Seasonal flu activity in the U.S. typically peaks between December and February, and the Centers for Disease Control and Prevention (CDC) recommends everyone aged 6 months and older get their annual shot by Halloween. However, Dr. Robert Hopkins Jr., medical director of the National Foundation for Infectious Diseases, encourages older adults to protect themselves a bit earlier.

“For older adults and for folks who’ve got chronic medical problems, I usually recommend getting the flu vaccine around anytime from the middle of September through early October,” Hopkins previously told Fortune. “Because in those of us with immune systems that aren’t quite as robust, we want to make sure that we vaccinate at a time that’s likely to give us immune protection throughout the flu season.”

In addition to young children, pregnant people, and those with chronic conditions such as diabetes, older adults are at increased risk of serious flu complications. That’s because as you age, your immune system slows and weakens. In recent years, the CDC estimates, people 65 and older have represented 70% to 85% of seasonal-flu-related deaths and 50% to 70% of seasonal-flu-related hospitalizations.

Aging immune systems also may not work as well in combination with standard-dose flu vaccines to protect you from severe infection, hospitalization, and death—which is why the CDC recommends the following specialized flu vaccines for those over 64. All 2024–25 vaccines are trivalent, meaning they protect against three influenza viruses.

Fluzone High-Dose flu vaccine

The Fluzone High-Dose vaccine, manufactured by Sanofi, contains four times the protection of standard-dose vaccines. Medicare Part B, most Medicare Advantage plans, and most private insurance plans cover Fluzone High-Dose, according to Sanofi.

The Food and Drug Administration (FDA) has approved the immunization only for people 65 and older. A randomized controlled trial published in 2014 in the New England Journal of Medicine showed that high-dose vaccines were 24% more effective than standard-dose vaccines in preventing flu among older adults.

Don’t be alarmed if you experience more side effects than usual after receiving a high-dose shot, advises Dr. Priya Sampathkumar, an infectious disease specialist at the Mayo Clinic.

“This vaccine has four times the amount of antigen—which is the active part of the vaccine—as the other vaccines,” she tells Fortune. “Since it has more antigen, it does produce a better immune response. It also means that there may be a little bit more in terms of local side effects, pain, low-grade fever, et cetera.”

Fluad adjuvanted flu vaccine

Fluad, manufactured by Seqirus, is the only other flu vaccine FDA-approved exclusively for people 65 and older. While a standard-dose vaccine, Fluad contains an adjuvant, an added ingredient that helps your body develop a stronger immune response. Fluad’s adjuvant is called MF59, an oil-in-water emulsion containing squalene, a substance naturally occurring in plants, humans, and animals.

Medicare Part B and most Medicare Advantage plans cover Fluad without a co-pay, Seqirus says.

While research has shown Fluad to better protect older adults from flu than standard-dose, unadjuvanted vaccines, the CDC says no randomized studies to date have compared the efficacy of high-dose and adjuvanted flu vaccines.

Flublok recombinant flu vaccine

Flu vaccine production has historically involved the replication of viruses inside chicken eggs—a process that takes time and requires large quantities of eggs. Since 2013, the U.S. has also begun manufacturing vaccines more quickly and synthetically via recombinant technology. Flublok, a vaccine manufactured by Sanofi and FDA-approved for adults 18 and older, is a recombinant flu vaccine.

Flublok isn’t exclusive to older adults, but may offer them enhanced protection because it contains three times the antigen of standard-dose vaccines. Medicare considers Flublok among its preferred vaccines for people 65 and older.

If Fluzone, Fluad, or Flublok aren’t available to you, the CDC says some protection is better than no protection and advises older adults to get a standard-dose vaccine.

For more on vaccinations:

• Yes, you can get the COVID, flu, and RSV vaccines at once. Here are the pros and cons
• New flu shots have arrived. Here’s the best time to get your annual vaccine
• RSV can be deadly, especially for older adults. What to know about symptoms and the new vaccine
• New COVID vaccines are here. What to know about the latest shots as summer surge continues
• COVID can cause mental illness—but being vaccinated helps, new study shows