent," says Dr. Seidman. "But even though a person can still get the flu after being vaccinated, there is evidence that the severity of the infection can be significantly less in patients who were vaccinated compared to those who were not."

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