What it prevents: No doubt you’re familiar with the un-fun symptoms of the flu – fever, sore throat, cough, and body aches – but you may not realize that this highly contagious virus can occasionally be deadly, killing about 36,000 people each year. This illness can also cause ear infections and pneumonia.

When and how it’s given: It’s recommended that children ages 6 months to 18 years be vaccinated against the flu once a year, ideally before the November to May flu season. There are two types of flu immunization: an inactivated version (given by injection) and a live version of the vaccine, which contains a weakened, but real form of the flu virus (given as a nasal spray). Children should receive an annual flu shot starting at 6 months of age. Once your child is two, he may be eligible to receive the nasal spray vaccination, assuming that he doesn’t have asthma and isn’t prone to episodes of wheezing when he’s sick. (If he does, he may be eligible for the flu spray after he turns five.) Parents and caregivers of babies should receive an annual flu immunization as well, not only to protect themselves, but also to create a protective “cocoon” of immunity around unvaccinated infants.

What you may have heard: Since the flu virus regularly changes, the vaccine is reformulated each year to try to match the current strains of the flu. However, even if the strains are not an exact match, the vaccine still offers some protection. Many flu shots—unlike other vaccines— still use thimerosal as a preservative. While research has disputed any concerns regarding side effects of thimerosal, you can ask your doctor about receiving a thimerosal-free version of the flu shot. The nasal spray does not contain thimerosal.

Risk of a reaction: The flu virus is grown in eggs. Children with a severe egg allergy generally do not receive a flu shot. If your child has an allergy to eggs (or you have a family history of egg allergy), talk to your doctor. Mild side effects of the injection include redness, soreness and swelling at the injection site, as well as mild cold symptoms (cough, fever, aches) that can last one to two days. Mild side effects of the spray include flu-like symptoms such as a cough, runny nose, fever, wheezing or upset stomach.