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Cooler temperatures mean flu.

The cold-weather illness crops up every fall and winter without fail, which is why it's not too early to start thinking about your 2015-2016 flu shot.

The U.S. Centers for Disease Control and Prevention — along with every other major U.S. health organization — recommends all individuals over the age of six months get the vaccine every year. It's particularly important for people 65 and up, young children and pregnant women, who might be more at-risk for serious, even deadly, complications from the virus.

Last year, the flu shot formulation, which changes annually based on which strains of the virus are circulating throughout the world, was one of the least-effective in the past decade, preventing just 13 percent of illnesses caused by the dominant strain of the flu. (However, it fought other strains of the virus, such as H1N1, more effectively.) The issue arose because of this dominant strain — H3N2 — mutated, rendering the vaccine, which contained a slightly different variety of H3N2, ineffective.

(MORE: 7 Steps to Dodge the Flu — Without a Shot)

“Immunity to the flu is complex,” Wendy Sue Swanson, M.D., M.B.E., a pediatrician and the executive director of digital health at Seattle Children's Hospital, told weather.com. “We know influenza year to year is unpredictable. The only thing we can predict is its unpredictability.”

These issues — plus that less than half of the U.S. population got vaccinated — meant it was a deadly year. Dr. Swanson said 145 deaths in children were reported to the CDC, though the actual figure is likely much higher.

CDC chief Thomas Friedan, M.D., M.P.H., said in a National Foundation for Infectious Diseases press conference on Sept. 17 that the 2014-2015 season had the highest hospitalization rate among seniors that we've ever documented. “The H3N2 type tends to be harder on seniors,” he said.

But so far this year, the new vaccine formulation looks promising. “The [strains] that are causing very early disease are exactly as predicted, and it looks to me like the vaccine is going to be well protective,” Dr. William Schaffner, infectious disease specialist at the Vanderbilt University School of Medicine and the past president of the NFID, said at the press conference.

Regardless of the effectiveness of the shot year to year, it's still without a doubt the best protection against the deadly disease that we have, public health experts say. Dr. Friedan emphasized that though the public might think it's a mild sickness, in reality flu sickens millions, hospitalizes hundreds of thousands and kills tens of thousands of Americans each year.

Dr. Swanson echoed this. “For me, it's a no brainer. It's a decreased risk of having a potentially deadly disease.”

MORE ON WEATHER.COM: Bogus Excuses Not to Get a Flu Shot