The flu season is from november to april, with most cases occurring between late december and early march, but the vaccine is usually offered between september and mid-november (and may be given at other times of the year)

The Flu Vaccine
Theseason is from November to April, with most cases occurring between late December and early March,
but the vaccine is usually offered between September and mid-November (and may be given at other times of Getting the shot before the flu season is in full force gives the body a chance to build up immunity to, or protection from, the virus. Although you can get a flu shot well into flu season, it's best to try to get it earlier rather tha However, even when there are only 2 or 3 months left in the flu season, it's still a good idea to get protection if the vaccine is available in your area. In times when the vaccine is in short supply, certain people need it more than others. The Centers for Disease Control and Prevention (CDC) often will recommend that certain high-risk groups be given priority when flu shot supplies are limited. Call your doctor or local public health department about vaccine availability in your area. A non-shot option, the nasal mist vaccine, is now available, but because it contains weakened live flu viruses it is not for people with weakened immune systems or certain health conditions. The nasal mist vaccine is only for healthy, non-pregnant people between the ages of 2 and 49 years. Check with your doctor to see if your child can — or should — get this type of flu vaccine.
Who Should Get the Flu Shot?
Federal health officials now urge flu vaccination for all kids 6 months of age and older (instead of just the
youngest, as before). Although young tots (from 6 months to 5 years old) are still considered the group of kids who need the flu vaccine the most, updated guidelines from the CDC now recommend that all older kids and
teens get it, too (as long as enough is available).
People who should not get a flu shot include:
-anyone who's severely allergic and egg products (ingredients for flu shots are grown inside eggs, so tell your doctor if your child is allergic to eggs or egg products before he or she gets a flu shot) -anyone who's ever had a severe reaction to a flu vaccination (although most people do not
experience any side effects from the flu shot) Kids under 9 who get a flu shot for the first time will receive two separate shots a month apart. It can take about 1 or 2 weeks after the shot for the body to build up protection to the flu.
How the Flu Vaccine Works
Flu vaccines are available as a shot or nasal mist. Given as an injection, the flu shot contains killed flu viruses
that will not cause the flu, but will prepare the body to fight off infection by the live flu virus. Getting a shot of the killed virus means a person is protected against that particular type of live flu virus if he or she comes into contact with it. The nasal mist vaccine contains weakened live flu viruses. Because it contains live viruses, the mist is not for people with weakened immune systems or certain health conditions. Even if you or your child may have gotten the vaccine last year, that won't protect you from getting the flu this year because the protection wears off and flu viruses constantly change. That's why the vaccine is updated each year to include the most current strains of the virus. The flu vaccine reduces the average person's chances of catching the flu by up to 80% during the season. Because the vaccine prevents infection with only a few of the viruses that can cause flu-like symptoms, it isn't a guarantee against getting sick. But even if someone who's gotten the shot gets the flu, symptoms usually will be
Side Effects
Again, most people do not experience any side effects from the flu shot. According to the CDC, the flu shot
rarely causes serious harm. Some of those vaccinated may have soreness or swelling at the site of the injection or mild side effects, such as headache or low-grade fever. Although these side effects may last for a day, the flu can knock you off your feet for 2 to 3 weeks and can cause complications such as pneumonia, ear infections, sinus infections, and other respiratory problems. If your child does have symptoms after getting the flu shot, put a warm compress on the injection site to ease soreness or swelling, and give acetaminophen or ibuprofen for headache or low-grade fever. Do not give your
child aspirin unless your doctor instructs you to do so. A comabout the flu shot is that it can actually cause the flu. But the flu shot used in the United States is made from killed influenza viruses, which means that it's impossible to catch the flu by receiving it. However, because the nasal spray flu vaccine is made from live viruses, it may cause mild flu-like symptoms, including runny nose, headache, vomiting, muscle aches, and fever.

Source: http://www.qbs.edu.hk/sites/qbs/files/TheFluVaccine.pdf