Flu activity tends to rise in October, according to the Centers for Disease Prevention and Control, prompting many health officials to advise getting a flu shot this time of year. Millions of people in the U.S. get the flu annually, with thousands of people dying from it each year, the CDC reported.

If you got a flu vaccine a year or more ago, you might think you're still protected from the flu this year. Not so.

It's important to get the flu shot each year to fight different illnesses and also because the vaccines can weaken over time, the CDC said:

"The first reason is that because flu viruses are constantly changing, flu vaccines may be updated from one season to the next to protect against the viruses research indicates may be most common during the upcoming flu season. The second reason that annual vaccination is recommended is that a person’s immune protection from the vaccine declines over time. Annual vaccination is needed for optimal protection."

Annual flu vaccines are recommended for most people, but people of certain age groups and people with certain conditions cannot get it.

Children under six months of age and people with extreme allergies to the flu vaccine should not get the flu vaccine, according to the CDC. People with allergies to eggs may also want to talk to a doctor before getting the shot, which usually includes a small portion of an egg.

The best time to get a flu shot is in October, and people over 65 might want to wait until Halloween or Thanksgiving to get a shot so the vaccine does not fade for them during the "really bad flu season," Laura Haynes, a professor of immunology at the University of Connecticut, recently told NPR:

"[The] immune response [for people over 65] isn't as good to the vaccine. So the protection that's induced wanes more quickly. So, therefore, you'd want to wait a little bit longer than you would if you were a younger adult. That way, you're protected throughout the majority of the really bad flu season, which happens from, say, January to April."

The flu shot isn't as effective for the elderly and very young children, but it is still very important for the elderly to get, Haynes added. "People who are elderly should still get the flu vaccine, because even if it doesn't keep them from getting the flu, it's been shown that it will keep them from getting sick enough that they have to go to the hospital. And staying out of the hospital is always a good thing."