The short answer: No one really knows.

The long answer: This year’s flu shot is looking promising—so far.

The effectiveness of the flu shot varies each year, because there are multiple strains of the flu that spread each season, according to the FDA. Each vaccine is formulated to target three or four of the “most likely to circulate” strains each year, but figuring out which strains to include in the seasonal flu shot is somewhat of an educated guessing game.

And, while the flu shot doesn’t become a topic of conversation until September for most people, researchers from several federal agencies spend the entire year studying the flu season prior and preparing for the next.

The FDA, World Health Organization, and the Centers for Disease Control and Prevention review global data each year to determine which strains to include in the vaccine—a decision that has to be made months in advance of the next flu season. The strains for this year’s flu shot were picked in March.

“One of the challenges in fighting flu is that the viruses can change their genetic make-up rapidly—not only between flu seasons, but also during the course of a single season,” FDA Commissioner Scott Gottlieb, M.D., said in a statement. “As a result, the seasonal influenza vaccine needs to be evaluated annually to see whether its composition needs to be adjusted.”

And that is what researchers believe is one of the reasons last year’s flu season was so deadly—the virus mutated.

“We guessed right about the common strains that dominated last year’s flu season,” Gottlieb said. “It’s now believed that part of the reason that the vaccine was not as protective is that the flu strain used to manufacture the vaccine mutated very subtly during the development process.”

Those who received the flu shot in 2017 still developed antibodies to ward off the virus, but due to the slight changes the virus had already undergone, those antibodies weren’t enough of a match for the mutated virus.

“This isn’t the first time that the vaccine strains have deviated in some important ways from the influenza strain that ends up circulating during the flu season,” Gottlieb said. “But we’ve learned a lot of lessons from these highly unfortunate incidents. And we’ve improved our scientific methods to reduce the odds that it happens again.”

Improvements made to the 2018 flu vaccine include replacing two of last year’s influenza strains with potentially better matches. And, so far, that new vaccine seems to be on target.

Based on current flu patterns in the Southern hemisphere, Gottlieb says, the 2018 flu vaccine should be effective.

So, should you get the flu shot this year? Yes. The FDA stresses that the seasonal shot is still the most effective way to fight off the flu—not only for yourself, but for those around you.

Last year, most of the children who died from the flu had not been vaccinated, the agency said, adding that even if a person contracts the flu, a flu shot could lessen its severity.

The flu shot is recommended by both the FDA and CDC for those ages 6 months and older, especially those in high-risk groups: pregnant women, senior citizens, and young children.