A: Elena, it’s good that you got your son the flu (influenza) vaccine to help prevent him from catching and spreading the flu. Even though we mostly think of the flu as an illness in elderly people, young children also often catch it. Sometimes they get very sick with the flu, and they can also spread it to others who are more likely to get sick including infants, pregnant women, the elderly and people with chronic health problems.

There are now two different types of flu vaccine: an injected one and a nasal spray. To get the injected vaccine, a child must be at least 6 months old; to get the nasal spray vaccine, a child must be at least 5 years old and not have a chronic illness. For both vaccines, it’s recommended that children under 9 years of age receive two doses the first time they get the flu vaccine, at least four weeks apart for the injected vaccine and six weeks for the nasal spray. This recommendation was developed because studies found that young children’s immune systems had little experience with influenza and needed the second booster to get good immune protection against the flu. However, once a child’s immune system has been “primed” by a previous flu vaccine or a flu infection, one dose each year is sufficient.

It’s unfortunate that your doctor didn’t tell you last year that your son should have two doses. This might have left your son a little more vulnerable to catching the flu last year. But there’s no need to worry about it now. This year and in subsequent years, he should only need one dose since his immune system was primed by the vaccine last year.