Perhaps there is no health issue more polarizing than the flu shot. But whether you get vaccinated every year or not at all, here are five things you need to know about getting immunized.

Turns out you're better off getting the flu shot than you are without says Dr. Ian Gemmill, medical officer of health for KFL&A Public Health in Kingston, Ont. "It's always worth getting the influenza vaccine to get protection against the three or four strains that are predicted to circulate," he says. "We don't always know what changes will occur with the virus at the beginning of the season, but the vaccine, which offers protection the majority of the time, remains the safest, longest-lasting and most effective way to prevent influenza."

1. When to get the shot
In most provinces, the flu vaccine is available in October, and the earlier you can get immunized, the better, says Dr. Jeff Kwong, family physician and flu researcher at the Institute for Clinical Evaluative Sciences in Toronto. That's because there's no way to predict when the virus will start circulating, and it takes your immune system two weeks to respond to the vaccine.

2. Number of doses
One per season—there's no evidence that doubling up offers added protection. The only exception: Kids under nine who are getting their first flu shot should get a second dose four weeks later.

3. Who shouldn't get the shot
If you've had a severe reaction to a flu shot in the past, you'll want to check with your doctor before getting this year's vaccine. Ditto if your doctor has advised you to avoid the jab. But one new development: An egg allergy is no longer considered a reason to avoid the vaccine (but if you've had an allergic reaction to eggs, check with your doctor first).

4. After-effects
While a couple of days of soreness at the injection site is normal, unlike what you might have heard, the shot can't cause an influenza infection, says Dr. Gemmill. That said, some first-timers may feel achy or tired for a day or so, as their immune systems are triggered into flu-fighting mode. These mild side-effects don't last long, especially when compared to the symptoms of a full-blown flu.

5. What else you can do
Getting a vaccination is the first step in stopping the spread of influenza, says Dr. Kwong. For the best protection, follow it up by washing your hands regularly—and stay home when you're sick.