The signs start popping up earlier and earlier each year. "Get your flu shot now!" urge posters at pharmacies, even while they're still pushing sunblock and bug spray.

How smart of those early birds to get protected weeks or even months before flu season rears its aching, nasty head. Or is it?

The Centers for Disease Control and Prevention tells doctors they can start vaccinating patients as soon as the flu shot becomes available each season. These days, that can be as early as August in many places across the United States.

However, in the agency's recommendations for influenza vaccination practices, the CDC points out that evidence from some clinical trials has shown that protection from viruses similar to those found in the flu vaccine often lasts for six to eight months. Sometimes, immunity from viruses can last longer, especially if the influenza virus strains remain similar for multiple flu seasons. Some studies have noted a drop in vaccine effectiveness in adults 65 and older.

So, do the math. If you get the vaccine in early August and flu season can last through May of the following year, you may not be as protected in late spring when the flu may still be spreading.

Better early than never

But the key, points out infectious disease expert Dr. Kathleen Neuzil, M.D., is that even if you get the flu shot early, you will still be protected later in the season.

"It doesn’t mean it works for six months and stops working at six months and one day," says Neuzil, who is director of the Center for Vaccine Development at the University of Maryland School of Medicine and wrote an editorial on flu shots for the Journal of Infectious Diseases..

"There have been certain years where the vaccine has looked like it has waned for certain people. But for the majority of seasons and the majority of people, the protection is going to last for the flu season."

Neuzil says that in the past 30 years, flu season has most commonly peaked in February. However, in the past five years, it has peaked in December three times.

"One of the real challenges with influenza season is that we don't know when it's going to start," she says. So if you're going in for your physical in September, go ahead and get the vaccine, Neuzil suggests. Don't risk forgetting about it later or being susceptible if flu season starts earlier than usual.

senior getting flu shot People over 65 are more likely to have their vaccines lose effectiveness as flu season tapers off. (Photo: JPC-PROD/Shutterstock)

For groups like nursing homes, where all the residents are older and living together, it may make sense to give flu shots in October, closer to the traditional start of flu season, says Neuzil.

Older people are the ones who might experience a loss in effectiveness as the season progresses.

“If you’re over 65, don’t get the flu vaccine in September. Or August. It’s a marketing scheme,” Laura Haynes, an immunologist at the University of Connecticut Center on Aging, tells Kaiser Health News.

In addition to becoming less effective more quickly, various factors make it harder for older people's immune systems to respond to the vaccination.

Hayes, who specializes in working with aging populations, says the ideal time for seniors to get their flu shots is between Halloween and Thanksgiving.

Will the flu vaccine improve?

No. There's no reason to hold out in hopes that a new and improved vaccine to be developed some time later in the flu season.

The flu vaccine is made for the virus that health experts believe will be circulating that particular season. Sometimes their forecasting powers are dead-on and the vaccine is accurate; other times, not so much. But the vaccine isn't reformulated throughout the season, Neuzil says. The flu shot you get in August is the same one you'll get in October or January.

The key is that it's better to get it early than not at all.

The CDC recommends that everyone 6 months and older get the flu vaccine each season by the end of October. It takes about two weeks for the body's immune system to mount a full response so you're fully protected.

Children who are getting their first flu shot and need two doses of the vaccine should get the first dose as soon as possible, according to the CDC, so that they have the second dose (which is given 28 days later) before the start of the flu season.

"It would be great if we knew the influenza season was going to start on a specific date," Neuzil says. "It's obviously better to have the shot than not to get the shot at all."

Mary Jo DiLonardo Mary Jo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.