It's time to get your flu shot. Or is it?
The answer to that question depends on whom you ask.
The sign in the pharmacy window might suggest flu season is already here, and the Centers for Disease Control and Prevention is urging everyone at least 6 months old to get a dose of prevention "as soon as possible."
But according to some infectious disease experts, the beneficial effects of a flu shot received in August or September could start to fade just as the virus kicks into high gear.
While doctors can never predict exactly when it will strike, the official start of flu season has long been October, with the number of cases typically peaking in February or even later.
Medical professionals agree that a flu vaccine loses its potency over time, a phenomenon called "waning immunity." What they don't agree on is how long the vaccine can be expected to protect a person against particular strains of the virus.
"Getting the vaccine early beats not getting the vaccine at all," said Dr. Jorge Parada, director of infection control and prevention at Loyola University Health System. "But if you get it very early, you may have your waning immunity in February, which is peak flu season. I personally see no reason to get the vaccine until October."
But to keep pace with other providers, Loyola officials decided to begin offering influenza vaccinations to patients earlier this September. The hospital is waiting to vaccinate its employees, however, until October.
"There are two sides of the coin," said Dr. Michael Jhung of the CDC's influenza division in Atlanta. "Do you vaccinate now and have that be too early … or do you wait, and find out that is too late? (If) you wait too long, you might not need the vaccine because you've already got the flu."
Influenza is a highly contagious virus that attacks the upper respiratory system. While still viewed by many as a routine part of winter, only moderately more punishing than the common cold, the flu causes 3,000 to about 49,000 deaths in the U.S. each year, according to the CDC, which based its estimate on numbers gathered between 1976 and 2007. The agency notes the "variability and unpredictability of flu" in making its estimate.
Each year, a vaccine is engineered to protect against three different strains of flu. Experts try to identify the strains most likely to spread across the U.S. They typically zero in on strains active in the Southern Hemisphere, where countries experience winter when it is summer in the Northern Hemisphere.
"Sometimes they are spot-on, and the vaccine is very effective … and sometimes they miss," Parada said.
Last year, the seasonal flu arrived later than usual, peaking around March, and the toll it took in the U.S. was relatively mild, according to the CDC.
While the flu changes year to year, the predominant strains of the virus in the 2011-12 season were the exact same as those targeted the year before. But flu strains most likely to crop up this season are different from last year, officials say.
This year's vaccine includes a strain of the influenza A virus, which is capable of major mutations, and a strain of the influenza B virus, which mutates just enough to make people sick year after year. The cocktail also includes the same H1N1 virus first discovered in pigs in 2009.
The secret to preventing the spread of influenza, as well as some other contagious diseases, is something called "herd immunity." According to doctors, when enough people in a community are immunized, the chances for an outbreak become slim, and even those who fail to get a flu shot are protected.
"The main thing is to remind people early," said Dr. Lisa Grohskopf, medical officer in the CDC's influenza division. "(The timing) is not something we can predict with absolute certainty. That makes it difficult for us to tell people to wait."
More people have been getting influenza inoculations than at any time in the past, the CDC reports. In addition to a shot, the vaccine can be administered through a mist.
When the agency issued its first influenza advisories in the 1960s, the flu shot was recommended only for people 65 or older and those suffering from chronic diseases. That's the way it stayed until 2000, when the agency decided that healthy adults as young as 50 also should be inoculated.
A few years after that, the CDC expanded its recommendation to include all children under 2, pregnant women and day care workers. As of 2010, the CDC advisory includes everyone except babies under 6 months old.
That's good news, Parada said, but rising demand for flu vaccine has increased pressure on pharmaceutical companies to produce more of the vaccine earlier. Almost 73 million doses of vaccine had been distributed in the U.S. as of Sept. 7, according to the CDC.
The early start to vaccine campaigns in recent years could be linked to the swine flu outbreak that swept across the country in 2009, Parada said. In order to focus on the emerging outbreak, many vaccine manufacturers fast tracked production of the seasonal flu shots in the spring of that year.
Supply of the seasonal flu formula dwindled, and health professionals were forced to ration available vaccine to those most at risk.
"The CDC has been working very hard to make sure we get good (flu vaccine) production early (and) in large amounts; they have been very successful at that," Parada said.
"The flip side is now there is a lot of vaccine early in the season and people who have it want to sell it. Then you get a race to start early, which is absurd."
Many providers begin marketing the flu shot soon after shipments of the vaccine arrive.
Walgreens pharmacies around the country have been offering the vaccine since Aug. 6, said spokesman Robert Elfinger.
"Over the years, it has gotten earlier. (This year) we are able to make it available for back-to-school shopping," Elfinger said Wednesday from the company's Deerfield headquarters.
And a new swine flu strain has infected more than 300 people in the U.S. this year, according to the CDC. About 80 percent of those infected were reported in Ohio and Indiana. Only four people in Illinois have confirmed infections.
"One of the things that is driving people to get vaccinated (for the seasonal flu) earlier this year is the buzz about this swine flu outbreak," Parada said.
He added that people should be aware that the seasonal flu vaccine will not protect them from the new mutated H1N1 virus. Most victims have been farmworkers or other people exposed to pigs, and most experts say the new H1N1 virus will "burn itself out."