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With colder weather comes the flu — and the flu shot.

But this year, a new study in children has cast confusion over whether it's best to get the annual shot. The research, from the Marshfield Clinic Research Foundation, tested flu vaccine efficacy in children and found that kids who were vaccinated annually were more likely to contract the flu than children who only received the shot in the year of study.

“The vaccine was significantly more effective … if they had not been vaccinated in the previous five years,” Edward Belongia, M.D., an epidemiologist at the Marshfield Center, told the website StatNews.

But the findings raise more questions than they offer answers, Dr. Belongia said. There's no reason to buck the U.S. Centers for Disease Control and Prevention's recommendations, which state that individuals six months and older should receive an annual vaccine every year.

“In every scenario, it is better for people to be vaccinated than not vaccinated,” Dr. Belongia emphasized to StatNews. “It would not be, I think, accurate or helpful for people to take away from this, ‘Oh, well, I shouldn’t get vaccinated because I got vaccinated in the past and that’s a bad thing.’”

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

Really, the study simply reinforces what's already known: The influenza vaccine is complicated and requires more research to perfect.

“We like to say [the flu shot] is a good, but not perfect vaccine,” William Schaffner, M.D., an infectious disease specialist at the Vanderbilt University Medical Center and the medical director of the National Foundation for Infectious Diseases, told weather.com. “This issue of ... whether an annual immunization is optimal has been a matter of investigation going back 20 years,” he added.

Regardless of the new findings, your best bet is to get the shot every year. “Annual vaccination continues to be the order of the day — we're all pretty comfortable with that,” Dr. Schaffner explained, referring to infectious disease doctors across the world. “At the same time, we say 'let's keep investigating the vaccine.'”

As for the mechanism at play in the new study, researchers aren't sure. There's a theory that dates back several years, which states that some of the flu-fighting antibodies produced one year might wage war against the next year's shot. These old antibodies then reduce some of the immune system's response to the new vaccine, leaving the patient vulnerable to the virus.

But Dr. Schaffner said this hypothesis is just that for now — a hypothesis — and that it “seems a bit of a stretch to me.”

Annual immunization remains crucial because the virus mutates year after year. The vaccine is constantly updated to fight against new strains. This year, for example, the formulation includes protection against the deadly H3N2 strain that circulated throughout the Northern Hemisphere last year and also spread across the Southern Hemisphere during winter there. (Flu activity in the Southern Hemisphere during our summer is an indication of what might happen here the following winter.)

“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, summarized to weather.com in September. “We know influenza year to year is unpredictable. The only thing we can predict is its unpredictability.

“For me, [the shot] is a no brainer. It's a decreased risk of having a potentially deadly disease,” she said.

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