The Centers for Disease Control and Prevention said doctors are seeing the earliest flu season in nearly a decade.
A doctor at St. Luke's Hospital said his office ordered more than 600 doses of vaccine this year and they're already gone.
Dr. Brent Beasley, an internist who specializes in geriatric medicine, said he encourages all of his patients to get vaccinations.
"You really have to watch older people when they get the flu because it can get worse and it can progress a lot worse," he said.
In Missouri, there are already more than 3,100 confirmed flu cases around the state. It was only about 218 this same time last year.
"It looks like we're seeing cases earlier this year than we did last year and starting to see a trend upward," Beasley said. "We don't yet know what's going to happen with that trend or what it's going to look like throughout the rest of the year."
Flu season can start as early as October and typically peaks in January or February.
No one can predict how severe a flu season can be.
"We're right at the tip of this, so we don't know what it's going to be over the next few months," Beasley said.
While the vaccine is only 60 percent effective, experts hope that someone who gets the vaccine and still catches the flu will have a less severe case.
"The other benefit in making sure the healthy younger population is vaccinated is to try to protect our older population and older family members from picking it up from us," Beasley said.
He said the good news about the early flu season is that this season's flu vaccine seems to cover the predominant strain of the flu that's going around this year.